{"history":[{"id":"190f2b2b-f581-40d7-a48c-c66b46dfa524","wordChanged":"all","newWord":":::summary\n![Visualization of the observable universe. The scale is such that the fine grains represent collections of large numbers of superclusters. The Virgo Supercluster—home of Milky Way—is marked at the center, but is too small to be seen.](https://upload.wikimedia.org/wikipedia/commons/9/98/Observable///Universe///with///Measurements///01.png)\n**Bodystyle:** width:25em; \n**Title:** Observable universe \n**Label 1:** Diameter \n**Data 1:** 8.8 m or 880 [Ym](https://bliptext.com/articles/yotta) (28.5 Gpc or 93 Gly) \n**Label 2:** Circumference \n**Data 2:** 2.764 m or 2.764 [Rm](https://bliptext.com/articles/ronna) (89.6 Gpc or 292.2 Gly) \n**Label 3:** Volume \n**Data 3:** 3.566 m3 \n**Label 4:** Mass (ordinary matter) \n**Data 4:** 1.5 kg \n**Label 5:** Density (of total energy) \n**Data 5:** 9.9 kg/m3 (equivalent to 6 [protons](https://bliptext.com/articles/proton) per cubic meter of space) \n**Label 6:** Age \n**Data 6:** 13.787 billion years \n**Label 7:** Average temperature \n**Data 7:** 2.725 [K](https://bliptext.com/articles/kelvin) \n**Label 8:** Contents \n**Data 8:** [Ordinary (baryonic)](https://bliptext.com/articles/baryon) [matter](https://bliptext.com/articles/matter) (4.9%) · Dark [matter](https://bliptext.com/articles/matter) (26.8%) · [Dark energy](https://bliptext.com/articles/dark-energy) (68.3%) \n:::\n\n# \nThe **observable [universe](https://bliptext.com/articles/universe)** is a [spherical](https://bliptext.com/articles/ball-mathematics) region of the [universe](https://bliptext.com/articles/universe) consisting of no [matter](https://bliptext.com/articles/matter) that can be [observed](https://bliptext.com/articles/observation) from [Earth](https://bliptext.com/articles/earth) or its space-based telescopes and exploratory probes at the present time; the [electromagnetic radiation](https://bliptext.com/articles/electromagnetic-radiation) from these [objects](https://bliptext.com/articles/astronomical-object) has had time to reach the [Solar System](https://bliptext.com/articles/solar-system) and [Earth](https://bliptext.com/articles/earth) since the beginning of the [cosmological expansion](https://bliptext.com/articles/metric-expansion-of-space).\nAssuming the universe is [isotropic](https://bliptext.com/articles/isotropy), the distance to the edge of the observable universe is roughly [the same](https://bliptext.com/articles/equidistant) in every direction.\nThat is, the observable universe is a [spherical](https://bliptext.com/articles/sphere) region centered on the observer.\nEvery location in the universe has its own observable universe, which may or may not overlap with the one centered on Earth.\nThe word observable in this sense does not refer to the capability of modern technology to detect [light](https://bliptext.com/articles/light) or other information from an object, or whether there is anything to be detected.\nIt refers to the physical limit created by the [speed of light](https://bliptext.com/articles/speed-of-light) itself.\nNo signal can travel faster than light, hence there is a maximum distance, called the [particle horizon](https://bliptext.com/articles/particle-horizon), beyond which nothing can be detected, as the signals could not have reached us yet.\nSometimes astrophysicists distinguish between the observable universe and the visible universe.\nThe former includes signals since the end of the [inflationary epoch](https://bliptext.com/articles/inflationary-epoch), while the latter includes only signals emitted since [recombination](https://bliptext.com/articles/recombination-cosmology).\nAccording to calculations, the current [comoving distance](https://bliptext.com/articles/comoving-distance) to particles from which the [cosmic microwave background radiation](https://bliptext.com/articles/cosmic-microwave-background-radiation) (CMBR) was emitted, which represents the radius of the visible universe, is about 14.0 billion [parsecs](https://bliptext.com/articles/parsec) (about 45.7 billion light-years).\nThe comoving distance to the edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light-years), about 2% larger.\nThe [radius](https://bliptext.com/articles/radius) of the observable universe is therefore estimated to be about 46.5 billion light-years.\nUsing the [critical density](https://bliptext.com/articles/friedmann-equations) and the diameter of the observable universe, the total mass of ordinary matter in the universe can be calculated to be about 1.5 kg. In November 2018, astronomers reported that [extragalactic background light](https://bliptext.com/articles/extragalactic-background-light) (EBL) amounted to 4 photons.\nAs the universe's expansion is accelerating, all currently observable objects, outside the local [supercluster](https://bliptext.com/articles/supercluster), will eventually appear to freeze in time, while emitting progressively redder and fainter light.\nFor instance, objects with the current [redshift](https://bliptext.com/articles/redshift) [z](https://bliptext.com/articles/redshift) from 5 to 10 will only be observable up to an age of 4–6 billion years.\nIn addition, light emitted by objects currently situated beyond a certain comoving distance (currently about 19 Gpc) will never reach Earth.\n\n# Overview\nThe universe's size is unknown, and it may be infinite in extent.\nSome parts of the universe are too far away for the light emitted since the [Big Bang](https://bliptext.com/articles/big-bang) to have had enough time to reach Earth or space-based instruments, and therefore lie outside the observable universe.\nIn the future, light from distant galaxies will have had more time to travel, so one might expect that additional regions will become observable.\nRegions distant from observers (such as us) are expanding away faster than the speed of light, at rates estimated by [Hubble's law](https://bliptext.com/articles/hubble-s-law).\nThe [expansion rate appears to be accelerating](https://bliptext.com/articles/accelerating-expansion-of-the-universe), which [dark energy](https://bliptext.com/articles/dark-energy) was proposed to explain.\nAssuming dark energy remains constant (an unchanging [cosmological constant](https://bliptext.com/articles/cosmological-constant)) so that the expansion rate of the universe continues to accelerate, there is a \"future visibility limit\" beyond which objects will never enter the observable universe at any time in the future because light emitted by objects outside that limit could never reach the Earth.\nNote that, because the [Hubble parameter](https://bliptext.com/articles/hubble-s-law) is decreasing with time, there can be cases where a galaxy that is receding from Earth only slightly faster than light emits a signal that eventually reaches Earth.\nThis future visibility limit is calculated at a [comoving distance](https://bliptext.com/articles/comoving-distance) of 19 billion parsecs (62 billion light-years), assuming the universe will keep expanding forever, which implies the number of galaxies that can ever be theoretically observed in the infinite future is only larger than the number currently observable by a factor of 2.36 (ignoring redshift effects).\nIn principle, more galaxies will become observable in the future; in practice, an increasing number of galaxies will become extremely [redshifted](https://bliptext.com/articles/redshift) due to ongoing expansion, so much so that they will seem to disappear from view and become invisible.\nA galaxy at a given comoving distance is defined to lie within the \"observable universe\" if we can receive signals emitted by the galaxy at any age in its history, say, a signal sent from the galaxy only 500 million years after the Big Bang.