{"history":[],"article":{"id":"7a585ef2-df0a-41e4-86ac-6a8f7b5069ee","title":"BKL singularity","slug":"bkl-singularity","content":":::summary\n:::\n\n# \nA **Belinski–Khalatnikov–Lifshitz (BKL) singularity** is a model of the dynamic [evolution of the universe](https://bliptext.com/articles/evolution-of-the-universe) near the initial [gravitational singularity](https://bliptext.com/articles/gravitational-singularity), described by an [anisotropic](https://bliptext.com/articles/anisotropic), [chaotic](https://bliptext.com/articles/chaos-physics) [solution of the Einstein field equation of gravitation](https://bliptext.com/articles/solutions-of-the-einstein-field-equations).\nAccording to this model, the universe is chaotically oscillating around a gravitational singularity in which time and space become equal to zero or, equivalently, the spacetime curvature becomes infinitely big.\nThis singularity is physically real in the sense that it is a necessary property of the solution, and will appear also in the [exact solution](https://bliptext.com/articles/exact-solutions-in-general-relativity) of those equations.\nThe singularity is not artificially created by the assumptions and simplifications made by the other special solutions such as the [Friedmann–Lemaître–Robertson–Walker](https://bliptext.com/articles/friedmann-lema-tre-robertson-walker-metric), quasi-isotropic, and [Kasner](https://bliptext.com/articles/kasner-metric) solutions.\nThe model is named after its authors [Vladimir Belinski](https://bliptext.com/articles/vladimir-belinski), [Isaak Khalatnikov](https://bliptext.com/articles/isaak-markovich-khalatnikov), and [Evgeny Lifshitz](https://bliptext.com/articles/evgeny-lifshitz), then working at the [Landau Institute for Theoretical Physics](https://bliptext.com/articles/landau-institute-for-theoretical-physics).\nThe picture developed by BKL has several important elements.\nThese are:\nThe study of the dynamics of the universe in the vicinity of the cosmological singularity has become a rapidly developing field of modern theoretical and mathematical physics.\nThe generalization of the BKL model to the cosmological singularity in multidimensional ([Kaluza–Klein type](https://bliptext.com/articles/kaluza-klein-theory)) cosmological models has a chaotic character in the spacetimes whose dimensionality is not higher than ten, while in the spacetimes of higher dimensionalities a universe after undergoing a finite number of oscillations enters into monotonic Kasner-type contracting regime.\nThe development of cosmological studies based on [superstring models](https://bliptext.com/articles/superstring-theory) has revealed some new aspects of the dynamics in the vicinity of the singularity.\nIn these models, mechanisms of changing of Kasner epochs are provoked not by the gravitational interactions but by the influence of other fields present.\nIt was proved that the cosmological models based on six main superstring models plus [eleven-dimensional supergravity](https://bliptext.com/articles/eleven-dimensional-supergravity) model exhibit the chaotic BKL dynamics towards the singularity.\nA connection was discovered between oscillatory BKL-like cosmological models and a special subclass of infinite-dimensional [Lie algebras](https://bliptext.com/articles/lie-algebra) – the so-called hyperbolic [Kac–Moody algebras](https://bliptext.com/articles/kac-moody-algebra).\n\n# Introduction\nThe basis of modern [cosmology](https://bliptext.com/articles/cosmology) are the special [solutions of the Einstein field equations](https://bliptext.com/articles/solutions-of-the-einstein-field-equations) found by [Alexander Friedmann](https://bliptext.com/articles/alexander-friedmann) in 1922–1924.\nThe Universe is assumed [homogeneous](https://bliptext.com/articles/homogeneous-space) (space has the same metric properties (measures) in all points) and [isotropic](https://bliptext.com/articles/isotropic) (space has the same measures in all directions).\nFriedmann's solutions allow two possible geometries for space: closed model with a ball-like, outwards-bowed space ([positive curvature](https://bliptext.com/articles/positive-curvature)) and open model with a saddle-like, inwards-bowed space ([negative curvature](https://bliptext.com/articles/negative-curvature)).\nIn both models, the Universe is not standing still, it is constantly either expanding (becoming larger) or contracting (shrinking, becoming smaller).\nThis was confirmed by [Edwin Hubble](https://bliptext.com/articles/edwin-hubble) who established the [Hubble redshift](https://bliptext.com/articles/hubble-redshift) of receding galaxies.\nThe present consensus is that the [isotropic model](https://bliptext.com/articles/big-bang), in general, gives an adequate description of the present state of the Universe; however, isotropy of the present Universe by itself is not a reason to expect that it is adequate for describing the early stages of [Universe evolution](https://bliptext.com/articles/cosmic-evolution).\nAt the same time, it is obvious that in the real world [homogeneity](https://bliptext.com/articles/homogeneity) is, at best, only an approximation.\nEven if one can speak about a homogeneous distribution of matter density at distances that are large compared to the intergalactic space, this homogeneity vanishes at smaller scales.\nOn the other hand, the homogeneity assumption goes very far in a mathematical aspect: it makes the solution highly [symmetric](https://bliptext.com/articles/symmetric) which can impart specific properties that disappear when considering a more general case.\nAnother important property of the isotropic model is the inevitable existence of a [time singularity](https://bliptext.com/articles/gravitational-singularity): time flow is not continuous, but stops or reverses after time reaches some very large or very small value.\nBetween singularities, time flows in one direction: away from the singularity ([arrow of time](https://bliptext.com/articles/arrow-of-time)).\nIn the open model, there is one time singularity so time is limited at one end but unlimited at the other, while in the closed model there are two singularities that limit time at both ends (the [Big Bang](https://bliptext.com/articles/big-bang) and [Big Crunch](https://bliptext.com/articles/big-crunch)).\nThe only physically interesting properties of [spacetimes](https://bliptext.com/articles/spacetime) (such as singularities) are those which are [stable](https://bliptext.com/articles/stability-theory), i.e., those properties which still occur when the initial data is perturbed slightly.\nIt is possible for a singularity to be stable and yet be of no physical interest: stability is a necessary but not a sufficient condition for physical relevance.\nFor example, a singularity could be stable only in a neighbourhood of initial data sets corresponding to highly [anisotropic](https://bliptext.com/articles/anisotropy) universes.\nSince the actual universe is now apparently almost isotropic such a singularity could not occur in our universe.\nA sufficient condition for a stable singularity to be of physical interest is the requirement that the singularity be [generic](https://bliptext.com/articles/generic-property) (or general).\nRoughly speaking, a stable singularity is generic if it occurs near every set of initial conditions and the non-gravitational fields are restricted in some specified way to \"physically realistic\" fields so that the Einstein equations, various equations of state, etc., are assumed to hold on the evolved spacetimes.\nIt might happen that a singularity is stable under small variations of the true gravitational [degrees of freedom](https://bliptext.com/articles/degrees-of-freedom-physics-and-chemistry), and yet it is not generic because the singularity depends in some way on the [coordinate system](https://bliptext.com/articles/coordinate-system), or rather on the choice of the initial [hypersurface](https://bliptext.com/articles/hypersurface) from which the spacetime is evolved.\nFor a system of [non-linear differential equations](https://bliptext.com/articles/non-linear-differential-equation), such as the [Einstein equations](https://bliptext.com/articles/einstein-field-equations), a [general solution](https://bliptext.com/articles/linear-differential-equation) is not unambiguously defined.\nIn principle, there may be multiple [general integrals](https://bliptext.com/articles/antiderivative), and each of those may contain only a finite [subset](https://bliptext.com/articles/subset) of all possible [initial conditions](https://bliptext.com/articles/initial-condition).\nEach of those [integrals](https://bliptext.com/articles/integral) may contain all required [independent](https://bliptext.com/articles/algebraic-independence) [functions](https://bliptext.com/articles/function-mathematics) which, however, may be subject to some conditions (e.g., some [inequalities](https://bliptext.com/articles/inequality-mathematics)).\nExistence of a general solution with a singularity, therefore, does not preclude the existence of other additional general solutions that do not contain a singularity.\nFor example, there is no reason to doubt the existence of a general solution without a singularity that describes an isolated body with a relatively small mass.\nIt is impossible to find a general integral for all space and for all time.\nHowever, this is not necessary for resolving the problem: it is sufficient to study the solution near the singularity.\nThis would also resolve another aspect of the problem: the characteristics of [spacetime metric](https://bliptext.com/articles/spacetime-metric) evolution in the general solution when it reaches the physical singularity, understood as a point where [matter density](https://bliptext.com/articles/matter-density) and [invariants](https://bliptext.com/articles/invariants-of-tensors) of the [Riemann curvature tensor](https://bliptext.com/articles/riemann-curvature-tensor) become infinite.\n\n# Existence of physical time singularity\nOne of the principal problems studied by the [Landau group](https://bliptext.com/articles/lev-davidovich-landau) (to which BKL belong) was whether [relativistic](https://bliptext.com/articles/general-relativity) [cosmological models](https://bliptext.com/articles/cosmological-model) necessarily contain a time singularity or whether the time singularity is an artifact of the assumptions used to simplify these models.\nThe independence of the singularity on symmetry assumptions would mean that time singularities exist not only in the special, but also in the general solutions of the Einstein equations.\nIt is reasonable to suggest that if a singularity is present in the general solution, there must be some indications that are based only on the most general properties of the Einstein equations, although those indications by themselves might be insufficient for characterizing the singularity.\nA criterion for generality of solutions is the number of independent space coordinate functions that they contain.\nThese include only the \"physically independent\" functions whose number cannot be reduced by any choice of [reference frame](https://bliptext.com/articles/frame-of-reference).\nIn the general solution, the number of such functions must be enough to fully define the [initial conditions](https://bliptext.com/articles/initial-conditions) (distribution and movement of matter, distribution of [gravitational field](https://bliptext.com/articles/gravitational-field)) at some moment of time chosen as initial.\nThis number is four for an empty (vacuum) space, and eight for a matter and/or radiation-filled space.\nPrevious work by the Landau group; reviewed in ) led to the conclusion that the general solution does not contain a physical singularity. This search for a broader class of solutions with a singularity has been done, essentially, by a trial-and-error method, since a systematic approach to the study of the Einstein equations was lacking. A negative result, obtained in this way, is not convincing by itself; a solution with the necessary degree of generality would invalidate it, and at the same time would confirm any positive results related to the specific solution.\nAt that time, the only known indication for the existence of physical singularity in the general solution was related to the form of the Einstein equations written in a [synchronous frame](https://bliptext.com/articles/synchronous-frame), that is, in a frame in which the proper time x0 = t is synchronized throughout the whole space; in this frame the space distance element dl is separate from the time interval dt.\nThe Einstein equation written in synchronous frame gives a result in which the metric [determinant](https://bliptext.com/articles/determinant) g inevitably becomes zero in a finite time irrespective of any assumptions about matter distribution.\nHowever, the efforts to find a general physical singularity were foregone after it became clear that the singularity mentioned above is linked with a specific geometric property of the synchronous frame: the crossing of time line coordinates.\nThis crossing takes place on some encircling [hypersurfaces](https://bliptext.com/articles/hypersurface) which are four-dimensional analogs of the [caustic surfaces](https://bliptext.com/articles/caustic-optics) in [geometrical optics](https://bliptext.com/articles/geometrical-optics); g becomes zero exactly at this crossing.\nTherefore, although this singularity is general, it is fictitious, and not a physical one; it disappears when the reference frame is changed.\nThis, apparently, dissuaded the researchers for further investigations along these lines.\nSeveral years passed before the interest in this problem waxed again when published his theorems that linked the existence of a singularity of unknown character with some very general assumptions that did not have anything in common with a choice of reference frame.\nOther similar theorems were found later on by [Hawking](https://bliptext.com/articles/stephen-hawking) and Geroch (see Penrose–[Hawking](https://bliptext.com/articles/stephen-hawking) singularity theorems).\nThis revived interest in the search for singular solutions.\n\n# Generalized homogeneous solution\nIn a space that is both homogeneous and isotropic the metric is determined completely, leaving free only the sign of the curvature.\nAssuming only space homogeneity with no additional symmetry such as isotropy leaves considerably more freedom in choosing the metric.\nThe following pertains to the space part of the metric at a given instant of time t assuming a synchronous frame so that t is the same synchronised time for the whole space.\n\n## The BKL conjecture\nIn their 1970 work, BKL stated that as one approaches a singularity, terms containing time derivatives in Einstein's equations dominate over those containing spatial derivatives.\nThis has since been known as the **BKL conjecture** and implies that Einstein's [partial differential equations](https://bliptext.com/articles/partial-differential-equations) (PDE) are well approximated by [ordinary differential equations](https://bliptext.com/articles/ordinary-differential-equations) (ODEs), whence the dynamics of general relativity effectively become local and oscillatory.\nThe time evolution of fields at each spatial point is well approximated by the homogeneous cosmologies in the Bianchi classification.\nBy separating the time and space derivatives in the Einstein equations, for example, in the way used for the[ classification of homogeneous spaces](https://bliptext.com/articles/bianchi-classification), and then setting the terms containing space derivatives equal to zero, one can define the so-called truncated theory of the system (truncated equations).\nThen, the BKL conjecture can be made more specific:\nWeak conjecture: As the singularity is approached the terms containing space derivatives in the Einstein equations are negligible in comparison to the terms containing time derivatives.\nThus, as the singularity is approached the Einstein equations approach those found by setting derivative terms to zero.\nThus, the weak conjecture says that the Einstein equations can be well approximated by the truncated equations in the vicinity of the singularity.\nNote that this does not imply that the solutions of the full equations of motion will approach the solutions to the truncated equations as the singularity is approached.\nThis additional condition is captured in the strong version as follows.\nStrong conjecture: As the singularity is approached the Einstein equations approach those of the truncated theory and in addition the solutions to the full equations are well approximated by solutions to the truncated equations.\nIn the beginning, the BKL conjecture seemed to be coordinate-dependent and rather implausible.\nBarrow and Tipler, for example, among the ten criticisms of BKL studies, include the inappropriate (according to them) choice of synchronous frame as a means to separate time and space derivatives.\nThe BKL conjecture was sometimes rephrased in the literature as a statement that near the singularity only the time derivatives are important.\nSuch a statement, taken at face value, is wrong or at best misleading since, as shown in the BKL analysis itself, space-like gradients of the metric tensor cannot be neglected for generic solutions of pure Einstein gravity in four spacetime dimensions, and in fact play a crucial role in the appearance of the oscillatory regime.\nHowever, there exist reformulations of Einstein theory in terms of new variables involving the relevant gradients, for example in Ashtekar-like variables, for which the statement about the dominant role of the time derivatives is correct.\nIt is true that one gets at each spatial point an effective description of the singularity in terms of a finite dimensional dynamical system described by ordinary differential equations with respect to time, but the spatial gradients do enter these equations non-trivially.\nSubsequent analysis by a large number of authors has shown that the BKL conjecture can be made precise and by now there is an impressive body of numerical and analytical evidence in its support.\nIt is fair to say that we are still quite far from a proof of the strong conjecture.\nBut there has been outstanding progress in simpler models.\nIn particular, Berger, Garfinkle, Moncrief, Isenberg, Weaver, and others showed that, in a class of models, as the singularity is approached the solutions to the full Einstein field equations approach the \"velocity term dominated\" (truncated) ones obtained by neglecting spatial derivatives.\nAndersson and Rendall showed that for gravity coupled to a massless scalar field or a stiff fluid, for every solution to the truncated equations there exists a solution to the full field equations that converges to the truncated solution as the singularity is approached, even in the absence of symmetries.\nThese results were generalized to also include p-form [gauge fields](https://bliptext.com/articles/gauge-theory).\nIn these truncated models the dynamics are simpler, allowing a precise statement of the conjecture that could be proven.