\nBecause of the universe's expansion, there may be some later age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future, so, for example, we might never see what the galaxy looked like 10 billion years after the Big Bang, even though it remains at the same comoving distance less than that of the observable universe.\nThis can be used to define a type of cosmic [event horizon](https://bliptext.com/articles/event-horizon) whose distance from the Earth changes over time.\nFor example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening at present can eventually reach the Earth if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is further away.\nThe space before this cosmic event horizon can be called \"reachable universe\", that is all galaxies closer than that could be reached if we left for them today, at the speed of light; all galaxies beyond that are unreachable.\nSimple observation will show the future visibility limit (62 billion light-years) is exactly equal to the reachable limit (16 billion light-years) added to the current visibility limit (46 billion light-years).\n# [Observable Universe as a function of time and distance, in context of the expanding Universe | upload.wikimedia.org/wikipedia/commons/f/f2/Home///in///Relation///to///Everything-Observable///Universe.png]\n# [The reachable Universe as a function of time and distance, in context of the expanding Universe. | upload.wikimedia.org/wikipedia/commons/8/88/Home///in///Relation///to///Everything-Reachable///Universe.png]\n\n# \"The universe\" versus \"the observable universe\"\nBoth popular and professional research articles in cosmology often use the term \"universe\" to mean \"observable universe\".\nThis can be justified on the grounds that we can never know anything by direct observation about any part of the universe that is [causally disconnected](https://bliptext.com/articles/causality-physics) from the Earth, although many credible theories require a total universe much larger than the observable universe.\nNo evidence exists to suggest that the boundary of the observable universe constitutes a boundary on the universe as a whole, nor do any of the mainstream cosmological models propose that the universe has any physical boundary in the first place.\nHowever, some models propose it could be finite but unbounded, like a higher-dimensional analogue of the 2D surface of a sphere that is finite in area but has no edge.\nIt is plausible that the [galaxies](https://bliptext.com/articles/galaxy) within the observable universe represent only a minuscule fraction of the [galaxies](https://bliptext.com/articles/galaxy) in the universe.\nAccording to the theory of [cosmic inflation](https://bliptext.com/articles/cosmic-inflation) initially introduced by [Alan Guth](https://bliptext.com/articles/alan-guth) and [D. Kazanas](https://bliptext.com/articles/d-kazanas), if it is assumed that inflation began about 10−37 seconds after the Big Bang and that the pre-inflation size of the universe was approximately equal to the speed of light times its age, that would suggest that at present the entire universe's size is at least 1.5 light-years—at least 3 times the radius of the observable universe.\nIf the universe is finite but unbounded, it is also possible that the universe is smaller than the observable universe.\nIn this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated the universe.\nIt is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different.\nBielewicz et al. claim to establish a lower bound of 27.9 gigaparsecs (91 billion light-years) on the diameter of the last scattering surface.\nThis value is based on matching-circle analysis of the [WMAP](https://bliptext.com/articles/wmap) 7-year data.\nThis approach has been disputed.\n\n# Size\nThe [comoving distance](https://bliptext.com/articles/comoving-and-proper-distances) from Earth to the edge of the observable universe is about 14.26 giga[parsecs](https://bliptext.com/articles/parsec) (46.5 [billion](https://bliptext.com/articles/1000000000-number) [light-years](https://bliptext.com/articles/light-year) or 14.26 Gpc) in any direction.\nThe observable universe is thus a sphere with a [diameter](https://bliptext.com/articles/diameter) of about 28.5 gigaparsecs (93 billion light-years or 28.5 Gpc).\nAssuming that space is roughly [flat](https://bliptext.com/articles/shape-of-the-universe) (in the sense of being a [Euclidean space](https://bliptext.com/articles/euclidean-space)), this size corresponds to a comoving volume of about 1.22 Gpc3 (4.22 Gly3 or 3.57 m3).\nThese are distances now (in [cosmological time](https://bliptext.com/articles/cosmological-time)), not distances at the time the light was emitted.\nFor example, the cosmic microwave background radiation that we see right now was emitted at the [time of photon decoupling](https://bliptext.com/articles/recombination-cosmology), estimated to have occurred about 380,000 years after the Big Bang, which occurred around 13.8 billion years ago.\nThis radiation was emitted by matter that has, in the intervening time, mostly condensed into galaxies, and those galaxies are now calculated to be about 46 billion light-years from Earth.\nTo estimate the distance to that matter at the time the light was emitted, we may first note that according to the [Friedmann–Lemaître–Robertson–Walker metric](https://bliptext.com/articles/friedmann-lema-tre-robertson-walker-metric), which is used to model the expanding universe, if we receive light with a [redshift](https://bliptext.com/articles/redshift) of z, then the [scale factor](https://bliptext.com/articles/scale-factor-cosmology) at the time the light was originally emitted is given by \".\"\n[WMAP nine-year results](https://bliptext.com/articles/wilkinson-microwave-anisotropy-probe) combined with other measurements give the redshift of photon [decoupling](https://bliptext.com/articles/decoupling-cosmology) as z = 1,091.64, which implies that the scale factor at the time of photon [decoupling](https://bliptext.com/articles/decoupling-cosmology) would be 1/undefined.\nSo if the matter that originally emitted the oldest CMBR [photons](https://bliptext.com/articles/photon) has a present distance of 46 billion light-years, then the distance would have been only about 42 million light-years at the time of decoupling.\nThe [light-travel distance](https://bliptext.com/articles/light-travel-distance) to the edge of the observable universe is the [age of the universe](https://bliptext.com/articles/age-of-the-universe) times the [speed of light](https://bliptext.com/articles/speed-of-light), 13.8 billion light years.\nThis is the distance that a photon emitted shortly after the Big Bang, such as one from the [cosmic microwave background](https://bliptext.com/articles/cosmic-microwave-background), has traveled to reach observers on Earth.\nBecause [spacetime](https://bliptext.com/articles/spacetime) is curved, corresponding to the [expansion of space](https://bliptext.com/articles/expansion-of-the-universe), this distance does not correspond to the true distance at any moment in time.\n# [Hubble Ultra-Deep Field image of a region of the observable universe (equivalent sky area size shown in bottom left corner), near the constellation Fornax. Each spot is a galaxy, consisting of billions of stars. The light from the smallest, most redshifted galaxies originated nearly 13.8 billion years ago. | upload.wikimedia.org/wikipedia/commons/e/e4/HubbleUltraDeepFieldwithScaleComparison.jpg]\n\n# Matter and mass\n\n## Number of galaxies and stars\nThe observable universe contains as many as an estimated 2 trillion galaxies and, overall, as many as an estimated 1024 stars – more stars (and, potentially, [Earth](https://bliptext.com/articles/earth)-like planets) than all the [grains of beach sand](https://bliptext.com/articles/sand) on planet [Earth](https://bliptext.com/articles/earth).\nOther estimates are in the hundreds of billions rather than trillions.\nThe estimated total number of stars in an [inflationary universe](https://bliptext.com/articles/inflation-cosmology) (observed and unobserved) is 10100.\n\n## Matter content—number of atoms\nAssuming the mass of ordinary matter is about 1.45 kg as discussed above, and assuming all atoms are [hydrogen atoms](https://bliptext.com/articles/hydrogen-atom) (which are about 74% of all atoms in the Milky Way by mass), the estimated total number of atoms in the observable universe is obtained by dividing the mass of ordinary matter by the mass of a hydrogen atom.\nThe result is approximately 1080 hydrogen atoms, also known as the [Eddington number](https://bliptext.com/articles/eddington-number).\n\n## Mass of ordinary matter\nThe mass of the observable universe is often quoted as 1053 kg. In this context, mass refers to ordinary (baryonic) matter and includes the [interstellar medium](https://bliptext.com/articles/interstellar-medium) (ISM) and the [intergalactic medium](https://bliptext.com/articles/intergalactic-medium) (IGM).\nHowever, it excludes [dark matter](https://bliptext.com/articles/dark-matter) and [dark energy](https://bliptext.com/articles/dark-energy).\nThis quoted value for the mass of ordinary matter in the universe can be estimated based on critical density.\nThe calculations are for the observable universe only as the volume of the whole is unknown and may be infinite.\n\n## Estimates based on critical density\nCritical density is the energy density for which the universe is flat.\nIf there is no dark energy, it is also the [density](https://bliptext.com/articles/density) for which the expansion of the universe is poised between continued expansion and collapse.\nFrom the [Friedmann equations](https://bliptext.com/articles/friedmann-equations), the value for critical density, is:\nwhere G is the [gravitational constant](https://bliptext.com/articles/gravitational-constant) and H = H0 is the present value of the [Hubble constant](https://bliptext.com/articles/hubble-constant).\nThe value for H0, as given by the European Space Agency's Planck Telescope, is H0 = 67.15 kilometres per second per megaparsec.\nThis gives a critical density of 0.85 kg/m3, or about 5 hydrogen atoms per cubic metre.\nThis density includes four significant types of energy/mass: ordinary matter (4.8%), neutrinos (0.1%), [cold dark matter](https://bliptext.com/articles/cold-dark-matter) (26.8%), and [dark energy](https://bliptext.com/articles/dark-energy) (68.3%).\nAlthough neutrinos are [Standard Model](https://bliptext.com/articles/standard-model) particles, they are listed separately because they are [ultra-relativistic](https://bliptext.com/articles/scale-factor-cosmology) and hence [behave](https://bliptext.com/articles/equation-of-state-cosmology) like radiation rather than like matter.\nThe density of ordinary matter, as measured by Planck, is 4.8% of the total critical density or 4.08 kg/m3.\nTo convert this density to mass we must multiply by volume, a value based on the radius of the \"observable universe\".\nSince the universe has been expanding for 13.8 billion years, the [comoving distance](https://bliptext.com/articles/comoving-and-proper-distances) (radius) is now about 46.6 billion light-years.\nThus, volume (4⁄3πr3) equals 3.58 m3 and the mass of ordinary matter equals density (4.08 kg/m3) times volume (3.58 m3) or 1.46 kg.\n\n# Large-scale structure\n[Sky surveys](https://bliptext.com/articles/redshift-survey) and mappings of the various [wavelength](https://bliptext.com/articles/wavelength) bands of [electromagnetic radiation](https://bliptext.com/articles/electromagnetic-radiation) (in particular [21-cm emission](https://bliptext.com/articles/hydrogen-line)) have yielded much information on the content and character of the [universe](https://bliptext.com/articles/universe)'s structure.\nThe organization of structure appears to follow a [hierarchical](https://bliptext.com/articles/hierarchy) model with organization up to the [scale](https://bliptext.com/articles/scale-spatial) of [superclusters](https://bliptext.com/articles/supercluster) and [filaments](https://bliptext.com/articles/galaxy-filament).\nLarger than this (at scales between 30 and 200 megaparsecs), there seems to be no continued structure, a phenomenon that has been referred to as the End of Greatness.\n# [Galaxy clusters, like RXC J0142.9+4438, are the nodes of the cosmic web that permeates the entire Universe. | upload.wikimedia.org/wikipedia/commons/1/16/Galactic///treasure///chest///RXC///J0142.9%2B4438.jpg]\n\n## Walls, filaments, nodes, and voids\nThe organization of structure arguably begins at the stellar level, though most cosmologists rarely address [astrophysics](https://bliptext.com/articles/astrophysics) on that scale.\n[Stars](https://bliptext.com/articles/star) are organized into [galaxies](https://bliptext.com/articles/galaxy), which in turn form [galaxy groups](https://bliptext.com/articles/galaxy-group), [galaxy clusters](https://bliptext.com/articles/galaxy-cluster), [superclusters](https://bliptext.com/articles/supercluster), sheets, [walls and filaments](https://bliptext.com/articles/galaxy-filament), which are separated by immense [voids](https://bliptext.com/articles/void-astronomy), creating a vast foam-like structure sometimes called the \"cosmic web\".\nPrior to 1989, it was commonly assumed that [virialized](https://bliptext.com/articles/virial-theorem) galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction.\nHowever, since the early 1980s, more and more structures have been discovered.\nIn 1983, Adrian Webster identified the [Webster LQG](https://bliptext.com/articles/webster-lqg), a [large quasar group](https://bliptext.com/articles/large-quasar-group) consisting of 5 quasars.\nThe discovery was the first identification of a large-scale structure, and has expanded the information about the known grouping of matter in the universe.\nIn 1987, [Robert Brent Tully](https://bliptext.com/articles/r-brent-tully) identified the [Pisces–Cetus Supercluster Complex](https://bliptext.com/articles/pisces-cetus-supercluster-complex), the galaxy filament in which the Milky Way resides.\nIt is about 1 billion light-years across.\nThat same year, an unusually large region with a much lower than average distribution of galaxies was discovered, the [Giant Void](https://bliptext.com/articles/giant-void), which measures 1.3 billion light-years across.\nBased on [redshift survey](https://bliptext.com/articles/redshift-survey) data, in 1989 [Margaret Geller](https://bliptext.com/articles/margaret-geller) and [John Huchra](https://bliptext.com/articles/john-huchra) discovered the \"[Great Wall](https://bliptext.com/articles/cfa2-great-wall)\", a sheet of galaxies more than 500 million [light-years](https://bliptext.com/articles/light-year) long and 200 million [light-years](https://bliptext.com/articles/light-year) wide, but only 15 million [light-years](https://bliptext.com/articles/light-year) thick.\nThe existence of this structure escaped notice for so long because it requires locating the position of galaxies in three dimensions, which involves combining location information about the galaxies with distance information from [redshifts](https://bliptext.com/articles/redshift).\nTwo years later, astronomers Roger G. Clowes and Luis E. Campusano discovered the [Clowes–Campusano LQG](https://bliptext.com/articles/clowes-campusano-lqg), a [large quasar group](https://bliptext.com/articles/large-quasar-group) measuring two billion light-years at its widest point, which was the largest known structure in the universe at the time of its announcement.\nIn April 2003, another large-scale structure was discovered, the [Sloan Great Wall](https://bliptext.com/articles/sloan-great-wall).