\nIn the general case, the strongest evidence to date comes from numerical evolutions.\nBerger and Moncrief began a program to analyze generic cosmological singularities.\nWhile the initial work focused on symmetry reduced cases, more recently Garfinkle performed numerical evolution of space-times with no symmetries in which, again, the mixmaster behavior is apparent.\nFinally, additional support for the conjecture has come from a numerical study of the behavior of test fields near the singularity of a Schwarzschild black hole.\n\n## Kasner solution\nThe BKL approach to anisotropic (as opposed to isotropic) homogeneous spaces starts with a generalization of an exact [particular solution](https://bliptext.com/articles/ordinary-differential-equation) derived by Kasner for a field in vacuum, in which the space is homogeneous and has a [Euclidean metric](https://bliptext.com/articles/euclidean-metric) that depends on time according to the [Kasner metric](https://bliptext.com/articles/kasner-metric)\n(dl is the [line element](https://bliptext.com/articles/line-element); dx, dy, dz are [infinitesimal](https://bliptext.com/articles/infinitesimal) [displacements](https://bliptext.com/articles/differential-infinitesimal) in the three [spatial dimensions](https://bliptext.com/articles/spatial-dimensions), and t is time period passed since some initial moment t0 = 0).\nHere, p1, p2, p3 are any three numbers that satisfy the following Kasner conditions\nBecause of these relations, only one of the three numbers is [independent](https://bliptext.com/articles/independent-variable) (two [equations](https://bliptext.com/articles/equations) with three [unknowns](https://bliptext.com/articles/unknown-mathematics)).\nAll three numbers are never the same; two numbers are the same only in the [sets](https://bliptext.com/articles/set-mathematics) of values and (0, 0, 1).\nIn all other cases the numbers are different, one number is negative and the other two are positive.\nThis is partially proved by squaring both sides of the first condition and developing the square:\nThe term is equal to 1 by dint of the second condition and therefore the term with the mixed products should be zero.\nThis is possible if at least one of the p1, p2, p3 is negative.\nIf the numbers are arranged in increasing order, p1 < p2 < p3, they change in the [intervals](https://bliptext.com/articles/interval-mathematics) (Fig. 4)\nThe Kasner metric corresponds to a flat homogenous but anisotropic space in which all volumes increase with time in such a way that the linear distances along two axes y and z increase while the distance along the axis x decreases.\nThe moment t = 0 causes a singularity in the solution; the singularity in the metric at t = 0 cannot be avoided by any reference frame transformation.\nAt the singularity, the invariants of the four-dimensional curvature tensor go to infinity.\nAn exception is the case p1 = р2 = 0, р3 = 1; these values correspond to a flat spacetime: the transformation t sh z = ζ, t ch z = τ turns the Kasner metric into [Galilean](https://bliptext.com/articles/galilean-transformation).\nBKL [parametrize](https://bliptext.com/articles/parametrization-geometry) the numbers p1, p2, p3 in terms of a single independent (real) [parameter](https://bliptext.com/articles/parameter) u (Lifshitz-Khalatnikov [parameter](https://bliptext.com/articles/parameter)) as follows\nThe Kasner index parametrization appears mysterious until one thinks about the two constraints on the indices.\nBoth constraints fix the overall scale of the indices so that only their [ratios](https://bliptext.com/articles/ratios) can vary.\nIt is natural to pick one of those ratios as a new parameter, which can be done in six different ways.\nPicking u = u32 = p3 / p2, for example, it is trivial to express all six possible ratios in terms of it.\nEliminating p3 = up2 first, and then using the linear constraint to eliminate p1 = 1 − p2 − up2 = 1 − (1 + u)p2, the quadratic constraint reduces to a [quadratic equation](https://bliptext.com/articles/quadratic-equation) in p2\nwith [roots](https://bliptext.com/articles/root-of-an-equation) p2 = 0 (obvious) and p2 = (1 + u) / (1 + u + u2), from which p1 and p3 are then obtained by [back substitution](https://bliptext.com/articles/back-substitution).\nOne can define six such parameters uab = pa / pb, for which pc ≤ pb ≤ pa when (c, b, a) is a [cyclic permutation](https://bliptext.com/articles/cyclic-permutation) of (1, 2, 3).\nAll different values of p1, p2, p3 ordered as above are obtained with u running in the range u ≥ 1.\nThe values u < 1 are brought into this range according to\nIn the generalized solution, the form corresponding to applies only to the [asymptotic](https://bliptext.com/articles/asymptote) metric (the metric close to the singularity t = 0), respectively, to the major terms of its [series expansion by powers](https://bliptext.com/articles/power-series) of t.\nIn the synchronous reference frame it is written in the form of with a space distance element\nwhere\nThe three-di**m**e**n**sio**n**a**l** [vectors](https://bliptext.com/articles/vector-mathematics-and-physics) **l**, **m**, **n** defi**n**e the directio**n**s at which space dista**n**ce cha**n**ges with ti**m**e by the power **l**aws.\nThese vectors, as well as the numbers pl, pm, pn which, as before, are related by, are functions of the space coordinates.\nThe powers pl, pm, pn are not arranged in increasing order, reserving the symbols p1, p2, p3 for the numbers in that remain arranged in increasing order.\nThe [determinant](https://bliptext.com/articles/determinant) of the metric of is\nwhere v = **l**[**mn**].\nIt is convenient to introduce the following quantities\nThe space metric in is anisotropic because the powers of t in cannot have the same values.\nOn approaching the singularity at t = 0, the linear distances in each space element decrease in two directions and increase in the third direction.\nThe volume of the element decreases in proportion to t.\nThe Kasner metric is introduced in the Einstein equations by substituting the respective metric tensor γαβ from without defining a priori the dependence of a, b, c from t:\nwhere the dot above a symbol designates differentiation with respect to time.\nThe Einstein equation takes the form\nAll its terms are to a second order for the large (at t → 0) quantity 1/t.\nIn the Einstein equations, terms of such order appear only from terms that are time-differentiated.\nIf the components of Pαβ do not include terms of order higher than two, then\nwhere i**n**dices **l**, **m**, **n** desig**n**ate te**n**sor co**m**po**n**e**n**ts i**n** the directio**n**s **l**, **m**, **n**.\nThese equations together with give the expressions with powers that satisfy.\nHowever, the presence of one negative power among the 3 powers pl, pm, pn results in appearance of terms from Pαβ with an order greater than t−2.\nIf the negative power is pl (pl = p1 < 0), then Pαβ contains the coordinate function λ and become\nHere, the second terms are of order t−2(pm + pn − pl) whereby pm + pn − pl = 1 + 2 |pl| > 1.\nTo remove these terms and restore the metric, it is necessary to impose on the coordinate functions the condition λ = 0.\nThe remaining three Einstein equations contain only [first order time derivatives](https://bliptext.com/articles/first-order-derivative-expression) of the metric tensor.\nThey give three time-independent relations that must be imposed as necessary conditions on the coordinate functions in.\nThis, together with the condition λ = 0, makes four conditions.\nThese co**n**ditio**n**s bi**n**d te**n** differe**n**t coordi**n**ate fu**n**ctio**n**s: three co**m**po**n**e**n**ts of each of the vectors **l**, **m**, **n**, a**n**d o**n**e fu**n**ctio**n** i**n** the powers of t (a**n**y o**n**e of the fu**n**ctio**n**s p**l**, p**m**, p**n**, which are bou**n**d by the co**n**ditio**n**s ).\nWhen calculating the number of physically arbitrary functions, it must be taken into account that the synchronous system used here allows time-independent arbitrary [transformations](https://bliptext.com/articles/transformation-function) of the three space coordinates.\nTherefore, the final solution contains overall 10 − 4 − 3 = 3 physically arbitrary functions which is one less than what is needed for the general solution in vacuum.\nThe degree of generality reached at this point is not lessened by introducing matter; matter is written into the metric and contributes four new coordinate functions necessary to describe the initial distribution of its density and the three components of its velocity.\nThis makes possible to determine matter evolution merely from the laws of its movement in an a priori given gravitational field which are the [hydrodynamic](https://bliptext.com/articles/fluid-dynamics) equations\nwhere ui is the 4-dimensional velocity, ε and σ are the densities of energy and [entropy](https://bliptext.com/articles/entropy) of matter (cf. and; also; for details see ).\nFor the [ultrarelativistic](https://bliptext.com/articles/ultrarelativistic-limit) [equation of state](https://bliptext.com/articles/equation-of-state) p = ε/3 the entropy σ ~ ε1/4.\nThe major terms in and are those that contain time [derivatives](https://bliptext.com/articles/derivative).\nFrom and the space components of one has\nresulting in\nwhere 'const' are time-independent quantities.\nAdditionally, from the identity uiui = 1 one has (because all covariant components of uα are to the same order)\nwhere u**n** is the velocity compo**n**e**n**t alo**n**g the directio**n** of **n** that is co**n****n**ected with the highest (positive) power of t (supposi**n**g that p**n** = p3).\nFrom the above relations, it follows that\nor\nThe above equations can be used to confirm that the components of the matter [stress-energy-momentum tensor](https://bliptext.com/articles/stress-energy-momentum-tensor) standing in the right hand side of the equations\nare, indeed, to a lower order by 1/t than the major terms in their left hand sides.\nIn the equations the presence of matter results only in the change of relations imposed on their constituent coordinate functions.\nThe fact that ε becomes infinite by the law confirms that in the solution to one deals with a physical singularity at any values of the powers p1, p2, p3 excepting only (0, 0, 1).\nFor these last values, the singularity is non-physical and can be removed by a change of reference frame.\nThe fictional singularity corresponding to the powers (0, 0, 1) arises as a result of time line coordinates crossing over some 2-dimensional \"[focal surface](https://bliptext.com/articles/focal-surface)\".\nAs pointed out in, a synchronous reference frame can always be chosen in such a way that this inevitable time line crossing occurs exactly on such surface (instead of a 3-dimensional caustic surface).\nTherefore, a solution with such simultaneous for the whole space fictional singularity must exist with a full set of arbitrary functions needed for the general solution.\nClose to the point t = 0 it allows a regular expansion by whole powers of t.\nFor an analysis of this case, see.\n# [Plot of p1, p2, p3 with an argument 1/u. The numbers p1(u) and p3(u) are monotonously increasing while p2(u) is a monotonously decreasing function of u. | upload.wikimedia.org/wikipedia/commons/c/c9/Bkl///fig1.svg]\n\n# Oscillating mode towards the singularity\nThe general solution by definition is completely stable; otherwise the Universe would not exist.\nAny [perturbation](https://bliptext.com/articles/perturbation-astronomy) is equivalent to a change in the initial conditions in some moment of time; since the general solution allows arbitrary initial conditions, the [perturbation](https://bliptext.com/articles/perturbation-astronomy) is not able to change its character.\nLooked at such angle, the four conditions imposed on the coordinate functions in the solution are of different types: three conditions that arise from the equations = 0 are \"natural\"; they are a consequence of the structure of Einstein equations.\nHowever, the additional condition λ = 0 that causes the loss of one derivative function, is of entirely different type: instability caused by perturbations can break this condition.\nThe action of such perturbation must bring the model to another, more general, mode.\nThe perturbation cannot be considered as small: a transition to a new mode exceeds the range of very small perturbations.\nThe analysis of the behavior of the model under perturbative action, performed by BKL, delineates a complex [oscillatory](https://bliptext.com/articles/oscillation) mode on approaching the singularity.\nThey could not give all details of this mode in the broad frame of the general case.\nHowever, BKL explained the most important properties and character of the solution on specific models that allow far-reaching analytical study.\nThese models are based on a [homogeneous space](https://bliptext.com/articles/homogeneous-space) metric of a particular type.\nSupposing a homogeneity of space without any additional symmetry leaves a great freedom in choosing the metric.\nAll possible homogeneous (but anisotropic) spaces are classified, according to [Bianchi](https://bliptext.com/articles/luigi-bianchi), in several [Bianchi](https://bliptext.com/articles/luigi-bianchi) types (Type I to IX).\n(see also [Generalized homogeneous solution](https://bliptext.com/articles/)) BKL investigate only spaces of Bianchi Types VIII and IX.\nIf the **m**etric has the for**m** of, for each type of ho**m**oge**n**eous spaces exists so**m**e fu**n**ctio**n**a**l** re**l**atio**n** betwee**n** the refere**n**ce vectors **l**, **m**, **n** a**n**d the space coordi**n**ates.\nThe specific form of this relation is not important.\nThe i****m****porta**n**t fact is that for Type VIII a**n**d IX spaces, the qua**n**tities λ, μ, ν are co**n**sta**n**ts whi******l******e a******l************l****** \"****m****ixed\" products ******l****** rot ****m****, ******l****** rot **n**, ****m**** rot ******l******, etc..\nare zeros.\nFor Type IX spaces, the quantities λ, μ, ν have the same sign and one can write λ = μ = ν = 1 (the simultaneous sign change of the 3 constants does not change anything).\nFor Type VIII spaces, 2 constants have a sign that is opposite to the sign of the third constant; one can write, for example, λ = − 1, μ = ν = 1.\nThe study of the effect of the perturbation on the \"Kasner mode\" is thus confined to a study on the effect of the λ-containing terms in the Einstein equations.\nType VIII and IX spaces are the most suitable models for such a study.\nSi**n**ce a**l****l** 3 qua**n**tities λ, μ, ν i**n** those Bia**n**chi types differ fro**m** zero, the co**n**ditio**n** λ = 0 does **n**ot ho**l**d irrespective of which directio**n** **l**, **m**, **n** has **n**egative power **l**aw ti**m**e depe**n**de**n**ce.\nThe Einstein equations for the Type VIII and Type IX space models are\n(the remaining components R_l^0, R_m^0, R_n^0, R_l^m, R_l^n, R_m^n are identically zeros).\nThese equations contain only functions of time; this is a condition that has to be fulfilled in all homogeneous spaces.\nHere, the and are exact and their validity does not depend on how near one is to the singularity at t = 0.\nThe time derivatives in and take a simpler form if а, b, с are substituted by their logarithms α, β, γ:\nsubstituting the variable t for τ according to:\nThen (subscripts denote differentiation by τ):\nAdding together equations and substituting in the left hand side the sum (α + β + γ)τ τ according to, one obtains an equation containing only first derivatives which is the [first integral](https://bliptext.com/articles/first-integral) of the system :\nThis equation plays the role of a binding condition imposed on the initial state of.\nThe Kasner mode is a solution of when ignoring all terms in the right hand sides.\nBut such situation cannot go on (at t → 0) indefinitely because among those terms there are always some that grow.\nThus, if the negative power is in the function a(t) (pl = p1) then the perturbation of the Kasner mode will arise by the terms λ2a4; the rest of the terms will decrease with decreasing t.\nIf only the growing terms are left in the right hand sides of, one obtains the system:\n(compare ; below it is substituted λ2 = 1).\nThe solution of these equations must describe the metric evolution from the initial state, in which it is described by with a given set of powers (with pl < 0); let pl = р1, pm = р2, pn = р3 so that\nThen\nwhere Λ is constant.\nInitial conditions for are redefined as\nEquations are easily integrated; the solution that satisfies the condition is\nwhere b0 and c0 are two more constants.\nIt can easily be seen that the asymptotic of functions at t → 0 is.\nThe asymptotic expressions of these functions and the function t(τ) at τ → −∞ is\nExpressing a, b, c as functions of t, one has\nwhere\nThen\nThe above shows that perturbation acts in such a way that it changes one Kasner **m**ode with another Kasner **m**ode, and in this process the negative power of t f**l**ips fro**m** direction **l** to direction **m**: if before it was p**l** < 0, now it is p'**m** < 0.\nDuring this change the function a(t) passes through a maximum and b(t) passes through a minimum; b, which before was decreasing, now increases: a from increasing becomes decreasing; and the decreasing c(t) decreases further.\nThe perturbation itself (λ2a4α in ), which before was increasing, now begins to decrease and die away.\nFurther evolution similarly causes an increase in the perturbation from the terms with μ2 (instead of λ2) in, next change of the Kasner mode, and so on.\nIt is convenient to write the power substitution rule with the help of the parametrization :\nThe greater of the two positive powers remains positive.\nBKL call this flip of negative power between directions a Kasner [epoch](https://bliptext.com/articles/epoch-astronomy).\nThe key to understanding the character of metric evolution on approaching singularity is exactly this process of Kasner epoch alternation with flipping of powers pl, pm, pn by the rule.\nThe successive a**l**ternations with f**l**ipping of the negative power p1 between directions **l** and **m** (Kasner epochs) continues by dep**l**etion of the who**l**e part of the initia**l** u unti**l** the **m**o**m**ent at which u < 1.\nThe value u < 1 transforms into u > 1 according to ; in this moment the negative power is pl or pm while pn becomes the lesser of two positive numbers (pn = p2).\nThe ****n****ext series of Kas****n****er epochs the****n**** f**l**ips the ****n****egative power betwee****n**** directio****n****s ****n**** a****n****d **l** or betwee****n**** ****n**** a****n****d **m**.\nAt an arbitrary ([irrational](https://bliptext.com/articles/irrational-number)) initial value of u this process of alternation continues unlimited.\nIn the exact solution of the Einstein equations, the powers pl, pm, pn lose their original precise sense.\nThis circumstance introduces some \"fuzziness\" in the determination of these numbers (and together with them, to the parameter u) which, although small, makes meaningless the analysis of any definite (for example, [rational](https://bliptext.com/articles/rational-number)) values of u.