\nIn August 2007, a possible supervoid was detected in the constellation [Eridanus](https://bliptext.com/articles/eridanus-constellation).\nIt coincides with the '[CMB cold spot](https://bliptext.com/articles/cmb-cold-spot)', a cold region in the microwave sky that is highly improbable under the currently favored cosmological model.\nThis supervoid could cause the cold spot, but to do so it would have to be improbably big, possibly a billion light-years across, almost as big as the Giant Void mentioned above.\nAnother large-scale structure is the [SSA22 Protocluster](https://bliptext.com/articles/ssa22-protocluster), a collection of galaxies and enormous gas bubbles that measures about 200 million light-years across.\nIn 2011, a large quasar group was discovered, [U1.11](https://bliptext.com/articles/u1-11), measuring about 2.5 billion light-years across.\nOn January 11, 2013, another large quasar group, the [Huge-LQG](https://bliptext.com/articles/huge-lqg), was discovered, which was measured to be four billion light-years across, the largest known structure in the universe at that time.\nIn November 2013, astronomers discovered the [Hercules–Corona Borealis Great Wall](https://bliptext.com/articles/hercules-corona-borealis-great-wall), an even bigger structure twice as large as the former.\nIt was defined by the mapping of [gamma-ray bursts](https://bliptext.com/articles/gamma-ray-burst).\nIn 2021, the [American Astronomical Society](https://bliptext.com/articles/american-astronomical-society) announced the detection of the [Giant Arc](https://bliptext.com/articles/the-giant-arc); a crescent-shaped string of galaxies that span 3.3 billion light years in length, located 9.2 billion light years from Earth in the constellation [Boötes](https://bliptext.com/articles/bo-tes) from observations captured by the [Sloan Digital Sky Survey](https://bliptext.com/articles/sloan-digital-sky-survey).\n# [Map of the cosmic web generated from a slime mould-inspired algorithm | upload.wikimedia.org/wikipedia/commons/2/26/Map///of///the///Cosmic///Web///Generated///from///Slime///Mould///Algorithm.jpg]\n# [Computer simulated image of an area of space more than 50 million light-years across, presenting a possible large-scale distribution of light sources in the universe—precise relative contributions of galaxies and quasars are unclear. | upload.wikimedia.org/wikipedia/commons/6/6d/Large-scale///structure///of///light///distribution///in///the///universe.jpg]\n\n## End of Greatness\nThe End of Greatness is an observational scale discovered at roughly 100 [Mpc](https://bliptext.com/articles/megaparsec) (roughly 300 million light-years) where the lumpiness seen in the large-scale structure of the [universe](https://bliptext.com/articles/universe) is [homogenized](https://bliptext.com/articles/homogeneous) and [isotropized](https://bliptext.com/articles/isotropic) in accordance with the [cosmological principle](https://bliptext.com/articles/cosmological-principle).\nAt this scale, no pseudo-random [fractalness](https://bliptext.com/articles/fractal) is apparent.\nThe [superclusters](https://bliptext.com/articles/supercluster) and [filaments](https://bliptext.com/articles/galaxy-filament) seen in smaller surveys are [randomized](https://bliptext.com/articles/random) to the extent that the smooth distribution of the universe is visually apparent.\nIt was not until the [redshift surveys](https://bliptext.com/articles/redshift-survey) of the 1990s were completed that this scale could accurately be observed.\n\n## Observations\nAnother indicator of large-scale structure is the '[Lyman-alpha forest](https://bliptext.com/articles/lyman-alpha-forest)'.\nThis is a collection of [absorption lines](https://bliptext.com/articles/spectral-line) that appear in the spectra of light from [quasars](https://bliptext.com/articles/quasar), which are interpreted as indicating the existence of huge thin sheets of intergalactic (mostly [hydrogen](https://bliptext.com/articles/hydrogen)) gas.\nThese sheets appear to collapse into filaments, which can feed galaxies as they grow where filaments either cross or are dense.\nAn early direct evidence for this cosmic web of gas was the 2019 detection, by astronomers from the RIKEN Cluster for Pioneering Research in Japan and Durham University in the U.K., of light from the brightest part of this web, surrounding and illuminated by a cluster of forming galaxies, acting as cosmic flashlights for intercluster medium hydrogen fluorescence via Lyman-alpha emissions.\nIn 2021, an international team, headed by Roland Bacon from the Centre de Recherche Astrophysique de Lyon (France), reported the first observation of diffuse extended Lyman-alpha emission from redshift 3.1 to 4.5 that traced several cosmic web filaments on scales of 2.5−4 [cMpc](https://bliptext.com/articles/cmpc) (comoving mega-parsecs), in filamentary environments outside massive structures typical of web nodes.\nSome caution is required in describing structures on a cosmic scale because they are often different from how they appear.\n[Gravitational lensing](https://bliptext.com/articles/gravitational-lens) can make an image appear to originate in a different direction from its real source, when foreground objects curve surrounding spacetime (as predicted by [general relativity](https://bliptext.com/articles/general-relativity)) and deflect passing light rays.\nRather usefully, strong gravitational lensing can sometimes magnify distant galaxies, making them easier to detect.\n[Weak lensing](https://bliptext.com/articles/weak-gravitational-lensing) by the intervening universe in general also subtly changes the observed large-scale structure.\nThe large-scale structure of the universe also looks different if only redshift is used to measure distances to galaxies.\nFor example, galaxies behind a galaxy cluster are attracted to it and fall towards it, and so are [blueshifted](https://bliptext.com/articles/blueshift) (compared to how they would be if there were no cluster).\nOn the near side, objects are redshifted.\nThus, the environment of the cluster looks somewhat pinched if using redshifts to measure distance.\nThe opposite effect is observed on galaxies already within a cluster: the galaxies have some random motion around the cluster center, and when these random motions are converted to redshifts, the cluster appears elongated.\nThis creates a \"[finger of God](https://bliptext.com/articles/redshift-space-distortions)\"—the illusion of a long chain of galaxies pointed at Earth.\n# [\"Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. The image is derived from the 2MASS Extended Source Catalog (XSC)—more than 1.5 million galaxies, and the Point Source Catalog (PSC)—nearly 0.5 billion Milky Way stars. The galaxies are color-coded by 'redshift' obtained from the UGC, CfA, Tully NBGC, LCRS, 2dF, 6dFGS, and SDSS surveys (and from various observations compiled by the NASA Extragalactic Database), or photo-metrically deduced from the K band (2.2 μm). Blue are the nearest sources (z < 0.01); green are at moderate distances (0.01 < z < 0.04) and red are the most distant sources that 2MASS resolves (0.04 < z < 0.1). The map is projected with an equal area Aitoff in the Galactic system (Milky Way at center).\" | upload.wikimedia.org/wikipedia/commons/7/7d/2MASS///LSS///chart-NEW///Nasa.jpg]\n# [Constellations grouped in galactic quadrants (N/S, 1–4) and their approximate divisions vis-a-vis celestial quadrants (NQ/SQ) | upload.wikimedia.org/wikipedia/commons/7/74/Galactic%2Bcelestial///quads.jpg]\n\n## Cosmography of Earth's cosmic neighborhood\nAt the centre of the [Hydra–Centaurus Supercluster](https://bliptext.com/articles/hydra-centaurus-supercluster), a gravitational anomaly called the [Great Attractor](https://bliptext.com/articles/great-attractor) affects the motion of galaxies over a region hundreds of millions of light-years across.\nThese galaxies are all [redshifted](https://bliptext.com/articles/redshift), in accordance with [Hubble's law](https://bliptext.com/articles/hubble-s-law).\nThis indicates that they are receding from us and from each other, but the variations in their redshift are sufficient to reveal the existence of a concentration of mass equivalent to tens of thousands of galaxies.\nThe Great Attractor, discovered in 1986, lies at a distance of between 150 million and 250 million light-years in the direction of the [Hydra](https://bliptext.com/articles/hydra-constellation) and [Centaurus](https://bliptext.com/articles/centaurus) [constellations](https://bliptext.com/articles/constellation).\nIn its vicinity there is a preponderance of large old galaxies, many of which are colliding with their neighbours, or radiating large amounts of radio waves.\nIn 1987, [astronomer](https://bliptext.com/articles/astronomer) [R. Brent Tully](https://bliptext.com/articles/r-brent-tully) of the [University of Hawaii](https://bliptext.com/articles/university-of-hawaii)'s Institute of Astronomy identified what he called the [Pisces–Cetus Supercluster Complex](https://bliptext.com/articles/pisces-cetus-supercluster-complex), a structure one billion [light-years](https://bliptext.com/articles/light-year) long and 150 million [light-years](https://bliptext.com/articles/light-year) across in which, he claimed, the Local Supercluster is embedded.\n\n# Most distant objects\nThe most distant [astronomical object](https://bliptext.com/articles/astronomical-object) identified (as of August of 2024) is a galaxy classified as [JADES-GS-z14-0](https://bliptext.com/articles/jades-gs-z14-0).\nIn 2009, a [gamma ray burst](https://bliptext.com/articles/gamma-ray-burst), [GRB 090423](https://bliptext.com/articles/grb-090423), was found to have a [redshift](https://bliptext.com/articles/redshift) of 8.2, which indicates that the collapsing star that caused it exploded when the universe was only 630 million years old.\nThe burst happened approximately 13 billion years ago, so a distance of about 13 billion light-years was widely quoted in the media, or sometimes a more precise figure of 13.035 billion light-years.\nThis would be the \"light travel distance\" (see [Distance measures (cosmology)](https://bliptext.com/articles/distance-measures-cosmology)) rather than the \"[proper distance](https://bliptext.com/articles/comoving-and-proper-distances)\" used in both [Hubble's law](https://bliptext.com/articles/hubble-s-law) and in defining the size of the observable universe.\nCosmologist [Ned Wright](https://bliptext.com/articles/edward-l-wright) argues against using this measure.\nThe proper distance for a redshift of 8.2 would be about 9.2 [Gpc](https://bliptext.com/articles/megaparsecs), or about 30 billion light-years.\n\n# Horizons\nThe limit of observability in the universe is set by cosmological horizons which limit—based on various physical constraints—the extent to which information can be obtained about various events in the universe.\nThe most famous horizon is the [particle horizon](https://bliptext.com/articles/particle-horizon) which sets a limit on the precise distance that can be seen due to the finite [age of the universe](https://bliptext.com/articles/age-of-the-universe).\nAdditional horizons are associated with the possible future extent of observations, larger than the particle horizon owing to the [expansion of space](https://bliptext.com/articles/expansion-of-space), an \"optical horizon\" at the [surface of last scattering](https://bliptext.com/articles/cosmic-microwave-background), and associated horizons with the [surface of last scattering](https://bliptext.com/articles/cosmic-microwave-background) for [neutrinos](https://bliptext.com/articles/cosmic-neutrino-background) and [gravitational waves](https://bliptext.com/articles/gravitational-wave-background).\n\n# Gallery","wordIndex":12,"createdAt":"2025-10-16T20:42:02.241Z","user":{"id":"MIwiIJQRGA2d9vGNZNngxPQCC0mSGcOP","name":"Natalie Fenselon","image":"https://lh3.googleusercontent.com/a/ACg8ocJcGgaEzeWc1eDUIFlGnWyUzAfCYiRCcBcHaXxOIgKzYZ8nxz_t=s96-c"}}],"article":{"id":"87498bb9-b6e0-4773-9386-38138487f3a3","title":"Observable universe","slug":"observable-universe","content":":::summary\n![Visualization of the observable universe. The scale is such that the fine grains represent collections of large numbers of superclusters. The Virgo Supercluster—home of Milky Way—is marked at the center, but is too small to be seen.](https://upload.wikimedia.org/wikipedia/commons/9/98/Observable///Universe///with///Measurements///01.png)\n**Bodystyle:** width:25em; \n**Title:** Observable universe \n**Label 1:** Diameter \n**Data 1:** 8.8 m or 880 [Ym](https://bliptext.com/articles/yotta) (28.5 Gpc or 93 Gly) \n**Label 2:** Circumference \n**Data 2:** 2.764 m or 2.764 [Rm](https://bliptext.com/articles/ronna) (89.6 Gpc or 292.2 Gly) \n**Label 3:** Volume \n**Data 3:** 3.566 m3 \n**Label 4:** Mass (ordinary matter) \n**Data 4:** 1.5 kg \n**Label 5:** Density (of total energy) \n**Data 5:** 9.9 kg/m3 (equivalent to 6 [protons](https://bliptext.com/articles/proton) per cubic meter of space) \n**Label 6:** Age \n**Data 6:** 13.787 billion years \n**Label 7:** Average temperature \n**Data 7:** 2.725 [K](https://bliptext.com/articles/kelvin) \n**Label 8:** Contents \n**Data 8:** [Ordinary (baryonic)](https://bliptext.com/articles/baryon) [matter](https://bliptext.com/articles/matter) (4.9%) · Dark [matter](https://bliptext.com/articles/matter) (26.8%) · [Dark energy](https://bliptext.com/articles/dark-energy) (68.3%) \n:::\n\n# \nThe **observable [universe](https://bliptext.com/articles/universe)** is a [spherical](https://bliptext.com/articles/ball-mathematics) region of the [universe](https://bliptext.com/articles/universe) consisting of no [matter](https://bliptext.com/articles/matter) that can be [observed](https://bliptext.com/articles/observation) from [Earth](https://bliptext.com/articles/earth) or its space-based telescopes and exploratory probes at the present time; the [electromagnetic radiation](https://bliptext.com/articles/electromagnetic-radiation) from these [objects](https://bliptext.com/articles/astronomical-object) has had time to reach the [Solar System](https://bliptext.com/articles/solar-system) and [Earth](https://bliptext.com/articles/earth) since the beginning of the [cosmological expansion](https://bliptext.com/articles/metric-expansion-of-space).\nAssuming the universe is [isotropic](https://bliptext.com/articles/isotropy), the distance to the edge of the observable universe is roughly [the same](https://bliptext.com/articles/equidistant) in every direction.\nThat is, the observable universe is a [spherical](https://bliptext.com/articles/sphere) region centered on the observer.\nEvery location in the universe has its own observable universe, which may or may not overlap with the one centered on Earth.\nThe word observable in this sense does not refer to the capability of modern technology to detect [light](https://bliptext.com/articles/light) or other information from an object, or whether there is anything to be detected.\nIt refers to the physical limit created by the [speed of light](https://bliptext.com/articles/speed-of-light) itself.\nNo signal can travel faster than light, hence there is a maximum distance, called the [particle horizon](https://bliptext.com/articles/particle-horizon), beyond which nothing can be detected, as the signals could not have reached us yet.\nSometimes astrophysicists distinguish between the observable universe and the visible universe.\nThe former includes signals since the end of the [inflationary epoch](https://bliptext.com/articles/inflationary-epoch), while the latter includes only signals emitted since [recombination](https://bliptext.com/articles/recombination-cosmology).