\nTherefore, only these laws that concern arbitrary irrational values of u have any particular meaning.\nThe larger periods in which the scales of space distances along two axes oscillate while distances along the third axis decrease monotonously, are called [eras](https://bliptext.com/articles/era); volumes decrease by a law close to ~ t.\nOn transition from one era to the next, the direction in which distances decrease [monotonously](https://bliptext.com/articles/monotonic-function), flips from one axis to another.\nThe order of these transitions acquires the asymptotic character of a [random process](https://bliptext.com/articles/stochastic-process).\nThe same random order is also characteristic for the alternation of the lengths of successive eras (by era length, BKL understand the number of Kasner epoch that an era contains, and not a time interval).\nTo each era (s-th era) correspond a series of values of the parameter u starting from the greatest,, and through the values − 1, − 2, ..., reaching to the smallest, < 1.\nThen\nthat is, k(s) = [] where the brackets mean the whole part of the value.\nThe number k(s) is the era length, measured by the number of Kasner epochs that the era contains.\nFor the next era\nIn the limitless series of numbers u, composed by these rules, there are infinitesimally small (but never zero) values x(s) and correspondingly infinitely large lengths k(s).\nThe era series become denser on approaching t = 0.\nHowever, the natural variable for describing the time course of this evolution is not the world time t, but its logarithm, ln t, by which the whole process of reaching the singularity is extended to −∞.\nAccording to, one of the functions a, b, c, that passes through a maximum during a transition between Kasner epochs, at the peak of its maximum is\nwhere it is supposed that amax is large compared to b0 and c0; in u is the value of the parameter in the Kasner epoch before transition.\nIt can be seen from here that the peaks of consecutive maxima during each era are gradually lowered.\nIndeed, in the next Kasner epoch this parameter has the value u' = u − 1, and Λ is substituted according to with Λ' = Λ(1 − 2|p1(u)|).\nTherefore, the ratio of 2 consecutive maxima is\nand finally\nThe above are solutions to Einstein equations in vacuum.\nAs for the pure Kasner mode, matter does not change the qualitative properties of this solution and can be written into it disregarding its reaction on the field.\nHowever, if one does this for the model under discussion, understood as an exact solution of the Einstein equations, the resulting picture of matter evolution would not have a general character and would be specific for the high symmetry imminent to the present model.\nMathematically, this specificity is related to the fact that for the homogeneous space geometry discussed here, the Ricci tensor components R_\\alpha^0 are identically zeros and therefore the Einstein equations would not allow movement of matter (which gives non-zero stress energy-momentum tensor components T_\\alpha^0).\nIn other words, the synchronous frame must also be co-moving with respect to matter.\nIf one substitutes in uα = 0, u0 = 1, it becomes ε ~ (abc)−4/3 ~ t−4/3.\nThis difficulty is avoided if one includes in the model only the major terms of the limiting (at t → 0) metric and writes into it a matter with arbitrary initial distribution of densities and velocities.\nThen the course of evolution of matter is determined by its general laws of movement and that result in.\nDuring each Kasner epoch, density increases by the law\nwhere p3 is, as above, the greatest of the numbers p1, p2, p3.\nMatter density increases monotonously during all evolution towards the singularity.\n\n# Metric evolution\nVery large u values correspond to Kasner powers\nwhich are close to the values (0, 0, 1).\nTwo values that are close to zero, are also close to each other, and therefore the changes in two out of the three types of \"perturbations\" (the terms with λ, μ and ν in the right hand sides of ) are also very similar.\nIf in the beginning of such long era these terms are very close in absolute values in the moment of transition between two Kasner epochs (or made artificially such by assigning initial conditions) then they will remain close during the greatest part of the length of the whole era.\nIn this case (BKL call this the case of small oscillations), analysis based on the action of one type of perturbations becomes incorrect; one must take into account the simultaneous effect of two perturbation types.\n\n## Two perturbations\nConsider a long era, during which two of the functions a, b, c (let them be a and b) undergo small oscillations while the third function (c) decreases monotonously.\nThe latter function quickly becomes small; consider the solution just in the region where one can ignore c in comparison to a and b.\nThe calculations are first done for the Type IX space model by substituting accordingly λ = μ = ν = 1.\nAfter ignoring function c, the first 2 equations give\nand can be used as a third equation, which takes the form\nThe solution of is written in the form\nwhere α0, ξ0 are positive constants, and τ0 is the upper limit of the era for the variable τ.\nIt is convenient to introduce further a new variable (instead of τ)\nThen\nEquations and are transformed by introducing the variable χ = α − β:\nDecrease of τ from τ0 to −∞ corresponds to a decrease of ξ from ξ0 to 0.\nThe long era with close a and b (that is, with small χ), considered here, is obtained if ξ0 is a very large quantity.\nIndeed, at large ξ the solution of in the first approximation by 1/ξ is\nwhere A is constant; the multiplier makes χ a small quantity so it can be substituted in by sh 2χ ≈ 2χ.\nFrom one obtains\nAfter determining α and β from and and expanding eα and eβ in series according to the above approximation, one obtains finally:\nThe relation between the variable ξ and time t is obtained by integration of the definition dt = abc dτ which gives\nThe constant c0 (the value of с at ξ = ξ0) should be now c0 \\ll α0·\nLet us now consider the domain ξ \\ll 1.\nHere the major terms in the solution of are:\nwhere k is a constant in the range − 1 < k < 1; this condition ensures that the last term in is small (sh 2χ contains ξ2k and ξ−2k).\nThen, after determining α, β, and t, one obtains\nThis is again a Kasner mode with the negative t power present in the function c(t).\nThese results picture an evolution that is qualitatively similar to that, described above.\nDuring a long period of time that corresponds to a large decreasing ξ value, the two functions a and b oscillate, remaining close in magnitude ; in the same time, both functions a and b slowly decrease.\nThe period of oscillations is constant by the variable ξ : Δξ = 2π (or, which is the same, with a constant period by logarithmic time: Δ ln t = 2πΑ2).\nThe third function, c, decreases monotonously by a law close to c = c0t/t0.\nThis evolution continues until ξ ≈1 and formulas and are no longer applicable.\nIts time duration corresponds to change of t from t0 to the value t1, related to ξ0 according to\nThe relationship between ξ and t during this time can be presented in the form\nAfter that, as seen from, the decreasing function c starts to increase while functions a and b start to decrease.\nThis Kasner epoch continues until terms c2/a2b2 in become ~ t2 and a next series of oscillations begins.\nThe law for density change during the long era under discussion is obtained by substitution of in :\nWhen ξ changes from ξ0 to ξ ≈1, the density increases \\xi^2_0 times.\nIt must be stressed that although the function c(t) changes by a law, close to c ~ t, the metric does not correspond to a Kasner metric with powers (0, 0, 1).\nThe latter corresponds to an exact solution found by Taub which is allowed by eqs.\n– and in which\nwhere p, δ1, δ2 are constant.\nIn the asymptotic region τ → −∞, one can obtain from here a = b = const, c = const.t after the substitution ерτ = t.\nIn this metric, the singularity at t = 0 is non-physical.\nLet us now describe the analogous study of the Type VIII model, substituting in eqs.\n**–** λ = −1, μ = ν = 1.\nIf during the long era, the monotonically decreasing function is a, nothing changes in the foregoing analysis: ignoring a2 on the right side of equations and, goes back to the same equations and (with altered notation).\nSome changes occur, however, if the monotonically decreasing function is b or c; let it be c.\nAs before, one has equation with the same symbols, and, therefore, the former expressions for the functions a(ξ) and b(ξ), but equation is replaced by\nThe major term at large ξ now becomes\nso that\nThe value of c as a function of time t is again c = c0t/t0 but the time dependence of ξ changes.\nThe length of a long era depends on ξ0 according to\nOn the other hand, the value ξ0 determines the number of oscillations of the functions a and b during an era (equal to ξ0/2π).\nGiven the length of an era in logarithmic time (i.e., with given ratio t0/t1) the number of oscillations for Type VIII will be, generally speaking, less than for Type IX.\nFor the period of oscillations one gets now Δ ln t = πξ/2; contrary to Type IX, the period is not constant throughout the long era, and slowly decreases along with ξ.\n\n## The small-time domain\nLong eras violate the \"regular\" course of evolution which makes it difficult to study the evolution of time intervals spanning several eras.\nIt can be shown, however, that such \"abnormal\" cases appear in the spontaneous evolution of the model to a singular point in the asymptotically small times t at sufficiently large distances from a start point with arbitrary initial conditions.\nEven in long eras both oscillatory functions during transitions between Kasner epochs remain so different that the transition occurs under the influence of only one perturbation.\nAll results in this section relate equally to models of the types VIII and IX.\nDuring each Kasner epoch abc = Λt, i. e. α + β + γ = ln Λ + ln t.\nOn changing over from one epoch (with a given value of the parameter u) to the next epoch the constant Λ is multiplied by 1 + 2p1 = (1 – u + u2)/(1 + u + u2) < 1.\nThus a systematic decrease in Λ takes place.\nBut it is essential that the mean (with respect to the lengths k of eras) value of the entire variation of ln Λ during an era is finite.\nActually the divergence of the mean value could be due only to a too rapid increase of this variation with increasing k.\nFor large value of the parameter u, ln(1 + 2p1) ≈ −2/u.\nFor a large k the maximal value u(max) = k + x ≈ k. Hence the entire variation of ln Λ during an era is given by a sum of the form\nwith only the terms that correspond to large values of u written down.\nWhen k increases this sum increases as ln k.\nBut the probability for an appearance of an era of a large length k decreases as 1/k2 according to ; hence the mean value of the sum above is finite.\nConsequently, the systematic variation of the quantity ln Λ over a large number of eras will be proportional to this number.\nBut it is seen in that with t → 0 the number s increases merely as ln |ln t|.\nThus in the asymptotic limit of arbitrarily small t the term ln Λ can indeed be neglected as compared to ln t.\nIn this approximation\nwhere Ω denotes the \"logarithmic time\"\nand the process of epoch transitions can be regarded as a series of brief time flashes.\nThe magnitudes of maxima of the oscillating scale functions are also subject to a systematic variation.\nFrom for u ≫ 1 it follows that.\nIn the same way as it was done above for the quantity ln Λ, one can hence deduce that the mean decrease in the height of the maxima during an era is finite and the total decrease over a large number of eras increases with t → 0 merely as ln Ω.\nAt the same time the lowering of the minima, and by the same token the increase of the [amplitude](https://bliptext.com/articles/amplitude) of the oscillations, proceed proportional to Ω.\nIn correspondence with the adopted approximation the lowering of the maxima is neglected in comparison with the increase of the amplitudes so\nfor the maximal values of all oscillating functions and the quantities α, β, γ run only through negative values that are connected with one another at each instant of time by the relation.\nConsidering such instant change of epochs, the transition periods are ignored as small in comparison to the epoch length; this condition is actually fulfilled.\nReplacement of α, β, and γ maxima with zeros requires that quantities ln (|p1|Λ) be small in comparison with the amplitudes of oscillations of the respective functions.\nAs mentioned [above](https://bliptext.com/articles/), during transitions between eras |p1| values can become very small while their magnitude and probability for occurrence are not related to the oscillation amplitudes in the respective moment.\nTherefore, in principle, it is possible to reach so small |p1| values that the above condition (zero maxima) is violated.\nSuch drastic drop of\nαmax\ncan lead to various special situations in which the transition between Kasner epochs by the rule becomes incorrect (including the situations described [above](https://bliptext.com/articles/)).\nThese \"dangerous\" situations could break the laws used for the statistical analysis below.\nAs mentioned, however, the probability for such deviations converges asymptotically to zero; this issue will be discussed below.\nConsider an era that contains k Kasner epochs with a parameter u running through the values\nand let α and β are the oscillating functions during this era (Fig. 4).\nInitial moments of Kasner epochs with parameters un are Ωn.\nIn each initial moment, one of the values α or β is zero, while the other has a minimum.\nValues α or β in consecutive minima, that is, in moments Ωn are\n(not distinguishing minima α and β).\nValues δn that measure those minima in respective Ωn units can run between 0 and 1.\nFunction γ monotonously decreases during this era; according to its value in moment Ωn is\nDuring the epoch starting at moment Ωn and ending at moment Ωn+1 one of the functions α or β increases from −δnΩn to zero while the other decreases from 0 to −δn+1Ωn+1 by linear laws, respectively:\nresulting in the [recurrence relation](https://bliptext.com/articles/recurrence-relation)\nand for the logarithmic epoch length\nwhere, for short, f(u) = 1 + u + u2.\nThe sum of n epoch lengths is obtained by the formula\nIt can be seen from that |αn+1| > |αn|, i.e., the oscillation amplitudes of functions α and β increase during the whole era although the factors δn may be small.\nIf the minimum at the beginning of an era is deep, the next minima will not become shallower; in other words, the residue |α — β| at the moment of transition between Kasner epochs remains large.\nThis assertion does not depend upon era length k because transitions between epochs are determined by the common rule also for long eras.\nThe last oscillation amplitude of functions α or β in a given era is related to the amplitude of the first oscillation by the relationship |αk−1| = |α0| (k + x) / (1 + x).\nEven at k's as small as several units x can be ignored in comparison to k so that the increase of α and β oscillation amplitudes becomes proportional to the era length.\nFor functions a = eα and b = eβ this means that if the amplitude of their oscillations in the beginning of an era was A0, at the end of this era the amplitude will become.\nThe length of Kasner epochs (in logarithmic time) also increases inside a given era; it is easy to calculate from that Δn+1 > Δn.\nThe total era length is\n(the term with 1/x arises from the last, k-th, epoch whose length is great at small x; cf. Fig. 2).\nMoment Ωn when the k-th epoch of a given era ends is at the same time moment Ω'0 of the beginning of the next era.\nIn the first Kasner epoch of the new era function γ is the first to rise from the minimal value γk = − Ωk (1 − δk) that it reached in the previous era; this value plays the role of a starting amplitude δ'0Ω'0 for the new series of oscillations.\nIt is easily obtained that:\nIt is obvious that δ'0Ω'0 > δ0Ω0.\nEven at not very great k the amplitude increase is very significant: function c = eγ begins to oscillate from amplitude.\nThe issue about the above-mentioned \"dangerous\" cases of drastic lowering of the upper oscillation limit is left aside for now.\nAccording to the increase in matter density during the first (k − 1) epochs is given by the formula\nFor the last k epoch of a given era, at u = x < 1 the greatest power is p2(x) (not p3(x) ).\nTherefore, for the density increase over the whole era one obtains\nTherefore, even at not very great k values,.\nDuring the next era (with a length k ' ) density will increase faster because of the increased starting amplitude A0':, etc. These formulae illustrate the steep increase in matter density.\n# [Variation of α, β, and γ as functions of the logarithmic time Ω during one era. The vertical dash lines denote alterations of the Kasner epochs, corresponding to the linear segments of the curves. On the top are indicated the values of the parameter u that determine the Kasner exponents. The last epoch has a longer duration if x is small. In the first epoch of the next era, γ begins to increase and α becomes a monotonically decreasing function. | upload.wikimedia.org/wikipedia/commons/9/98/Kasner///epochs.svg]\n\n## Statistical analysis near the singularity\nThe sequence of era lengths k(s), measured by the number of Kasner epochs contained in them, acquires asymptotically the character of a random process.\nThe same pertains also to the sequence of the interchanges of the pairs of oscillating functions on going over from one era to the next (it depends on whether the numbers k(s) are even or odd).\nA source of this stochasticity is the rule – according to which the transition from one era to the next is determined in an infinite numerical sequence of u values.\nThis rule states, in other words, that if the entire infinite sequence begins with a certain initial value, then the lengths of the eras k(0), k(1), ..., are the numbers in the [simple continued fraction](https://bliptext.com/articles/simple-continued-fraction) expansion\nThis expansion corresponds to the mapping transformation of the interval [0, 1] onto itself by the formula Tx = {1/x}, i.e., xs+1 = {1/xs}.\nThis transformation belongs to the so-called expanding transformations of the interval [0, 1], i.e., transformations x → f(x) with |f′(x)| > 1.\nSuch transformations possess the property of exponential instability: if we take initially two close points their mutual distance increases exponentially under the iterations of the transformations.\nIt is well known that the exponential instability leads to the appearance of strong stochastic properties.\nIt is possible to change over to a probabilistic description of such a sequence by considering not a definite initial value x(0) but the values x(0) = x distributed in the interval from 0 to 1 in accordance with a certain [probabilistic distributional law](https://bliptext.com/articles/probability-distribution) w0(x).