\nAccording to calculations, the current [comoving distance](https://bliptext.com/articles/comoving-distance) to particles from which the [cosmic microwave background radiation](https://bliptext.com/articles/cosmic-microwave-background-radiation) (CMBR) was emitted, which represents the radius of the visible universe, is about 14.0 billion [parsecs](https://bliptext.com/articles/parsec) (about 45.7 billion light-years).\nThe comoving distance to the edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light-years), about 2% larger.\nThe [radius](https://bliptext.com/articles/radius) of the observable universe is therefore estimated to be about 46.5 billion light-years.\nUsing the [critical density](https://bliptext.com/articles/friedmann-equations) and the diameter of the observable universe, the total mass of ordinary matter in the universe can be calculated to be about 1.5 kg. In November 2018, astronomers reported that [extragalactic background light](https://bliptext.com/articles/extragalactic-background-light) (EBL) amounted to 4 photons.\nAs the universe's expansion is accelerating, all currently observable objects, outside the local [supercluster](https://bliptext.com/articles/supercluster), will eventually appear to freeze in time, while emitting progressively redder and fainter light.\nFor instance, objects with the current [redshift](https://bliptext.com/articles/redshift) [z](https://bliptext.com/articles/redshift) from 5 to 10 will only be observable up to an age of 4–6 billion years.\nIn addition, light emitted by objects currently situated beyond a certain comoving distance (currently about 19 Gpc) will never reach Earth.\n\n# Overview\nThe universe's size is unknown, and it may be infinite in extent.\nSome parts of the universe are too far away for the light emitted since the [Big Bang](https://bliptext.com/articles/big-bang) to have had enough time to reach Earth or space-based instruments, and therefore lie outside the observable universe.\nIn the future, light from distant galaxies will have had more time to travel, so one might expect that additional regions will become observable.\nRegions distant from observers (such as us) are expanding away faster than the speed of light, at rates estimated by [Hubble's law](https://bliptext.com/articles/hubble-s-law).\nThe [expansion rate appears to be accelerating](https://bliptext.com/articles/accelerating-expansion-of-the-universe), which [dark energy](https://bliptext.com/articles/dark-energy) was proposed to explain.\nAssuming dark energy remains constant (an unchanging [cosmological constant](https://bliptext.com/articles/cosmological-constant)) so that the expansion rate of the universe continues to accelerate, there is a \"future visibility limit\" beyond which objects will never enter the observable universe at any time in the future because light emitted by objects outside that limit could never reach the Earth.\nNote that, because the [Hubble parameter](https://bliptext.com/articles/hubble-s-law) is decreasing with time, there can be cases where a galaxy that is receding from Earth only slightly faster than light emits a signal that eventually reaches Earth.\nThis future visibility limit is calculated at a [comoving distance](https://bliptext.com/articles/comoving-distance) of 19 billion parsecs (62 billion light-years), assuming the universe will keep expanding forever, which implies the number of galaxies that can ever be theoretically observed in the infinite future is only larger than the number currently observable by a factor of 2.36 (ignoring redshift effects).\nIn principle, more galaxies will become observable in the future; in practice, an increasing number of galaxies will become extremely [redshifted](https://bliptext.com/articles/redshift) due to ongoing expansion, so much so that they will seem to disappear from view and become invisible.\nA galaxy at a given comoving distance is defined to lie within the \"observable universe\" if we can receive signals emitted by the galaxy at any age in its history, say, a signal sent from the galaxy only 500 million years after the Big Bang.\nBecause of the universe's expansion, there may be some later age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future, so, for example, we might never see what the galaxy looked like 10 billion years after the Big Bang, even though it remains at the same comoving distance less than that of the observable universe.\nThis can be used to define a type of cosmic [event horizon](https://bliptext.com/articles/event-horizon) whose distance from the Earth changes over time.\nFor example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening at present can eventually reach the Earth if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is further away.\nThe space before this cosmic event horizon can be called \"reachable universe\", that is all galaxies closer than that could be reached if we left for them today, at the speed of light; all galaxies beyond that are unreachable.\nSimple observation will show the future visibility limit (62 billion light-years) is exactly equal to the reachable limit (16 billion light-years) added to the current visibility limit (46 billion light-years).\n# [Observable Universe as a function of time and distance, in context of the expanding Universe | upload.wikimedia.org/wikipedia/commons/f/f2/Home///in///Relation///to///Everything-Observable///Universe.png]\n# [The reachable Universe as a function of time and distance, in context of the expanding Universe. | upload.wikimedia.org/wikipedia/commons/8/88/Home///in///Relation///to///Everything-Reachable///Universe.png]\n\n# \"The universe\" versus \"the observable universe\"\nBoth popular and professional research articles in cosmology often use the term \"universe\" to mean \"observable universe\".\nThis can be justified on the grounds that we can never know anything by direct observation about any part of the universe that is [causally disconnected](https://bliptext.com/articles/causality-physics) from the Earth, although many credible theories require a total universe much larger than the observable universe.\nNo evidence exists to suggest that the boundary of the observable universe constitutes a boundary on the universe as a whole, nor do any of the mainstream cosmological models propose that the universe has any physical boundary in the first place.\nHowever, some models propose it could be finite but unbounded, like a higher-dimensional analogue of the 2D surface of a sphere that is finite in area but has no edge.\nIt is plausible that the [galaxies](https://bliptext.com/articles/galaxy) within the observable universe represent only a minuscule fraction of the [galaxies](https://bliptext.com/articles/galaxy) in the universe.\nAccording to the theory of [cosmic inflation](https://bliptext.com/articles/cosmic-inflation) initially introduced by [Alan Guth](https://bliptext.com/articles/alan-guth) and [D. Kazanas](https://bliptext.com/articles/d-kazanas), if it is assumed that inflation began about 10−37 seconds after the Big Bang and that the pre-inflation size of the universe was approximately equal to the speed of light times its age, that would suggest that at present the entire universe's size is at least 1.5 light-years—at least 3 times the radius of the observable universe.\nIf the universe is finite but unbounded, it is also possible that the universe is smaller than the observable universe.\nIn this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated the universe.\nIt is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different.\nBielewicz et al. claim to establish a lower bound of 27.9 gigaparsecs (91 billion light-years) on the diameter of the last scattering surface.\nThis value is based on matching-circle analysis of the [WMAP](https://bliptext.com/articles/wmap) 7-year data.\nThis approach has been disputed.\n\n# Size\nThe [comoving distance](https://bliptext.com/articles/comoving-and-proper-distances) from Earth to the edge of the observable universe is about 14.26 giga[parsecs](https://bliptext.com/articles/parsec) (46.5 [billion](https://bliptext.com/articles/1000000000-number) [light-years](https://bliptext.com/articles/light-year) or 14.26 Gpc) in any direction.\nThe observable universe is thus a sphere with a [diameter](https://bliptext.com/articles/diameter) of about 28.5 gigaparsecs (93 billion light-years or 28.5 Gpc).\nAssuming that space is roughly [flat](https://bliptext.com/articles/shape-of-the-universe) (in the sense of being a [Euclidean space](https://bliptext.com/articles/euclidean-space)), this size corresponds to a comoving volume of about 1.22 Gpc3 (4.22 Gly3 or 3.57 m3).\nThese are distances now (in [cosmological time](https://bliptext.com/articles/cosmological-time)), not distances at the time the light was emitted.\nFor example, the cosmic microwave background radiation that we see right now was emitted at the [time of photon decoupling](https://bliptext.com/articles/recombination-cosmology), estimated to have occurred about 380,000 years after the Big Bang, which occurred around 13.8 billion years ago.\nThis radiation was emitted by matter that has, in the intervening time, mostly condensed into galaxies, and those galaxies are now calculated to be about 46 billion light-years from Earth.\nTo estimate the distance to that matter at the time the light was emitted, we may first note that according to the [Friedmann–Lemaître–Robertson–Walker metric](https://bliptext.com/articles/friedmann-lema-tre-robertson-walker-metric), which is used to model the expanding universe, if we receive light with a [redshift](https://bliptext.com/articles/redshift) of z, then the [scale factor](https://bliptext.com/articles/scale-factor-cosmology) at the time the light was originally emitted is given by \".\"\n[WMAP nine-year results](https://bliptext.com/articles/wilkinson-microwave-anisotropy-probe) combined with other measurements give the redshift of photon [decoupling](https://bliptext.com/articles/decoupling-cosmology) as z = 1,091.64, which implies that the scale factor at the time of photon [decoupling](https://bliptext.com/articles/decoupling-cosmology) would be 1/undefined.\nSo if the matter that originally emitted the oldest CMBR [photons](https://bliptext.com/articles/photon) has a present distance of 46 billion light-years, then the distance would have been only about 42 million light-years at the time of decoupling.\nThe [light-travel distance](https://bliptext.com/articles/light-travel-distance) to the edge of the observable universe is the [age of the universe](https://bliptext.com/articles/age-of-the-universe) times the [speed of light](https://bliptext.com/articles/speed-of-light), 13.8 billion light years.\nThis is the distance that a photon emitted shortly after the Big Bang, such as one from the [cosmic microwave background](https://bliptext.com/articles/cosmic-microwave-background), has traveled to reach observers on Earth.\nBecause [spacetime](https://bliptext.com/articles/spacetime) is curved, corresponding to the [expansion of space](https://bliptext.com/articles/expansion-of-the-universe), this distance does not correspond to the true distance at any moment in time.\n# [Hubble Ultra-Deep Field image of a region of the observable universe (equivalent sky area size shown in bottom left corner), near the constellation Fornax. Each spot is a galaxy, consisting of billions of stars. The light from the smallest, most redshifted galaxies originated nearly 13.8 billion years ago. | upload.wikimedia.org/wikipedia/commons/e/e4/HubbleUltraDeepFieldwithScaleComparison.jpg]\n\n# Matter and mass\n\n## Number of galaxies and stars\nThe observable universe contains as many as an estimated 2 trillion galaxies and, overall, as many as an estimated 1024 stars – more stars (and, potentially, [Earth](https://bliptext.com/articles/earth)-like planets) than all the [grains of beach sand](https://bliptext.com/articles/sand) on planet [Earth](https://bliptext.com/articles/earth).\nOther estimates are in the hundreds of billions rather than trillions.\nThe estimated total number of stars in an [inflationary universe](https://bliptext.com/articles/inflation-cosmology) (observed and unobserved) is 10100.\n\n## Matter content—number of atoms\nAssuming the mass of ordinary matter is about 1.45 kg as discussed above, and assuming all atoms are [hydrogen atoms](https://bliptext.com/articles/hydrogen-atom) (which are about 74% of all atoms in the Milky Way by mass), the estimated total number of atoms in the observable universe is obtained by dividing the mass of ordinary matter by the mass of a hydrogen atom.\nThe result is approximately 1080 hydrogen atoms, also known as the [Eddington number](https://bliptext.com/articles/eddington-number).\n\n## Mass of ordinary matter\nThe mass of the observable universe is often quoted as 1053 kg. In this context, mass refers to ordinary (baryonic) matter and includes the [interstellar medium](https://bliptext.com/articles/interstellar-medium) (ISM) and the [intergalactic medium](https://bliptext.com/articles/intergalactic-medium) (IGM).\nHowever, it excludes [dark matter](https://bliptext.com/articles/dark-matter) and [dark energy](https://bliptext.com/articles/dark-energy).\nThis quoted value for the mass of ordinary matter in the universe can be estimated based on critical density.\nThe calculations are for the observable universe only as the volume of the whole is unknown and may be infinite.\n\n## Estimates based on critical density\nCritical density is the energy density for which the universe is flat.\nIf there is no dark energy, it is also the [density](https://bliptext.com/articles/density) for which the expansion of the universe is poised between continued expansion and collapse.\nFrom the [Friedmann equations](https://bliptext.com/articles/friedmann-equations), the value for critical density, is:\nwhere G is the [gravitational constant](https://bliptext.com/articles/gravitational-constant) and H = H0 is the present value of the [Hubble constant](https://bliptext.com/articles/hubble-constant).\nThe value for H0, as given by the European Space Agency's Planck Telescope, is H0 = 67.15 kilometres per second per megaparsec.\nThis gives a critical density of 0.85 kg/m3, or about 5 hydrogen atoms per cubic metre.\nThis density includes four significant types of energy/mass: ordinary matter (4.8%), neutrinos (0.1%), [cold dark matter](https://bliptext.com/articles/cold-dark-matter) (26.8%), and [dark energy](https://bliptext.com/articles/dark-energy) (68.3%).\nAlthough neutrinos are [Standard Model](https://bliptext.com/articles/standard-model) particles, they are listed separately because they are [ultra-relativistic](https://bliptext.com/articles/scale-factor-cosmology) and hence [behave](https://bliptext.com/articles/equation-of-state-cosmology) like radiation rather than like matter.\nThe density of ordinary matter, as measured by Planck, is 4.8% of the total critical density or 4.08 kg/m3.\nTo convert this density to mass we must multiply by volume, a value based on the radius of the \"observable universe\".\nSince the universe has been expanding for 13.8 billion years, the [comoving distance](https://bliptext.com/articles/comoving-and-proper-distances) (radius) is now about 46.6 billion light-years.\nThus, volume (4⁄3πr3) equals 3.58 m3 and the mass of ordinary matter equals density (4.08 kg/m3) times volume (3.58 m3) or 1.46 kg.