\nThen the values of x(s) terminating each era will also have distributions that follow certain laws ws(x).\nLet ws(x)dx be the probability that the s-th era terminates with the value lying in a specified interval dx.\nThe value x(s) = x, which terminates the s-th era, can result from initial (for this era) values, where k = 1, 2, ...; these values of correspond to the values x(s–1) = 1/(k + x) for the preceding era.\nNoting this, one can write the following recurrence relation, which expresses the distribution of the probabilities ws(x) in terms of the distribution ws–1(x):\nor\nIf the distribution ws(x) tends with increasing s to a stationary (independent of s) limiting distribution w(x), then the latter should satisfy an equation obtained from by dropping the indices of the functions ws−1(x) and ws(x).\nThis equation has a solution\n(normalized to unity and taken to the first order of x).\nIn order for the s-th era to have a length k, the preceding era must terminate with a number x in the interval between 1/(k + 1) and 1/k.\nTherefore, the probability that the era will have a length k is equal to (in the stationary limit)\nAt large values of k\nIn relating the statistical properties of the cosmological model with the [ergodic](https://bliptext.com/articles/ergodicity) properties of the transformation xs+1 = {1/xs} an important point must be mentioned.\nIn an infinite sequence of numbers x constructed in accordance with this rule, arbitrarily small (but never vanishing) values of x will be observed corresponding to arbitrarily large lengths k. Such cases can (by no means necessarily!) give rise to certain specific situations when the notion of eras, as of sequences of Kasner epochs interchanging each other according to the rule, loses its meaning (although the oscillatory mode of evolution of the model still persists).\nSuch an \"anomalous\" situation can be manifested, for instance, in the necessity to retain in the right-hand side of terms not only with one of the functions a, b, c (say, a4), as is the case in the \"regular\" interchange of the Kasner epochs, but simultaneously with two of them (say, a4, b4, a2b2).\nOn emerging from an \"anomalous\" series of oscillations a succession of regular eras is restored.\nStatistical analysis of the behavior of the model which is entirely based on regular iterations of the transformations is corroborated by an important theorem: the probability of the appearance of anomalous cases tends asymptotically to zero as the number of iterations s → ∞ (i.e., the time t → 0) which is proved at the end of this section.\nThe validity of this assertion is largely due to a very rapid rate of increase of the oscillation amplitudes during every era and especially in transition from one era to the next one.\nThe process of the relaxation of the cosmological model to the \"stationary\" statistical regime (with t → 0 starting from a given \"initial instant\") is less interesting, however, than the properties of this regime itself with due account taken for the concrete laws of the variation of the physical characteristics of the model during the successive eras.\nAn idea of the rate at which the stationary distribution sets in is obtained from the following example.\nLet the initial values x(0) be distributed in a narrow interval of width δx(0) about some definite number.\nFrom the recurrence relation (or directly from the expansion ) it is easy to conclude that the widths of the distributions ws(x) (about other definite numbers) will then be equal to\n(this expression is valid only so long as it defines quantities δx(s) ≪ 1).\nThe mean value \\bar k, calculated from this distribution, diverges logarithmically.\nFor a sequence, cut off at a very large, but still finite number N, one has.\nThe usefulness of the mean in this case is very limited because of its instability: because of the slow decrease of W(k), fluctuations in k diverge faster than its mean.\nA more adequate characteristic of this sequence is the probability that a randomly chosen number from it belongs to an era of length K where K is large.\nThis probability is lnK / lnN.\nIt is small if.\nIn this respect one can say that a randomly chosen number from the given sequence belongs to the long era with a high probability.\nIt convenient to average expressions that depend simultaneously on k(s) and x(s).\nSince both these quantities are derived from the same quantity x(s–1) (which terminates the preceding era), in accordance with the formula k(s) + x(s) = 1/x(s–1), their [statistical distributions](https://bliptext.com/articles/probability-distribution) cannot be regarded as independent.\nThe joint distribution Ws(k,x)dx of both quantities can be obtained from the distribution ws–1(x)dx by making in the latter the substitution x → 1/(x + k).\nIn other words, the function Ws(k,x) is given by the very expression under the summation sign in the right side of.\nIn the stationary limit, taking w from, one obtains\nSummation of this distribution over k brings us back to, and integration with respect to dx to.\nThe recurrent formulas defining transitions between eras are re-written with index s numbering the successive eras (not the Kasner epochs in a given era!), beginning from some era (s = 0) defined as initial.\nΩ(s) and ε(s) are, respectively, the initial moment and initial matter density in the s-th era; δ(s)Ω(s) is the initial oscillation amplitude of that pair of functions α, β, γ, which oscillates in the given era: k(s) is the length of s-th era, and x(s) determines the length (number of Kasner epochs) of the next era according to k(s+1) = [1/x(s)].\nAccording to –\n(ξ(s) is introduced in to be used further on).\nThe quantities δ(s) have a stable stationary statistical distribution P(δ) and a stable (small relative fluctuations) mean value.\nFor their determination KL in coauthorship with [Ilya Lifshitz](https://bliptext.com/articles/ilya-lifshitz), the brother of Evgeny Lifshitz, used (with due reservations) an approximate method based on the assumption of statistical independence of the random quantity δ(s) and of the random quantities k(s), x(s).\nFor the function P(δ) an integral equation was set up which expressed the fact that the quantities δ(s+1) and δ(s) interconnected by the relation have the same distribution; this equation was solved numerically.\nIn a later work, Khalatnikov et al. showed that the distribution P(δ) can actually be found exactly by an analytical method (see **Fig. 5**).\nFor the statistical properties in the stationary limit, it is reasonable to introduce the so-called natural extension of the transformation Tx = {1/x} by continuing it without limit to negative indices.\nOtherwise stated, this is a transition from a one-sided infinite sequence of the numbers (x0, x1, x2, ...), connected by the equalities Tx = {1/x}, to a \"doubly infinite\" sequence X = (..., x−1, x0, x1, x2, ...) of the numbers which are connected by the same equalities for all –∞ < s < ∞.\nOf course, such expansion is not unique in the literal meaning of the word (since xs–1 is not determined uniquely by xs), but all statistical properties of the extended sequence are uniform over its entire length, i.e., are invariant with respect to arbitrary shift (and x0 loses its meaning of an \"initial\" condition).\nThe sequence X is equivalent to a sequence of integers K = (..., k−1, k0, k1, k2, ...), constructed by the rule ks = [1/xs–1].\nInversely, every number of X is determined by the integers of K as an infinite [continued fraction](https://bliptext.com/articles/continued-fraction)\n(the convenience of introducing the notation x_{s+1}^{+} with an index shifted by 1 will become clear in the following).\nFor concise notation the continuous fraction is denoted simply by enumeration (in square brackets) of its denominators; then the definition of x_{s}^{+} can be written as\nReverse quantities are defined by a continued fraction with a retrograde (in the direction of diminishing indices) sequence of denominators\nThe recurrence relation is transformed by introducing temporarily the notation ηs = (1 − δs)/δs.\nThen can be rewritten as\nBy iteration an infinite continuous fraction is obtained\nHence and finally\nThis expression for δs contains only two (instead of the three in ) random quantities x_{s}^{+} and x_{s}^{-}, each of which assumes values in the interval [0, 1].\nIt follows from the definition that.\nHence the shift of the entire sequence X by one step to the right means a joint transformation of the quantities x_{s}^{+} and x_{s}^{-} according to\nThis is a [one-to-one mapping](https://bliptext.com/articles/bijection) in the [unit square](https://bliptext.com/articles/unit-square).\nThus we have now a one-to-one transformation of two quantities instead of a not one-to-one transformation Tx = {1/x} of one quantity.\nThe quantities x_{s}^{+} and x_{s}^{-} have a joint stationary distribution P(x+, x−).\nSince is a one-to-one transformation, the condition for the distribution to be stationary is expressed simply by a function equation\nwhere J is the [Jacobian](https://bliptext.com/articles/jacobian-matrix-and-determinant) of the transformation.\nA shift of the sequence X by one step gives rise to the following transformation T of the unit square:\n(with, , cf. ). The density P(x, y) defines the invariant measure for this transformation.\nIt is natural to suppose that P(x, y) is a symmetric function of x and y.\nThis means that the measure is invariant with respect to the transformation S(x, y) = (y, x) and hence with respect to the product ST with ST(x, y) = (x″, y″) and\nEvidently ST has a first integral H = 1/x + y.\nOn the line H = const ≡ c the transformation has the form\nHence the invariant measure density of ST must be of the form\nAccounting for the symmetry P(x, y)= P(y, x), this becomes f(c)= c−2 and hence (after normalization)\n(its integration over x+ or x– yields the function w(x) ).\nThe reduction of the transformation to one-to-one mapping was used already by Chernoff and Barrow and they obtained a formula of the form of but for other variables; their paper does not contain applications to the problems which are considered in Khalatnikov et al.\nThe correctness of be verified also by a direct calculation; the Jacobian of the transformation is\n(in its calculation one must note that ).\nSince by δs is expressed in terms of the random quantities x+ and x−, the knowledge of their joint distribution makes it possible to calculate the statistical distribution P(δ) by integrating P(x+, x−) over one of the variables at a constant value of δ.\nDue to symmetry of the function with respect to the variables x+ and x−, P(δ) = P(1 − δ), i.e., the function P(δ) is symmetrical with respect to the point δ = 1/2.\nThen\nOn evaluating this integral (for 0 ≤ δ ≤ 1/2 and then making use of the aforementioned symmetry), finally\nThe mean value = 1/2 already as a result of the symmetry of the function P(δ).\nThus the mean value of the initial (in every era) amplitude of oscillations of the functions α, β, γ increases as Ω/2.\nThe statistical relation between large time intervals Ω and the number of eras s contained in them is found by repeated application of :\nDirect averaging of this equation, however, does not make sense: because of the slow decrease of function W(k), the average values of the quantity exp ξ(s) are unstable in the above sense – the fluctuations increase even more rapidly than the mean value itself with increasing region of averaging.\nThis instability is eliminated by taking the logarithm: the \"doubly-logarithmic\" time interval\nis expressed by the sum of quantities ξ(p) which have a stable statistical distribution.\nThe mean value of τ is.\nTo calculate \\bar{\\xi} note that can be rewritten as\nFor the stationary distribution, and in virtue of the symmetry of the function P(δ) also.\nHence\n(w(x) from ).\nThus\nwhich determines the mean doubly-logarithmic time interval containing s successive eras.\nFor large s the number of terms in the sum is large and according to general theorems of the ergodic theory the values of τs are distributed around according to [Gauss's law](https://bliptext.com/articles/gauss-distribution) with the density\nCalculation of the variance Dτ is more complicated since not only the knowledge of \\bar{\\xi} and are needed but also of the correlations.\nThe calculation can be simplified by rearranging the terms in the sum.\nBy using the sum can be rewritten as\nThe last two terms do not increase with increasing s; these terms can be omitted as the limiting laws for large s are dominating.\nThen\n(the expression for δp is taken into account).\nTo the same accuracy (i.e., up to the terms which do not increase with s) the equality\nis valid.\nIndeed, in virtue of\nand hence\nBy summing this identity over p is obtained.\nFinally again with the same accuracy x_p^{+} is changed for xp under the summation sign and thus represent τs as\nThe variance of this sum in the limit of large s is\nIt is taken into account that in virtue of the statistical homogeneity of the sequence X the correlations depend only on the differences |p − p′|.\nThe mean value ; the mean square\nBy taking into account also the values of correlations with p = 1, 2, 3\n(calculated numerically) the final result Dτ s = (3.5 ± 0.1)s is obtained.\nAt increasing s the relative fluctuation tends to zero as s−1/2.\nIn other words, the statistical relation becomes almost certain at large s.\nThis makes it possible to invert the relation, i.e., to represent it as the dependence of the average number of the eras sτ that are interchanged in a given interval τ of the double logarithmic time:\nThe statistical distribution of the exact values of sτ around its average is also Gaussian with the variance\nThe respective statistical distribution is given by the same Gaussian distribution in which the random variable is now sτ at a given τ:\nFrom this point of view, the source of the statistical behavior is the arbitrariness in the choice of the starting point of the interval τ superimposed on the infinite sequence of the interchanging eras.\nRespective to matter density, can be re-written with account of in the form\nand then, for the total energy change during s eras,\nThe term with the sum by p gives the main contribution to this expression because it contains an exponent with a large power.\nLeaving only this term and averaging, one gets in its right hand side the expression s\\bar{\\xi} which coincides with ; all other terms in the sum (also terms with ηs in their powers) lead only to corrections of a relative order 1/s.\nTherefore,\nBy virtue of the almost certain character of the relation between τs and s can be written as\nwhich determines the value of the double logarithm of density increase averaged by given double-logarithmic time intervals τ or by a given number of eras s.\nThese stable statistical relationships exist specifically for doubly-logarithmic time intervals and for the density increase.\nFor other characteristics, e.g., ln (ε(s)/ε(0)) or Ω(s) / Ω(0) = exp τs the relative fluctuation increase exponentially with the increase of the averaging range thereby voiding the term mean value of a stable meaning.\nThe origin of the statistical relationship can be traced already from the initial law governing the variation of the density during the individual Kasner epochs.\nAccording to, during the entire evolution we have\nwith 1 − p3(t) changing from epoch to epoch, running through values in the interval from 0 to 1.\nThe term ln Ω = ln ln (1/t) increases monotonically; on the other hand, the term ln2(1 − p3) can assume large values (comparable with ln Ω) only when values of p3 very close to unity appear (i.e., very small |p1|).\nThese are precisely the \"dangerous\" cases that disturb the regular course of evolution expressed by the recurrent relationships –.\nIt remains to show that such cases actually do not arise in the asymptotic limiting regime.\nThe spontaneous evolution of the model starts at a certain instant at which definite initial conditions are specified in an arbitrary manner.\nAccordingly, by \"asymptotic\" is meant a regime sufficiently far away from the chosen initial instant.\nDangerous cases are those in which excessively small values of the parameter u = x (and hence also |p1| ≈ x) appear at the end of an era.\nA criterion for selection of such cases is the inequality\nwhere | α(s) | is the initial minima depth of the functions that oscillate in era s (it would be more appropriate to choose the final amplitude, but that would only strengthen the selection criterion).\nThe value of x(0) in the first era is determined by the initial conditions.\nDangerous are values in the interval δx(0) ~ exp ( − |α(0)| ), and also in intervals that could result in dangerous cases in the next eras.\nIn order for x(s) to fall in the dangerous interval δx(s) ~ exp ( − | α(s) | ), the initial value x(0) should lie into an interval of a width δx(0) ~ δx(s) / k(1)^2 ... k(s)^2.\nTherefore, from a unit interval of all possible values of x(0), dangerous cases will appear in parts λ of this interval:\n(the inner sum is taken over all the values k(1), k(2), ..., k(s) from 1 to ∞).\nIt is easy to show that this era converges to the value λ \\ll 1 whose order of magnitude is determined by the first term in.\nThis can be shown by a strong majoration of the era for which one substitutes | α(s) | = (s + 1) | α(0) |, regardless of the lengths of eras k(1), k(2), ... (In fact | α(s) | increase much faster; even in the most unfavorable case k(1) = k(2) = ... = 1 values of | α(s) | increase as qs | α(0) | with q > 1.) Noting that\none obtains\nIf the initial value of x(0) lies outside the dangerous region λ there will be no dangerous cases.\nIf it lies inside this region dangerous cases occur, but upon their completion the model resumes a \"regular\" evolution with a new initial value which only occasionally (with a probability λ) may come into the dangerous interval.\nRepeated dangerous cases occur with probabilities λ2, λ3, ..., asymptotically converging to zero.\n# [The probability distribution function P(δ). Red line: exact function . Blue line: approximate solution of the integral equation in. Both curves appear to be strikingly similar and the means of both distributions are 0.50. | upload.wikimedia.org/wikipedia/commons/4/4e/Bkl///fig3a.svg]\n\n# General solution with small oscillations\nIn the above models, metric evolution near the singularity is studied on the example of homogeneous space metrics.\nIt is clear from the characteristic of this evolution that the analytic construction of the general solution for a singularity of such type should be made separately for each of the basic evolution components: for the Kasner epochs, for the process of transitions between epochs caused by \"perturbations\", for long eras with two perturbations acting simultaneously.\nDuring a Kasner epoch (i.e. at small perturbations), the metric is given by without the condition λ = 0.\nBKL further developed a matter distribution-independent model (homogeneous or non-homogeneous) for long era with small oscillations.\nThe time dependence of this solution turns out to be very similar to that in the particular case of homogeneous models; the latter can be obtained from the distribution-independent model by a special choice of the arbitrary functions contained in it.\nIt is convenient, however, to construct the general solution in a system of coordinates somewhat different from synchronous reference frame: g0α = 0 as in the synchronous frame, but instead of g00 = 1 it is now g00 = −g33.\nDefining again the space metric tensor γαβ = −gαβ one has, therefore\nThe special space coordinate is written as x3 = z and the time coordinate is written as x0 = ξ (as different from proper time t); it will be shown that ξ corresponds to the same variable defined in homogeneous models.\nDifferentiation by ξ and z is designated, respectively, by dot and prime.\nLatin indices a, b, c take values 1, 2, corresponding to space coordinates x1, x2 which will be also written as x, y.\nTherefore, the metric is\nThe required solution should satisfy the inequalities\n(these conditions specify that one of the functions a2, b2, c2 is small compared to the other two which was also the case with homogeneous models).\nInequality means that components γa3 are small in the sense that at any ratio of the shifts dxa and dz, terms with products dxadz can be omitted in the square of the spatial length element dl2.\nTherefore, the first approximation to a solution is a metric with γa3 = 0:\nOne can be easily convinced by calculating the Ricci tensor components R_0^0, R_3^0, R_3^3, R_a^b using metric and the condition that all terms containing derivatives by coordinates xa are small compared to terms with derivatives by ξ and z (their ratio is ~ γ33 / γab).\nIn other words, to obtain the equations of the main approximation, γ33 and γab in should be differentiated as if they do not depend on xa.\nDesignating\none obtains the following equations:\nIndex raising and lowering is done here with the help of γab.\nThe quantities \\varkappa and λ are the contractions and \\lambda_a^a whereby\nAs to the Ricci tensor components R_a^0, R_a^3, by this calculation they are identically zero.\nIn the next approximation (i.e., with account to small γa3 and derivatives by x, y), they determine the quantities γa3 by already known γ33 and γab.\nContraction of gives, and, hence,\nDifferent cases are possible depending on the G variable.\nIn the above case g00 = γ33 \\gg γab and.\nThe case N > 0 (quantity N is time-like) leads to time singularities of interest.\nSubstituting in f1 = 1/2 (ξ + z) sin y, f2 = 1/2 (ξ − z) sin y results in G of type\nThis choice does not diminish the generality of conclusions; it can be shown that generality is possible (in the first approximation) just on account of the remaining permissible transformations of variables.\nAt N < 0 (quantity N is space-like) one can substitute G = z which generalizes the well-known [Einstein–Rosen metric](https://bliptext.com/articles/einstein-rosen-metric).\nAt N = 0 one arrives at the Robinson–Bondi wave metric that depends only on ξ + z or only on ξ − z (cf. ). The factor sin y in is put for convenient comparison with homogeneous models.\nTaking into account, equations – become\nThe principal equations are defining the γab components; then, function ψ is found by a simple integration of –.\nThe variable ξ runs through the values from 0 to ∞.\nThe solution of is considered at two boundaries, ξ \\gg 1 and \\ll 1.\nAt large ξ values, one can look for a solution that takes the form of a 1 / √ξ decomposition:\nwhereby\n(equation needs condition to be true).\nSubstituting in, one obtains in the first order\nwhere quantities aac constitute a matrix that is inverse to matrix aac.\nThe solution of has the form\nwhere la, ma, ρ, are arbitrary functions of coordinates x, y bound by condition derived from.\nTo find higher terms of this decomposition, it is convenient to write the matrix of required quantities γab in the form\nwhere the symbol ~ means matrix transposition.\nMatrix H is symmetric and its trace is zero.\nPresentation ensures symmetry of γab and fulfillment of condition.\nIf exp H is substituted with 1, one obtains from γab = ξaab with aab from.\nIn other words, the first term of γab decomposition corresponds to H = 0; higher terms are obtained by powers decomposition of matrix H whose components are considered small.\nThe independent components of matrix H are written as σ and φ so that\nSubstituting in and leaving only terms linear by H, one derives for σ and φ\nIf one tries to find a solution to these equations as [Fourier series](https://bliptext.com/articles/fourier-series) by the z coordinate, then for the series coefficients, as functions of ξ, one obtains Bessel equations.\nThe major asymptotic terms of the solution at large ξ are\nCoefficients A and B are arbitrary complex functions of coordinates x, y and satisfy the necessary conditions for real σ and φ; the base frequency ω is an arbitrary real function of x, y.\nNow from – it is easy to obtain the first term of the function ψ:\n(this term vanishes if ρ = 0; in this case the major term is the one linear for ξ from the decomposition: ψ = ξq (x, y) where q is a positive function ).\nTherefore, at large ξ values, the components of the metric tensor γab oscillate upon decreasing ξ on the background of a slow decrease caused by the decreasing ξ factor in.\nThe component γ33 = eψ decreases quickly by a law close to exp (ρ2ξ2); this makes it possible for condition.\nNext BKL consider the case ξ \\ll 1.\nThe first approximation to a solution of is found by the assumption (confirmed by the result) that in these equations terms with derivatives by coordinates can be left out:\nThis equation together with the condition gives\nwhere λa, μa, s1, s2 are arbitrary functions of all 3 coordinates x, y, z, which are related with other conditions\nEquations – give now\nThe derivatives, calculated by , contain terms ~ ξ4s1 − 2 and ~ ξ4s2 − 2 while terms left in are ~ ξ−2.\nTherefore, application of instead of is permitted on conditions s1 > 0, s2 > 0; hence 1 − > 0.\nThus, at small ξ oscillations of functions γab cease while function γ33 begins to increase at decreasing ξ.\nThis is a Kasner mode and when γ33 is compared to γab, the above approximation is not applicable.\nIn order to check the compatibility of this analysis, BKL studied the equations = 0, = 0, and, calculating from them the components γa3, confirmed that the inequality takes place.\nThis study showed that in both asymptotic regions the components γa3 were ~ γ33.\nTherefore, correctness of inequality immediately implies correctness of inequality.\nThis solution contains, as it should for the general case of a field in vacuum, four arbitrary functions of the three space coordinates x, y, z.\nIn the region ξ \\ll 1 these functions are, e.g., λ1, λ2, μ1, s1.\nIn the region ξ \\gg 1 the four functions are defined by the Fourier series by coordinate z from with coefficients that are functions of x, y; although Fourier series decomposition (or integral?) characterizes a special class of functions, this class is large enough to encompass any finite subset of the set of all possible initial conditions.\nThe solution contains also a number of other arbitrary functions of the coordinates x, y.\nSuch two-dimensional arbitrary functions appear, generally speaking, because the relationships between three-dimensional functions in the solutions of the Einstein equations are differential (and not algebraic), leaving aside the deeper problem about the geometric meaning of these functions.\nBKL did not calculate the number of independent two-dimensional functions because in this case it is hard to make unambiguous conclusions since the three-dimensional functions are defined by a set of two-dimensional functions (cf. for more details).\nFinally, BKL go on to show that the general solution contains the particular solution obtained above for homogeneous models.\nSubstituting the basis vectors for Bianchi Type IX homogeneous space in the space-time metric of this model takes the form\nWhen c2 \\ll a2, b2, one can ignore c2 everywhere except in the term c2 dz2.\nTo move from the synchronous frame used in to a frame with conditions, the transformation dt = c dξ/2 and substitution z → z/2 are done.\nAssuming also that χ ≡ ln (a/b) \\ll 1, one obtains from in the first approximation:\nSimilarly, with the basis vectors of Bianchi Type VIII homogeneous space, one obtains\nAccording to the analysis of homogeneous spaces above, in both cases ab = ξ (simplifying a_0^2 = ξ0) and χ is from ; function c (ξ) is given by formulae and, respectively, for models of Types IX and VIII.\nIdentical metric for Type VIII is obtained from, , choosing two-dimensional vectors la and ma in the form\nand substituting\nTo obtain the metric for Type IX, one should substitute\n(for calculation of c (ξ) the approximation in is not sufficient and the term in ψ linear by ξ is calculated )\nThis analysis was done for empty space.\nIncluding matter does not make the solution less general and does not change its qualitative characteristics.\nA limitation of great importance for the general solution is that all 3-dimensional functions contained in the metrics and should have a single and common characteristic change interval.\nOnly this allows to approximate in the Einstein equations all metric spatial component derivatives with simple products of these components by a characteristic wave numbers which results in ordinary differential equations of the type obtained for the Type IX homogeneous model.\nThis is the reason for the coincidence between homogeneous and general solutions.\nIt follows that both Type IX model and its generalisation contain an oscillatory mode with a single spatial scale of an arbitrary magnitude which is not selected among others by any physical conditions.\nHowever, it is known that in non-linear systems with infinite degrees of freedom such mode is unstable and partially dissipates to smaller oscillations.\nIn the general case of small perturbations with an arbitrary spectrum, there will always be some whose amplitudes will increase feeding upon the total process energy.\nAs a result, a complicated picture arises of multi-scale movements with certain energy distribution and energy exchange between oscillations of different scales.\nIt doesn't occur only in the case when the development of small-scale oscillations is impossible because of physical conditions.\nFor the latter, some natural physical length must exist which determines the minimal scale at which energy exits from a system with dynamical degrees of freedom (which, for example, occurs in a liquid with a certain viscosity).\nHowever, there is no innate physical scale for a gravitational field in vacuum, and, therefore, there is no impediment for the development of oscillations of arbitrarily small scales.\n\n# Conclusions\nBKL describe singularities in the cosmologic solution of Einstein equations that have a complicated oscillatory character.\nAlthough these singularities have been studied primarily on spatially homogeneous models, there are convincing reasons to assume that singularities in the general solution of Einstein equations have the same characteristics; this circumstance makes the BKL model important for cosmology.\nA basis for such statement is the fact that the oscillatory mode in the approach to singularity is caused by the single perturbation that also causes instability in the generalized Kasner solution.\nA confirmation of the generality of the model is the analytic construction for long era with small oscillations.\nAlthough this latter behavior is not a necessary element of metric evolution close to the singularity, it has all principal qualitative properties: metric oscillation in two spatial dimensions and monotonous change in the third dimension with a certain perturbation of this mode at the end of some time interval.\nHowever, the transitions between Kasner epochs in the general case of non-homogeneous spatial metric have not been elucidated in details.\nThe problem connected with the possible limitations upon space geometry caused by the singularity was left aside for further study.\nIt is clear from the outset, however, that the original BKL model is applicable to both finite or infinite space; this is evidenced by the existence of oscillatory singularity models for both closed and open spacetimes.\nThe oscillatory mode of the approach to singularity gives a new aspect to the term 'finiteness of time'.\nBetween any finite moment of the world time t and the moment t = 0 there is an infinite number of oscillations.\nIn this sense, the process acquires an infinite character.\nInstead of time t, a more adequate variable for its description is ln t by which the process is extended to -\\infty.\nBKL consider metric evolution in the direction of decreasing time.\nThe Einstein equations are symmetric in respect to the time sign so that a metric evolution in the direction of increasing time is equally possible.\nHowever, these two cases are fundamentally different because past and future are not equivalent in the physical sense.\nFuture singularity can be physically meaningful only if it is possible at arbitrary initial conditions existing in a previous moment.\nMatter distribution and fields in some moment in the evolution of Universe do not necessarily correspond to the specific conditions required for the existence of a given special solution to the Einstein equations.\nThe choice of solutions corresponding to the real world is related to profound physical requirements which is impossible to find using only the existing relativity theory and which can be found as a result of future synthesis of physical theories.\nThus, it may turn out that this choice singles out some special (e.g., isotropic) type of singularity.\nNevertheless, it is more natural to assume that because of its general character, the oscillatory mode should be the main characteristic of the initial evolutionary stages.\nIn this respect, of considerable interest is the property of the [\"Mixmaster\" model](https://bliptext.com/articles/mixmaster-universe) shown by Misner, related to propagation of light signals.\nIn the isotropic model, a \"light horizon\" exists, meaning that for each moment of time, there is some longest distance, at which exchange of light signals and, thus, a causal connection, is impossible: the signal cannot reach such distances for the time since the singularity t = 0.\nSignal propagation is determined by the equation ds = 0.\nIn the isotropic model near the singularity t = 0 the interval element is, where d \\bar{l}^2 is a time-independent spatial differential form.\nSubstituting yields\nThe \"distance\" reached by the signal is\nSince η, like t, runs through values starting from 0, up to the \"moment\" η signals can propagate only at the distance which fixes the farthest distance to the horizon.\nThe existence of a light horizon in the isotropic model poses a problem in the understanding of the origin of the presently observed isotropy in the relic radiation.\nAccording to the isotropic model, the observed isotropy means isotropic properties of radiation that comes to the observer from such regions of space that can not be causally connected with each other.\nThe situation in the oscillatory evolution model near the singularity can be different.\nFor example, in the homogeneous model for Type IX space, a signal is propagated in a direction in which for a long era, scales change by a law close to ~ t.\nThe square of the distance element in this direction is dl2 = t2\\bar{l}^2, and the respective element of the four-dimensional interval is.\nThe substitution puts this in the form\nand for the signal propagation one has equation of the type again.\nThe important difference is that the variable η runs now through values starting from -\\infty (if metric is valid for all t starting from t = 0).\nTherefore, for each given \"moment\" η are found intermediate intervals Δη sufficient for the signal to cover each finite distance.\nIn this way, during a long era a light horizon is opened in a given space direction.\nAlthough the duration of each long era is still finite, during the course of the world evolution eras change an infinite number of times in different space directions.\nThis circumstance makes one expect that in this model a causal connection between events in the whole space is possible.\nBecause of this property, Misner named this model \"Mixmaster universe\" by a brand name of a dough-blending machine.\nAs time passes and one goes away from the singularity, the effect of matter on metric evolution, which was insignificant at the early stages of evolution, gradually increases and eventually becomes dominant.\nIt can be expected that this effect will lead to a gradual \"isotropisation\" of space as a result of which its characteristics come closer to the Friedman model which adequately describes the present state of the Universe.\nFinally, BKL pose the problem about the feasibility of considering a \"singular state\" of a world with infinitely dense matter on the basis of the existing relativity theory.\nThe physical application of the Einstein equations in their present form in these conditions can be made clear only in the process of a future synthesis of physical theories and in this sense the problem can not be solved at present.\nIt is important that the gravitational theory itself does not lose its logical cohesion (i.e., does not lead to internal controversies) at whatever matter densities.\nIn other words, this theory is not limited by the conditions that it imposes, which could make logically inadmissible and controversial its application at very large densities; limitations could, in principle, appear only as a result of factors that are \"external\" to the gravitational theory.\nThis circumstance makes the study of singularities in cosmological models formally acceptable and necessary in the frame of existing theory.","current_revision":null,"created_by":"kACnuQmH5k9Nr5VNVEpVeg7XpvUmMctD","created_at":"2025-01-26T08:18:35.925Z","updated_at":"2025-01-26T08:18:35.925Z","search_vector":"'bkl':1 'singular':2","search_content":" \n \n\n# \nA **Belinski–Khalatnikov–Lifshitz (BKL) singularity** is a model of the dynamic [evolution of the universe](https://bliptext.com/articles/evolution-of-the-universe) near the initial [gravitational singularity](https://bliptext.com/articles/gravitational-singularity), described by an [anisotropic](https://bliptext.com/articles/anisotropic), [chaotic](https://bliptext.com/articles/chaos-physics) [solution of the Einstein field equation of gravitation](https://bliptext.com/articles/solutions-of-the-einstein-field-equations).\nAccording to this model, the universe is chaotically oscillating around a gravitational singularity in which time and space become equal to zero or, equivalently, the spacetime curvature becomes infinitely big.\nThis singularity is physically real in the sense that it is a necessary property of the solution, and will appear also in the [exact solution](https://bliptext.com/articles/exact-solutions-in-general-relativity) of those equations.\nThe singularity is not artificially created by the assumptions and simplifications made by the other special solutions such as the [Friedmann–Lemaître–Robertson–Walker](https://bliptext.com/articles/friedmann-lema-tre-robertson-walker-metric), quasi-isotropic, and [Kasner](https://bliptext.com/articles/kasner-metric) solutions.\nThe model is named after its authors [Vladimir Belinski](https://bliptext.com/articles/vladimir-belinski), [Isaak Khalatnikov](https://bliptext.com/articles/isaak-markovich-khalatnikov), and [Evgeny Lifshitz](https://bliptext.com/articles/evgeny-lifshitz), then working at the [Landau Institute for Theoretical Physics](https://bliptext.com/articles/landau-institute-for-theoretical-physics).\nThe picture developed by BKL has several important elements.\nThese are:\nThe study of the dynamics of the universe in the vicinity of the cosmological singularity has become a rapidly developing field of modern theoretical and mathematical physics.\nThe generalization of the BKL model to the cosmological singularity in multidimensional ([Kaluza–Klein type](https://bliptext.com/articles/kaluza-klein-theory)) cosmological models has a chaotic character in the spacetimes whose dimensionality is not higher than ten, while in the spacetimes of higher dimensionalities a universe after undergoing a finite number of oscillations enters into monotonic Kasner-type contracting regime.\nThe development of cosmological studies based on [superstring models](https://bliptext.com/articles/superstring-theory) has revealed some new aspects of the dynamics in the vicinity of the singularity.\nIn these models, mechanisms of changing of Kasner epochs are provoked not by the gravitational interactions but by the influence of other fields present.\nIt was proved that the cosmological models based on six main superstring models plus [eleven-dimensional supergravity](https://bliptext.com/articles/eleven-dimensional-supergravity) model exhibit the chaotic BKL dynamics towards the singularity.\nA connection was discovered between oscillatory BKL-like cosmological models and a special subclass of infinite-dimensional [Lie algebras](https://bliptext.com/articles/lie-algebra) – the so-called hyperbolic [Kac–Moody algebras](https://bliptext.com/articles/kac-moody-algebra).\n\n# Introduction\nThe basis of modern [cosmology](https://bliptext.com/articles/cosmology) are the special [solutions of the Einstein field equations](https://bliptext.com/articles/solutions-of-the-einstein-field-equations) found by [Alexander Friedmann](https://bliptext.com/articles/alexander-friedmann) in 1922–1924.\nThe Universe is assumed [homogeneous](https://bliptext.com/articles/homogeneous-space) (space has the same metric properties (measures) in all points) and [isotropic](https://bliptext.com/articles/isotropic) (space has the same measures in all directions).\nFriedmann's solutions allow two possible geometries for space: closed model with a ball-like, outwards-bowed space ([positive curvature](https://bliptext.com/articles/positive-curvature)) and open model with a saddle-like, inwards-bowed space ([negative curvature](https://bliptext.com/articles/negative-curvature)).\nIn both models, the Universe is not standing still, it is constantly either expanding (becoming larger) or contracting (shrinking, becoming smaller).\nThis was confirmed by [Edwin Hubble](https://bliptext.com/articles/edwin-hubble) who established the [Hubble redshift](https://bliptext.com/articles/hubble-redshift) of receding galaxies.\nThe present consensus is that the [isotropic model](https://bliptext.com/articles/big-bang), in general, gives an adequate description of the present state of the Universe; however, isotropy of the present Universe by itself is not a reason to expect that it is adequate for describing the early stages of [Universe evolution](https://bliptext.com/articles/cosmic-evolution).\nAt the same time, it is obvious that in the real world [homogeneity](https://bliptext.com/articles/homogeneity) is, at best, only an approximation.\nEven if one can speak about a homogeneous distribution of matter density at distances that are large compared to the intergalactic space, this homogeneity vanishes at smaller scales.\nOn the other hand, the homogeneity assumption goes very far in a mathematical aspect: it makes the solution highly [symmetric](https://bliptext.com/articles/symmetric) which can impart specific properties that disappear when considering a more general case.\nAnother important property of the isotropic model is the inevitable existence of a [time singularity](https://bliptext.com/articles/gravitational-singularity): time flow is not continuous, but stops or reverses after time reaches some very large or very small value.\nBetween singularities, time flows in one direction: away from the singularity ([arrow of time](https://bliptext.com/articles/arrow-of-time)).\nIn the open model, there is one time singularity so time is limited at one end but unlimited at the other, while in the closed model there are two singularities that limit time at both ends (the [Big Bang](https://bliptext.com/articles/big-bang) and [Big Crunch](https://bliptext.com/articles/big-crunch)).\nThe only physically interesting properties of [spacetimes](https://bliptext.com/articles/spacetime) (such as singularities) are those which are [stable](https://bliptext.com/articles/stability-theory), i.e., those properties which still occur when the initial data is perturbed slightly.\nIt is possible for a singularity to be stable and yet be of no physical interest: stability is a necessary but not a sufficient condition for physical relevance.\nFor example, a singularity could be stable only in a neighbourhood of initial data sets corresponding to highly [anisotropic](https://bliptext.com/articles/anisotropy) universes.\nSince the actual universe is now apparently almost isotropic such a singularity could not occur in our universe.\nA sufficient condition for a stable singularity to be of physical interest is the requirement that the singularity be [generic](https://bliptext.com/articles/generic-property) (or general).\nRoughly speaking, a stable singularity is generic if it occurs near every set of initial conditions and the non-gravitational fields are restricted in some specified way to \"physically realistic\" fields so that the Einstein equations, various equations of state, etc., are assumed to hold on the evolved spacetimes.\nIt might happen that a singularity is stable under small variations of the true gravitational [degrees of freedom](https://bliptext.com/articles/degrees-of-freedom-physics-and-chemistry), and yet it is not generic because the singularity depends in some way on the [coordinate system](https://bliptext.com/articles/coordinate-system), or rather on the choice of the initial [hypersurface](https://bliptext.com/articles/hypersurface) from which the spacetime is evolved.\nFor a system of [non-linear differential equations](https://bliptext.com/articles/non-linear-differential-equation), such as the [Einstein equations](https://bliptext.com/articles/einstein-field-equations), a [general solution](https://bliptext.com/articles/linear-differential-equation) is not unambiguously defined.\nIn principle, there may be multiple [general integrals](https://bliptext.com/articles/antiderivative), and each of those may contain only a finite [subset](https://bliptext.com/articles/subset) of all possible [initial conditions](https://bliptext.com/articles/initial-condition).\nEach of those [integrals](https://bliptext.com/articles/integral) may contain all required [independent](https://bliptext.com/articles/algebraic-independence) [functions](https://bliptext.com/articles/function-mathematics) which, however, may be subject to some conditions (e.g., some [inequalities](https://bliptext.com/articles/inequality-mathematics)).\nExistence of a general solution with a singularity, therefore, does not preclude the existence of other additional general solutions that do not contain a singularity.\nFor example, there is no reason to doubt the existence of a general solution without a singularity that describes an isolated body with a relatively small mass.\nIt is impossible to find a general integral for all space and for all time.\nHowever, this is not necessary for resolving the problem: it is sufficient to study the solution near the singularity.\nThis would also resolve another aspect of the problem: the characteristics of [spacetime metric](https://bliptext.com/articles/spacetime-metric) evolution in the general solution when it reaches the physical singularity, understood as a point where [matter density](https://bliptext.com/articles/matter-density) and [invariants](https://bliptext.com/articles/invariants-of-tensors) of the [Riemann curvature tensor](https://bliptext.com/articles/riemann-curvature-tensor) become infinite.\n\n# Existence of physical time singularity\nOne of the principal problems studied by the [Landau group](https://bliptext.com/articles/lev-davidovich-landau) (to which BKL belong) was whether [relativistic](https://bliptext.com/articles/general-relativity) [cosmological models](https://bliptext.com/articles/cosmological-model) necessarily contain a time singularity or whether the time singularity is an artifact of the assumptions used to simplify these models.\nThe independence of the singularity on symmetry assumptions would mean that time singularities exist not only in the special, but also in the general solutions of the Einstein equations.\nIt is reasonable to suggest that if a singularity is present in the general solution, there must be some indications that are based only on the most general properties of the Einstein equations, although those indications by themselves might be insufficient for characterizing the singularity.\nA criterion for generality of solutions is the number of independent space coordinate functions that they contain.\nThese include only the \"physically independent\" functions whose number cannot be reduced by any choice of [reference frame](https://bliptext.com/articles/frame-of-reference).\nIn the general solution, the number of such functions must be enough to fully define the [initial conditions](https://bliptext.com/articles/initial-conditions) (distribution and movement of matter, distribution of [gravitational field](https://bliptext.com/articles/gravitational-field)) at some moment of time chosen as initial.\nThis number is four for an empty (vacuum) space, and eight for a matter and/or radiation-filled space.\nPrevious work by the Landau group; reviewed in ) led to the conclusion that the general solution does not contain a physical singularity. This search for a broader class of solutions with a singularity has been done, essentially, by a trial-and-error method, since a systematic approach to the study of the Einstein equations was lacking. A negative result, obtained in this way, is not convincing by itself; a solution with the necessary degree of generality would invalidate it, and at the same time would confirm any positive results related to the specific solution.\nAt that time, the only known indication for the existence of physical singularity in the general solution was related to the form of the Einstein equations written in a [synchronous frame](https://bliptext.com/articles/synchronous-frame), that is, in a frame in which the proper time x0 = t is synchronized throughout the whole space; in this frame the space distance element dl is separate from the time interval dt.\nThe Einstein equation written in synchronous frame gives a result in which the metric [determinant](https://bliptext.com/articles/determinant) g inevitably becomes zero in a finite time irrespective of any assumptions about matter distribution.\nHowever, the efforts to find a general physical singularity were foregone after it became clear that the singularity mentioned above is linked with a specific geometric property of the synchronous frame: the crossing of time line coordinates.\nThis crossing takes place on some encircling [hypersurfaces](https://bliptext.com/articles/hypersurface) which are four-dimensional analogs of the [caustic surfaces](https://bliptext.com/articles/caustic-optics) in [geometrical optics](https://bliptext.com/articles/geometrical-optics); g becomes zero exactly at this crossing.\nTherefore, although this singularity is general, it is fictitious, and not a physical one; it disappears when the reference frame is changed.\nThis, apparently, dissuaded the researchers for further investigations along these lines.\nSeveral years passed before the interest in this problem waxed again when published his theorems that linked the existence of a singularity of unknown character with some very general assumptions that did not have anything in common with a choice of reference frame.\nOther similar theorems were found later on by [Hawking](https://bliptext.com/articles/stephen-hawking) and Geroch (see Penrose–[Hawking](https://bliptext.com/articles/stephen-hawking) singularity theorems).\nThis revived interest in the search for singular solutions.\n\n# Generalized homogeneous solution\nIn a space that is both homogeneous and isotropic the metric is determined completely, leaving free only the sign of the curvature.\nAssuming only space homogeneity with no additional symmetry such as isotropy leaves considerably more freedom in choosing the metric.\nThe following pertains to the space part of the metric at a given instant of time t assuming a synchronous frame so that t is the same synchronised time for the whole space.\n\n## The BKL conjecture\nIn their 1970 work, BKL stated that as one approaches a singularity, terms containing time derivatives in Einstein's equations dominate over those containing spatial derivatives.\nThis has since been known as the **BKL conjecture** and implies that Einstein's [partial differential equations](https://bliptext.com/articles/partial-differential-equations) (PDE) are well approximated by [ordinary differential equations](https://bliptext.com/articles/ordinary-differential-equations) (ODEs), whence the dynamics of general relativity effectively become local and oscillatory.\nThe time evolution of fields at each spatial point is well approximated by the homogeneous cosmologies in the Bianchi classification.\nBy separating the time and space derivatives in the Einstein equations, for example, in the way used for the[ classification of homogeneous spaces](https://bliptext.com/articles/bianchi-classification), and then setting the terms containing space derivatives equal to zero, one can define the so-called truncated theory of the system (truncated equations).\nThen, the BKL conjecture can be made more specific:\nWeak conjecture: As the singularity is approached the terms containing space derivatives in the Einstein equations are negligible in comparison to the terms containing time derivatives.\nThus, as the singularity is approached the Einstein equations approach those found by setting derivative terms to zero.\nThus, the weak conjecture says that the Einstein equations can be well approximated by the truncated equations in the vicinity of the singularity.\nNote that this does not imply that the solutions of the full equations of motion will approach the solutions to the truncated equations as the singularity is approached.\nThis additional condition is captured in the strong version as follows.\nStrong conjecture: As the singularity is approached the Einstein equations approach those of the truncated theory and in addition the solutions to the full equations are well approximated by solutions to the truncated equations.\nIn the beginning, the BKL conjecture seemed to be coordinate-dependent and rather implausible.\nBarrow and Tipler, for example, among the ten criticisms of BKL studies, include the inappropriate (according to them) choice of synchronous frame as a means to separate time and space derivatives.\nThe BKL conjecture was sometimes rephrased in the literature as a statement that near the singularity only the time derivatives are important.\nSuch a statement, taken at face value, is wrong or at best misleading since, as shown in the BKL analysis itself, space-like gradients of the metric tensor cannot be neglected for generic solutions of pure Einstein gravity in four spacetime dimensions, and in fact play a crucial role in the appearance of the oscillatory regime.\nHowever, there exist reformulations of Einstein theory in terms of new variables involving the relevant gradients, for example in Ashtekar-like variables, for which the statement about the dominant role of the time derivatives is correct.\nIt is true that one gets at each spatial point an effective description of the singularity in terms of a finite dimensional dynamical system described by ordinary differential equations with respect to time, but the spatial gradients do enter these equations non-trivially.\nSubsequent analysis by a large number of authors has shown that the BKL conjecture can be made precise and by now there is an impressive body of numerical and analytical evidence in its support.\nIt is fair to say that we are still quite far from a proof of the strong conjecture.\nBut there has been outstanding progress in simpler models.\nIn particular, Berger, Garfinkle, Moncrief, Isenberg, Weaver, and others showed that, in a class of models, as the singularity is approached the solutions to the full Einstein field equations approach the \"velocity term dominated\" (truncated) ones obtained by neglecting spatial derivatives.\nAndersson and Rendall showed that for gravity coupled to a massless scalar field or a stiff fluid, for every solution to the truncated equations there exists a solution to the full field equations that converges to the truncated solution as the singularity is approached, even in the absence of symmetries.\nThese results were generalized to also include p-form [gauge fields](https://bliptext.com/articles/gauge-theory).\nIn these truncated models the dynamics are simpler, allowing a precise statement of the conjecture that could be proven.\nIn the general case, the strongest evidence to date comes from numerical evolutions.\nBerger and Moncrief began a program to analyze generic cosmological singularities.\nWhile the initial work focused on symmetry reduced cases, more recently Garfinkle performed numerical evolution of space-times with no symmetries in which, again, the mixmaster behavior is apparent.\nFinally, additional support for the conjecture has come from a numerical study of the behavior of test fields near the singularity of a Schwarzschild black hole.\n\n## Kasner solution\nThe BKL approach to anisotropic (as opposed to isotropic) homogeneous spaces starts with a generalization of an exact [particular solution](https://bliptext.com/articles/ordinary-differential-equation) derived by Kasner for a field in vacuum, in which the space is homogeneous and has a [Euclidean metric](https://bliptext.com/articles/euclidean-metric) that depends on time according to the [Kasner metric](https://bliptext.com/articles/kasner-metric)\n(dl is the [line element](https://bliptext.com/articles/line-element); dx, dy, dz are [infinitesimal](https://bliptext.com/articles/infinitesimal) [displacements](https://bliptext.com/articles/differential-infinitesimal) in the three [spatial dimensions](https://bliptext.com/articles/spatial-dimensions), and t is time period passed since some initial moment t0 = 0).\nHere, p1, p2, p3 are any three numbers that satisfy the following Kasner conditions\nBecause of these relations, only one of the three numbers is [independent](https://bliptext.com/articles/independent-variable) (two [equations](https://bliptext.com/articles/equations) with three [unknowns](https://bliptext.com/articles/unknown-mathematics)).\nAll three numbers are never the same; two numbers are the same only in the [sets](https://bliptext.com/articles/set-mathematics) of values and (0, 0, 1).\nIn all other cases the numbers are different, one number is negative and the other two are positive.\nThis is partially proved by squaring both sides of the first condition and developing the square:\nThe term is equal to 1 by dint of the second condition and therefore the term with the mixed products should be zero.\nThis is possible if at least one of the p1, p2, p3 is negative.\nIf the numbers are arranged in increasing order, p1 1.\nTo remove these terms and restore the metric, it is necessary to impose on the coordinate functions the condition λ = 0.\nThe remaining three Einstein equations contain only [first order time derivatives](https://bliptext.com/articles/first-order-derivative-expression) of the metric tensor.\nThey give three time-independent relations that must be imposed as necessary conditions on the coordinate functions in.\nThis, together with the condition λ = 0, makes four conditions.\nThese co**n**ditio**n**s bi**n**d te**n** differe**n**t coordi**n**ate fu**n**ctio**n**s: three co**m**po**n**e**n**ts of each of the vectors **l**, **m**, **n**, a**n**d o**n**e fu**n**ctio**n** i**n** the powers of t (a**n**y o**n**e of the fu**n**ctio**n**s p**l**, p**m**, p**n**, which are bou**n**d by the co**n**ditio**n**s ).\nWhen calculating the number of physically arbitrary functions, it must be taken into account that the synchronous system used here allows time-independent arbitrary [transformations](https://bliptext.com/articles/transformation-function) of the three space coordinates.\nTherefore, the final solution contains overall 10 − 4 − 3 = 3 physically arbitrary functions which is one less than what is needed for the general solution in vacuum.\nThe degree of generality reached at this point is not lessened by introducing matter; matter is written into the metric and contributes four new coordinate functions necessary to describe the initial distribution of its density and the three components of its velocity.\nThis makes possible to determine matter evolution merely from the laws of its movement in an a priori given gravitational field which are the [hydrodynamic](https://bliptext.com/articles/fluid-dynamics) equations\nwhere ui is the 4-dimensional velocity, ε and σ are the densities of energy and [entropy](https://bliptext.com/articles/entropy) of matter (cf. and; also; for details see ).\nFor the [ultrarelativistic](https://bliptext.com/articles/ultrarelativistic-limit) [equation of state](https://bliptext.com/articles/equation-of-state) p = ε/3 the entropy σ ~ ε1/4.\nThe major terms in and are those that contain time [derivatives](https://bliptext.com/articles/derivative).\nFrom and the space components of one has\nresulting in\nwhere 'const' are time-independent quantities.\nAdditionally, from the identity uiui = 1 one has (because all covariant components of uα are to the same order)\nwhere u**n** is the velocity compo**n**e**n**t alo**n**g the directio**n** of **n** that is co**n****n**ected with the highest (positive) power of t (supposi**n**g that p**n** = p3).\nFrom the above relations, it follows that\nor\nThe above equations can be used to confirm that the components of the matter [stress-energy-momentum tensor](https://bliptext.com/articles/stress-energy-momentum-tensor) standing in the right hand side of the equations\nare, indeed, to a lower order by 1/t than the major terms in their left hand sides.\nIn the equations the presence of matter results only in the change of relations imposed on their constituent coordinate functions.\nThe fact that ε becomes infinite by the law confirms that in the solution to one deals with a physical singularity at any values of the powers p1, p2, p3 excepting only (0, 0, 1).\nFor these last values, the singularity is non-physical and can be removed by a change of reference frame.\nThe fictional singularity corresponding to the powers (0, 0, 1) arises as a result of time line coordinates crossing over some 2-dimensional \"[focal surface](https://bliptext.com/articles/focal-surface)\".\nAs pointed out in, a synchronous reference frame can always be chosen in such a way that this inevitable time line crossing occurs exactly on such surface (instead of a 3-dimensional caustic surface).\nTherefore, a solution with such simultaneous for the whole space fictional singularity must exist with a full set of arbitrary functions needed for the general solution.\nClose to the point t = 0 it allows a regular expansion by whole powers of t.\nFor an analysis of this case, see.\n# [Plot of p1, p2, p3 with an argument 1/u. The numbers p1(u) and p3(u) are monotonously increasing while p2(u) is a monotonously decreasing function of u. | upload.wikimedia.org/wikipedia/commons/c/c9/Bkl///fig1.svg]\n\n# Oscillating mode towards the singularity\nThe general solution by definition is completely stable; otherwise the Universe would not exist.\nAny [perturbation](https://bliptext.com/articles/perturbation-astronomy) is equivalent to a change in the initial conditions in some moment of time; since the general solution allows arbitrary initial conditions, the [perturbation](https://bliptext.com/articles/perturbation-astronomy) is not able to change its character.\nLooked at such angle, the four conditions imposed on the coordinate functions in the solution are of different types: three conditions that arise from the equations = 0 are \"natural\"; they are a consequence of the structure of Einstein equations.\nHowever, the additional condition λ = 0 that causes the loss of one derivative function, is of entirely different type: instability caused by perturbations can break this condition.\nThe action of such perturbation must bring the model to another, more general, mode.\nThe perturbation cannot be considered as small: a transition to a new mode exceeds the range of very small perturbations.\nThe analysis of the behavior of the model under perturbative action, performed by BKL, delineates a complex [oscillatory](https://bliptext.com/articles/oscillation) mode on approaching the singularity.\nThey could not give all details of this mode in the broad frame of the general case.\nHowever, BKL explained the most important properties and character of the solution on specific models that allow far-reaching analytical study.\nThese models are based on a [homogeneous space](https://bliptext.com/articles/homogeneous-space) metric of a particular type.\nSupposing a homogeneity of space without any additional symmetry leaves a great freedom in choosing the metric.\nAll possible homogeneous (but anisotropic) spaces are classified, according to [Bianchi](https://bliptext.com/articles/luigi-bianchi), in several [Bianchi](https://bliptext.com/articles/luigi-bianchi) types (Type I to IX).\n(see also [Generalized homogeneous solution](https://bliptext.com/articles/)) BKL investigate only spaces of Bianchi Types VIII and IX.\nIf the **m**etric has the for**m** of, for each type of ho**m**oge**n**eous spaces exists so**m**e fu**n**ctio**n**a**l** re**l**atio**n** betwee**n** the refere**n**ce vectors **l**, **m**, **n** a**n**d the space coordi**n**ates.\nThe specific form of this relation is not important.\nThe i****m****porta**n**t fact is that for Type VIII a**n**d IX spaces, the qua**n**tities λ, μ, ν are co**n**sta**n**ts whi******l******e a******l************l****** \"****m****ixed\" products ******l****** rot ****m****, ******l****** rot **n**, ****m**** rot ******l******, etc..\nare zeros.\nFor Type IX spaces, the quantities λ, μ, ν have the same sign and one can write λ = μ = ν = 1 (the simultaneous sign change of the 3 constants does not change anything).\nFor Type VIII spaces, 2 constants have a sign that is opposite to the sign of the third constant; one can write, for example, λ = − 1, μ = ν = 1.\nThe study of the effect of the perturbation on the \"Kasner mode\" is thus confined to a study on the effect of the λ-containing terms in the Einstein equations.\nType VIII and IX spaces are the most suitable models for such a study.\nSi**n**ce a**l****l** 3 qua**n**tities λ, μ, ν i**n** those Bia**n**chi types differ fro**m** zero, the co**n**ditio**n** λ = 0 does **n**ot ho**l**d irrespective of which directio**n** **l**, **m**, **n** has **n**egative power **l**aw ti**m**e depe**n**de**n**ce.\nThe Einstein equations for the Type VIII and Type IX space models are\n(the remaining components R_l^0, R_m^0, R_n^0, R_l^m, R_l^n, R_m^n are identically zeros).\nThese equations contain only functions of time; this is a condition that has to be fulfilled in all homogeneous spaces.\nHere, the and are exact and their validity does not depend on how near one is to the singularity at t = 0.\nThe time derivatives in and take a simpler form if а, b, с are substituted by their logarithms α, β, γ:\nsubstituting the variable t for τ according to:\nThen (subscripts denote differentiation by τ):\nAdding together equations and substituting in the left hand side the sum (α + β + γ)τ τ according to, one obtains an equation containing only first derivatives which is the [first integral](https://bliptext.com/articles/first-integral) of the system :\nThis equation plays the role of a binding condition imposed on the initial state of.\nThe Kasner mode is a solution of when ignoring all terms in the right hand sides.\nBut such situation cannot go on (at t → 0) indefinitely because among those terms there are always some that grow.\nThus, if the negative power is in the function a(t) (pl = p1) then the perturbation of the Kasner mode will arise by the terms λ2a4; the rest of the terms will decrease with decreasing t.\nIf only the growing terms are left in the right hand sides of, one obtains the system:\n(compare ; below it is substituted λ2 = 1).\nThe solution of these equations must describe the metric evolution from the initial state, in which it is described by with a given set of powers (with pl 1 according to ; in this moment the negative power is pl or pm while pn becomes the lesser of two positive numbers (pn = p2).\nThe ****n****ext series of Kas****n****er epochs the****n**** f**l**ips the ****n****egative power betwee****n**** directio****n****s ****n**** a****n****d **l** or betwee****n**** ****n**** a****n****d **m**.\nAt an arbitrary ([irrational](https://bliptext.com/articles/irrational-number)) initial value of u this process of alternation continues unlimited.\nIn the exact solution of the Einstein equations, the powers pl, pm, pn lose their original precise sense.\nThis circumstance introduces some \"fuzziness\" in the determination of these numbers (and together with them, to the parameter u) which, although small, makes meaningless the analysis of any definite (for example, [rational](https://bliptext.com/articles/rational-number)) values of u.\nTherefore, only these laws that concern arbitrary irrational values of u have any particular meaning.\nThe larger periods in which the scales of space distances along two axes oscillate while distances along the third axis decrease monotonously, are called [eras](https://bliptext.com/articles/era); volumes decrease by a law close to ~ t.\nOn transition from one era to the next, the direction in which distances decrease [monotonously](https://bliptext.com/articles/monotonic-function), flips from one axis to another.\nThe order of these transitions acquires the asymptotic character of a [random process](https://bliptext.com/articles/stochastic-process).\nThe same random order is also characteristic for the alternation of the lengths of successive eras (by era length, BKL understand the number of Kasner epoch that an era contains, and not a time interval).\nTo each era (s-th era) correspond a series of values of the parameter u starting from the greatest,, and through the values − 1, − 2, ..., reaching to the smallest, |αn|, i.e., the oscillation amplitudes of functions α and β increase during the whole era although the factors δn may be small.\nIf the minimum at the beginning of an era is deep, the next minima will not become shallower; in other words, the residue |α — β| at the moment of transition between Kasner epochs remains large.\nThis assertion does not depend upon era length k because transitions between epochs are determined by the common rule also for long eras.\nThe last oscillation amplitude of functions α or β in a given era is related to the amplitude of the first oscillation by the relationship |αk−1| = |α0| (k + x) / (1 + x).\nEven at k's as small as several units x can be ignored in comparison to k so that the increase of α and β oscillation amplitudes becomes proportional to the era length.\nFor functions a = eα and b = eβ this means that if the amplitude of their oscillations in the beginning of an era was A0, at the end of this era the amplitude will become.\nThe length of Kasner epochs (in logarithmic time) also increases inside a given era; it is easy to calculate from that Δn+1 > Δn.\nThe total era length is\n(the term with 1/x arises from the last, k-th, epoch whose length is great at small x; cf. Fig. 2).\nMoment Ωn when the k-th epoch of a given era ends is at the same time moment Ω'0 of the beginning of the next era.\nIn the first Kasner epoch of the new era function γ is the first to rise from the minimal value γk = − Ωk (1 − δk) that it reached in the previous era; this value plays the role of a starting amplitude δ'0Ω'0 for the new series of oscillations.\nIt is easily obtained that:\nIt is obvious that δ'0Ω'0 > δ0Ω0.\nEven at not very great k the amplitude increase is very significant: function c = eγ begins to oscillate from amplitude.\nThe issue about the above-mentioned \"dangerous\" cases of drastic lowering of the upper oscillation limit is left aside for now.\nAccording to the increase in matter density during the first (k − 1) epochs is given by the formula\nFor the last k epoch of a given era, at u = x 1.\nSuch transformations possess the property of exponential instability: if we take initially two close points their mutual distance increases exponentially under the iterations of the transformations.\nIt is well known that the exponential instability leads to the appearance of strong stochastic properties.\nIt is possible to change over to a probabilistic description of such a sequence by considering not a definite initial value x(0) but the values x(0) = x distributed in the interval from 0 to 1 in accordance with a certain [probabilistic distributional law](https://bliptext.com/articles/probability-distribution) w0(x).\nThen the values of x(s) terminating each era will also have distributions that follow certain laws ws(x).\nLet ws(x)dx be the probability that the s-th era terminates with the value lying in a specified interval dx.\nThe value x(s) = x, which terminates the s-th era, can result from initial (for this era) values, where k = 1, 2, ...; these values of correspond to the values x(s–1) = 1/(k + x) for the preceding era.\nNoting this, one can write the following recurrence relation, which expresses the distribution of the probabilities ws(x) in terms of the distribution ws–1(x):\nor\nIf the distribution ws(x) tends with increasing s to a stationary (independent of s) limiting distribution w(x), then the latter should satisfy an equation obtained from by dropping the indices of the functions ws−1(x) and ws(x).\nThis equation has a solution\n(normalized to unity and taken to the first order of x).\nIn order for the s-th era to have a length k, the preceding era must terminate with a number x in the interval between 1/(k + 1) and 1/k.\nTherefore, the probability that the era will have a length k is equal to (in the stationary limit)\nAt large values of k\nIn relating the statistical properties of the cosmological model with the [ergodic](https://bliptext.com/articles/ergodicity) properties of the transformation xs+1 = {1/xs} an important point must be mentioned.\nIn an infinite sequence of numbers x constructed in accordance with this rule, arbitrarily small (but never vanishing) values of x will be observed corresponding to arbitrarily large lengths k. Such cases can (by no means necessarily!) give rise to certain specific situations when the notion of eras, as of sequences of Kasner epochs interchanging each other according to the rule, loses its meaning (although the oscillatory mode of evolution of the model still persists).\nSuch an \"anomalous\" situation can be manifested, for instance, in the necessity to retain in the right-hand side of terms not only with one of the functions a, b, c (say, a4), as is the case in the \"regular\" interchange of the Kasner epochs, but simultaneously with two of them (say, a4, b4, a2b2).\nOn emerging from an \"anomalous\" series of oscillations a succession of regular eras is restored.\nStatistical analysis of the behavior of the model which is entirely based on regular iterations of the transformations is corroborated by an important theorem: the probability of the appearance of anomalous cases tends asymptotically to zero as the number of iterations s → ∞ (i.e., the time t → 0) which is proved at the end of this section.\nThe validity of this assertion is largely due to a very rapid rate of increase of the oscillation amplitudes during every era and especially in transition from one era to the next one.\nThe process of the relaxation of the cosmological model to the \"stationary\" statistical regime (with t → 0 starting from a given \"initial instant\") is less interesting, however, than the properties of this regime itself with due account taken for the concrete laws of the variation of the physical characteristics of the model during the successive eras.\nAn idea of the rate at which the stationary distribution sets in is obtained from the following example.\nLet the initial values x(0) be distributed in a narrow interval of width δx(0) about some definite number.\nFrom the recurrence relation (or directly from the expansion ) it is easy to conclude that the widths of the distributions ws(x) (about other definite numbers) will then be equal to\n(this expression is valid only so long as it defines quantities δx(s) ≪ 1).\nThe mean value \\bar k, calculated from this distribution, diverges logarithmically.\nFor a sequence, cut off at a very large, but still finite number N, one has.\nThe usefulness of the mean in this case is very limited because of its instability: because of the slow decrease of W(k), fluctuations in k diverge faster than its mean.\nA more adequate characteristic of this sequence is the probability that a randomly chosen number from it belongs to an era of length K where K is large.\nThis probability is lnK / lnN.\nIt is small if.\nIn this respect one can say that a randomly chosen number from the given sequence belongs to the long era with a high probability.\nIt convenient to average expressions that depend simultaneously on k(s) and x(s).\nSince both these quantities are derived from the same quantity x(s–1) (which terminates the preceding era), in accordance with the formula k(s) + x(s) = 1/x(s–1), their [statistical distributions](https://bliptext.com/articles/probability-distribution) cannot be regarded as independent.\nThe joint distribution Ws(k,x)dx of both quantities can be obtained from the distribution ws–1(x)dx by making in the latter the substitution x → 1/(x + k).\nIn other words, the function Ws(k,x) is given by the very expression under the summation sign in the right side of.\nIn the stationary limit, taking w from, one obtains\nSummation of this distribution over k brings us back to, and integration with respect to dx to.\nThe recurrent formulas defining transitions between eras are re-written with index s numbering the successive eras (not the Kasner epochs in a given era!), beginning from some era (s = 0) defined as initial.\nΩ(s) and ε(s) are, respectively, the initial moment and initial matter density in the s-th era; δ(s)Ω(s) is the initial oscillation amplitude of that pair of functions α, β, γ, which oscillates in the given era: k(s) is the length of s-th era, and x(s) determines the length (number of Kasner epochs) of the next era according to k(s+1) = [1/x(s)].\nAccording to –\n(ξ(s) is introduced in to be used further on).\nThe quantities δ(s) have a stable stationary statistical distribution P(δ) and a stable (small relative fluctuations) mean value.\nFor their determination KL in coauthorship with [Ilya Lifshitz](https://bliptext.com/articles/ilya-lifshitz), the brother of Evgeny Lifshitz, used (with due reservations) an approximate method based on the assumption of statistical independence of the random quantity δ(s) and of the random quantities k(s), x(s).\nFor the function P(δ) an integral equation was set up which expressed the fact that the quantities δ(s+1) and δ(s) interconnected by the relation have the same distribution; this equation was solved numerically.\nIn a later work, Khalatnikov et al. showed that the distribution P(δ) can actually be found exactly by an analytical method (see **Fig. 5**).\nFor the statistical properties in the stationary limit, it is reasonable to introduce the so-called natural extension of the transformation Tx = {1/x} by continuing it without limit to negative indices.\nOtherwise stated, this is a transition from a one-sided infinite sequence of the numbers (x0, x1, x2, ...), connected by the equalities Tx = {1/x}, to a \"doubly infinite\" sequence X = (..., x−1, x0, x1, x2, ...) of the numbers which are connected by the same equalities for all –∞ 1.) Noting that\none obtains\nIf the initial value of x(0) lies outside the dangerous region λ there will be no dangerous cases.\nIf it lies inside this region dangerous cases occur, but upon their completion the model resumes a \"regular\" evolution with a new initial value which only occasionally (with a probability λ) may come into the dangerous interval.\nRepeated dangerous cases occur with probabilities λ2, λ3, ..., asymptotically converging to zero.\n# [The probability distribution function P(δ). Red line: exact function . Blue line: approximate solution of the integral equation in. Both curves appear to be strikingly similar and the means of both distributions are 0.50. | upload.wikimedia.org/wikipedia/commons/4/4e/Bkl///fig3a.svg]\n\n# General solution with small oscillations\nIn the above models, metric evolution near the singularity is studied on the example of homogeneous space metrics.\nIt is clear from the characteristic of this evolution that the analytic construction of the general solution for a singularity of such type should be made separately for each of the basic evolution components: for the Kasner epochs, for the process of transitions between epochs caused by \"perturbations\", for long eras with two perturbations acting simultaneously.\nDuring a Kasner epoch (i.e. at small perturbations), the metric is given by without the condition λ = 0.\nBKL further developed a matter distribution-independent model (homogeneous or non-homogeneous) for long era with small oscillations.\nThe time dependence of this solution turns out to be very similar to that in the particular case of homogeneous models; the latter can be obtained from the distribution-independent model by a special choice of the arbitrary functions contained in it.\nIt is convenient, however, to construct the general solution in a system of coordinates somewhat different from synchronous reference frame: g0α = 0 as in the synchronous frame, but instead of g00 = 1 it is now g00 = −g33.\nDefining again the space metric tensor γαβ = −gαβ one has, therefore\nThe special space coordinate is written as x3 = z and the time coordinate is written as x0 = ξ (as different from proper time t); it will be shown that ξ corresponds to the same variable defined in homogeneous models.\nDifferentiation by ξ and z is designated, respectively, by dot and prime.\nLatin indices a, b, c take values 1, 2, corresponding to space coordinates x1, x2 which will be also written as x, y.\nTherefore, the metric is\nThe required solution should satisfy the inequalities\n(these conditions specify that one of the functions a2, b2, c2 is small compared to the other two which was also the case with homogeneous models).\nInequality means that components γa3 are small in the sense that at any ratio of the shifts dxa and dz, terms with products dxadz can be omitted in the square of the spatial length element dl2.\nTherefore, the first approximation to a solution is a metric with γa3 = 0:\nOne can be easily convinced by calculating the Ricci tensor components R_0^0, R_3^0, R_3^3, R_a^b using metric and the condition that all terms containing derivatives by coordinates xa are small compared to terms with derivatives by ξ and z (their ratio is ~ γ33 / γab).\nIn other words, to obtain the equations of the main approximation, γ33 and γab in should be differentiated as if they do not depend on xa.\nDesignating\none obtains the following equations:\nIndex raising and lowering is done here with the help of γab.\nThe quantities \\varkappa and λ are the contractions and \\lambda_a^a whereby\nAs to the Ricci tensor components R_a^0, R_a^3, by this calculation they are identically zero.\nIn the next approximation (i.e., with account to small γa3 and derivatives by x, y), they determine the quantities γa3 by already known γ33 and γab.\nContraction of gives, and, hence,\nDifferent cases are possible depending on the G variable.\nIn the above case g00 = γ33 \\gg γab and.\nThe case N > 0 (quantity N is time-like) leads to time singularities of interest.\nSubstituting in f1 = 1/2 (ξ + z) sin y, f2 = 1/2 (ξ − z) sin y results in G of type\nThis choice does not diminish the generality of conclusions; it can be shown that generality is possible (in the first approximation) just on account of the remaining permissible transformations of variables.\nAt N 0, s2 > 0; hence 1 − > 0.\nThus, at small ξ oscillations of functions γab cease while function γ33 begins to increase at decreasing ξ.\nThis is a Kasner mode and when γ33 is compared to γab, the above approximation is not applicable.\nIn order to check the compatibility of this analysis, BKL studied the equations = 0, = 0, and, calculating from them the components γa3, confirmed that the inequality takes place.\nThis study showed that in both asymptotic regions the components γa3 were ~ γ33.\nTherefore, correctness of inequality immediately implies correctness of inequality.\nThis solution contains, as it should for the general case of a field in vacuum, four arbitrary functions of the three space coordinates x, y, z.\nIn the region ξ \\ll 1 these functions are, e.g., λ1, λ2, μ1, s1.\nIn the region ξ \\gg 1 the four functions are defined by the Fourier series by coordinate z from with coefficients that are functions of x, y; although Fourier series decomposition (or integral?) characterizes a special class of functions, this class is large enough to encompass any finite subset of the set of all possible initial conditions.\nThe solution contains also a number of other arbitrary functions of the coordinates x, y.\nSuch two-dimensional arbitrary functions appear, generally speaking, because the relationships between three-dimensional functions in the solutions of the Einstein equations are differential (and not algebraic), leaving aside the deeper problem about the geometric meaning of these functions.\nBKL did not calculate the number of independent two-dimensional functions because in this case it is hard to make unambiguous conclusions since the three-dimensional functions are defined by a set of two-dimensional functions (cf. for more details).\nFinally, BKL go on to show that the general solution contains the particular solution obtained above for homogeneous models.\nSubstituting the basis vectors for Bianchi Type IX homogeneous space in the space-time metric of this model takes the form\nWhen c2 \\ll a2, b2, one can ignore c2 everywhere except in the term c2 dz2.\nTo move from the synchronous frame used in to a frame with conditions, the transformation dt = c dξ/2 and substitution z → z/2 are done.\nAssuming also that χ ≡ ln (a/b) \\ll 1, one obtains from in the first approximation:\nSimilarly, with the basis vectors of Bianchi Type VIII homogeneous space, one obtains\nAccording to the analysis of homogeneous spaces above, in both cases ab = ξ (simplifying a_0^2 = ξ0) and χ is from ; function c (ξ) is given by formulae and, respectively, for models of Types IX and VIII.\nIdentical metric for Type VIII is obtained from, , choosing two-dimensional vectors la and ma in the form\nand substituting\nTo obtain the metric for Type IX, one should substitute\n(for calculation of c (ξ) the approximation in is not sufficient and the term in ψ linear by ξ is calculated )\nThis analysis was done for empty space.\nIncluding matter does not make the solution less general and does not change its qualitative characteristics.\nA limitation of great importance for the general solution is that all 3-dimensional functions contained in the metrics and should have a single and common characteristic change interval.\nOnly this allows to approximate in the Einstein equations all metric spatial component derivatives with simple products of these components by a characteristic wave numbers which results in ordinary differential equations of the type obtained for the Type IX homogeneous model.\nThis is the reason for the coincidence between homogeneous and general solutions.\nIt follows that both Type IX model and its generalisation contain an oscillatory mode with a single spatial scale of an arbitrary magnitude which is not selected among others by any physical conditions.\nHowever, it is known that in non-linear systems with infinite degrees of freedom such mode is unstable and partially dissipates to smaller oscillations.\nIn the general case of small perturbations with an arbitrary spectrum, there will always be some whose amplitudes will increase feeding upon the total process energy.\nAs a result, a complicated picture arises of multi-scale movements with certain energy distribution and energy exchange between oscillations of different scales.\nIt doesn't occur only in the case when the development of small-scale oscillations is impossible because of physical conditions.\nFor the latter, some natural physical length must exist which determines the minimal scale at which energy exits from a system with dynamical degrees of freedom (which, for example, occurs in a liquid with a certain viscosity).\nHowever, there is no innate physical scale for a gravitational field in vacuum, and, therefore, there is no impediment for the development of oscillations of arbitrarily small scales.\n\n# Conclusions\nBKL describe singularities in the cosmologic solution of Einstein equations that have a complicated oscillatory character.\nAlthough these singularities have been studied primarily on spatially homogeneous models, there are convincing reasons to assume that singularities in the general solution of Einstein equations have the same characteristics; this circumstance makes the BKL model important for cosmology.\nA basis for such statement is the fact that the oscillatory mode in the approach to singularity is caused by the single perturbation that also causes instability in the generalized Kasner solution.\nA confirmation of the generality of the model is the analytic construction for long era with small oscillations.\nAlthough this latter behavior is not a necessary element of metric evolution close to the singularity, it has all principal qualitative properties: metric oscillation in two spatial dimensions and monotonous change in the third dimension with a certain perturbation of this mode at the end of some time interval.\nHowever, the transitions between Kasner epochs in the general case of non-homogeneous spatial metric have not been elucidated in details.\nThe problem connected with the possible limitations upon space geometry caused by the singularity was left aside for further study.\nIt is clear from the outset, however, that the original BKL model is applicable to both finite or infinite space; this is evidenced by the existence of oscillatory singularity models for both closed and open spacetimes.\nThe oscillatory mode of the approach to singularity gives a new aspect to the term 'finiteness of time'.\nBetween any finite moment of the world time t and the moment t = 0 there is an infinite number of oscillations.\nIn this sense, the process acquires an infinite character.\nInstead of time t, a more adequate variable for its description is ln t by which the process is extended to -\\infty.\nBKL consider metric evolution in the direction of decreasing time.\nThe Einstein equations are symmetric in respect to the time sign so that a metric evolution in the direction of increasing time is equally possible.\nHowever, these two cases are fundamentally different because past and future are not equivalent in the physical sense.\nFuture singularity can be physically meaningful only if it is possible at arbitrary initial conditions existing in a previous moment.\nMatter distribution and fields in some moment in the evolution of Universe do not necessarily correspond to the specific conditions required for the existence of a given special solution to the Einstein equations.\nThe choice of solutions corresponding to the real world is related to profound physical requirements which is impossible to find using only the existing relativity theory and which can be found as a result of future synthesis of physical theories.\nThus, it may turn out that this choice singles out some special (e.g., isotropic) type of singularity.\nNevertheless, it is more natural to assume that because of its general character, the oscillatory mode should be the main characteristic of the initial evolutionary stages.\nIn this respect, of considerable interest is the property of the [\"Mixmaster\" model](https://bliptext.com/articles/mixmaster-universe) shown by Misner, related to propagation of light signals.\nIn the isotropic model, a \"light horizon\" exists, meaning that for each moment of time, there is some longest distance, at which exchange of light signals and, thus, a causal connection, is impossible: the signal cannot reach such distances for the time since the singularity t = 0.\nSignal propagation is determined by the equation ds = 0.\nIn the isotropic model near the singularity t = 0 the interval element is, where d \\bar{l}^2 is a time-independent spatial differential form.\nSubstituting yields\nThe \"distance\" reached by the signal is\nSince η, like t, runs through values starting from 0, up to the \"moment\" η signals can propagate only at the distance which fixes the farthest distance to the horizon.\nThe existence of a light horizon in the isotropic model poses a problem in the understanding of the origin of the presently observed isotropy in the relic radiation.\nAccording to the isotropic model, the observed isotropy means isotropic properties of radiation that comes to the observer from such regions of space that can not be causally connected with each other.\nThe situation in the oscillatory evolution model near the singularity can be different.\nFor example, in the homogeneous model for Type IX space, a signal is propagated in a direction in which for a long era, scales change by a law close to ~ t.\nThe square of the distance element in this direction is dl2 = t2\\bar{l}^2, and the respective element of the four-dimensional interval is.\nThe substitution puts this in the form\nand for the signal propagation one has equation of the type again.\nThe important difference is that the variable η runs now through values starting from -\\infty (if metric is valid for all t starting from t = 0).\nTherefore, for each given \"moment\" η are found intermediate intervals Δη sufficient for the signal to cover each finite distance.\nIn this way, during a long era a light horizon is opened in a given space direction.\nAlthough the duration of each long era is still finite, during the course of the world evolution eras change an infinite number of times in different space directions.\nThis circumstance makes one expect that in this model a causal connection between events in the whole space is possible.\nBecause of this property, Misner named this model \"Mixmaster universe\" by a brand name of a dough-blending machine.\nAs time passes and one goes away from the singularity, the effect of matter on metric evolution, which was insignificant at the early stages of evolution, gradually increases and eventually becomes dominant.\nIt can be expected that this effect will lead to a gradual \"isotropisation\" of space as a result of which its characteristics come closer to the Friedman model which adequately describes the present state of the Universe.\nFinally, BKL pose the problem about the feasibility of considering a \"singular state\" of a world with infinitely dense matter on the basis of the existing relativity theory.\nThe physical application of the Einstein equations in their present form in these conditions can be made clear only in the process of a future synthesis of physical theories and in this sense the problem can not be solved at present.\nIt is important that the gravitational theory itself does not lose its logical cohesion (i.e., does not lead to internal controversies) at whatever matter densities.\nIn other words, this theory is not limited by the conditions that it imposes, which could make logically inadmissible and controversial its application at very large densities; limitations could, in principle, appear only as a result of factors that are \"external\" to the gravitational theory.\nThis circumstance makes the study of singularities in cosmological models formally acceptable and necessary in the frame of existing theory.","revisionCount":0},"hasMore":false,"nextCursor":null}

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