\n\n# Large-scale structure\n[Sky surveys](https://bliptext.com/articles/redshift-survey) and mappings of the various [wavelength](https://bliptext.com/articles/wavelength) bands of [electromagnetic radiation](https://bliptext.com/articles/electromagnetic-radiation) (in particular [21-cm emission](https://bliptext.com/articles/hydrogen-line)) have yielded much information on the content and character of the [universe](https://bliptext.com/articles/universe)'s structure.\nThe organization of structure appears to follow a [hierarchical](https://bliptext.com/articles/hierarchy) model with organization up to the [scale](https://bliptext.com/articles/scale-spatial) of [superclusters](https://bliptext.com/articles/supercluster) and [filaments](https://bliptext.com/articles/galaxy-filament).\nLarger than this (at scales between 30 and 200 megaparsecs), there seems to be no continued structure, a phenomenon that has been referred to as the End of Greatness.\n# [Galaxy clusters, like RXC J0142.9+4438, are the nodes of the cosmic web that permeates the entire Universe. | upload.wikimedia.org/wikipedia/commons/1/16/Galactic///treasure///chest///RXC///J0142.9%2B4438.jpg]\n\n## Walls, filaments, nodes, and voids\nThe organization of structure arguably begins at the stellar level, though most cosmologists rarely address [astrophysics](https://bliptext.com/articles/astrophysics) on that scale.\n[Stars](https://bliptext.com/articles/star) are organized into [galaxies](https://bliptext.com/articles/galaxy), which in turn form [galaxy groups](https://bliptext.com/articles/galaxy-group), [galaxy clusters](https://bliptext.com/articles/galaxy-cluster), [superclusters](https://bliptext.com/articles/supercluster), sheets, [walls and filaments](https://bliptext.com/articles/galaxy-filament), which are separated by immense [voids](https://bliptext.com/articles/void-astronomy), creating a vast foam-like structure sometimes called the \"cosmic web\".\nPrior to 1989, it was commonly assumed that [virialized](https://bliptext.com/articles/virial-theorem) galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction.\nHowever, since the early 1980s, more and more structures have been discovered.\nIn 1983, Adrian Webster identified the [Webster LQG](https://bliptext.com/articles/webster-lqg), a [large quasar group](https://bliptext.com/articles/large-quasar-group) consisting of 5 quasars.\nThe discovery was the first identification of a large-scale structure, and has expanded the information about the known grouping of matter in the universe.\nIn 1987, [Robert Brent Tully](https://bliptext.com/articles/r-brent-tully) identified the [Pisces–Cetus Supercluster Complex](https://bliptext.com/articles/pisces-cetus-supercluster-complex), the galaxy filament in which the Milky Way resides.\nIt is about 1 billion light-years across.\nThat same year, an unusually large region with a much lower than average distribution of galaxies was discovered, the [Giant Void](https://bliptext.com/articles/giant-void), which measures 1.3 billion light-years across.\nBased on [redshift survey](https://bliptext.com/articles/redshift-survey) data, in 1989 [Margaret Geller](https://bliptext.com/articles/margaret-geller) and [John Huchra](https://bliptext.com/articles/john-huchra) discovered the \"[Great Wall](https://bliptext.com/articles/cfa2-great-wall)\", a sheet of galaxies more than 500 million [light-years](https://bliptext.com/articles/light-year) long and 200 million [light-years](https://bliptext.com/articles/light-year) wide, but only 15 million [light-years](https://bliptext.com/articles/light-year) thick.\nThe existence of this structure escaped notice for so long because it requires locating the position of galaxies in three dimensions, which involves combining location information about the galaxies with distance information from [redshifts](https://bliptext.com/articles/redshift).\nTwo years later, astronomers Roger G. Clowes and Luis E. Campusano discovered the [Clowes–Campusano LQG](https://bliptext.com/articles/clowes-campusano-lqg), a [large quasar group](https://bliptext.com/articles/large-quasar-group) measuring two billion light-years at its widest point, which was the largest known structure in the universe at the time of its announcement.\nIn April 2003, another large-scale structure was discovered, the [Sloan Great Wall](https://bliptext.com/articles/sloan-great-wall).\nIn August 2007, a possible supervoid was detected in the constellation [Eridanus](https://bliptext.com/articles/eridanus-constellation).\nIt coincides with the '[CMB cold spot](https://bliptext.com/articles/cmb-cold-spot)', a cold region in the microwave sky that is highly improbable under the currently favored cosmological model.\nThis supervoid could cause the cold spot, but to do so it would have to be improbably big, possibly a billion light-years across, almost as big as the Giant Void mentioned above.\nAnother large-scale structure is the [SSA22 Protocluster](https://bliptext.com/articles/ssa22-protocluster), a collection of galaxies and enormous gas bubbles that measures about 200 million light-years across.\nIn 2011, a large quasar group was discovered, [U1.11](https://bliptext.com/articles/u1-11), measuring about 2.5 billion light-years across.\nOn January 11, 2013, another large quasar group, the [Huge-LQG](https://bliptext.com/articles/huge-lqg), was discovered, which was measured to be four billion light-years across, the largest known structure in the universe at that time.\nIn November 2013, astronomers discovered the [Hercules–Corona Borealis Great Wall](https://bliptext.com/articles/hercules-corona-borealis-great-wall), an even bigger structure twice as large as the former.\nIt was defined by the mapping of [gamma-ray bursts](https://bliptext.com/articles/gamma-ray-burst).\nIn 2021, the [American Astronomical Society](https://bliptext.com/articles/american-astronomical-society) announced the detection of the [Giant Arc](https://bliptext.com/articles/the-giant-arc); a crescent-shaped string of galaxies that span 3.3 billion light years in length, located 9.2 billion light years from Earth in the constellation [Boötes](https://bliptext.com/articles/bo-tes) from observations captured by the [Sloan Digital Sky Survey](https://bliptext.com/articles/sloan-digital-sky-survey).\n# [Map of the cosmic web generated from a slime mould-inspired algorithm | upload.wikimedia.org/wikipedia/commons/2/26/Map///of///the///Cosmic///Web///Generated///from///Slime///Mould///Algorithm.jpg]\n# [Computer simulated image of an area of space more than 50 million light-years across, presenting a possible large-scale distribution of light sources in the universe—precise relative contributions of galaxies and quasars are unclear. | upload.wikimedia.org/wikipedia/commons/6/6d/Large-scale///structure///of///light///distribution///in///the///universe.jpg]\n\n## End of Greatness\nThe End of Greatness is an observational scale discovered at roughly 100 [Mpc](https://bliptext.com/articles/megaparsec) (roughly 300 million light-years) where the lumpiness seen in the large-scale structure of the [universe](https://bliptext.com/articles/universe) is [homogenized](https://bliptext.com/articles/homogeneous) and [isotropized](https://bliptext.com/articles/isotropic) in accordance with the [cosmological principle](https://bliptext.com/articles/cosmological-principle).\nAt this scale, no pseudo-random [fractalness](https://bliptext.com/articles/fractal) is apparent.\nThe [superclusters](https://bliptext.com/articles/supercluster) and [filaments](https://bliptext.com/articles/galaxy-filament) seen in smaller surveys are [randomized](https://bliptext.com/articles/random) to the extent that the smooth distribution of the universe is visually apparent.\nIt was not until the [redshift surveys](https://bliptext.com/articles/redshift-survey) of the 1990s were completed that this scale could accurately be observed.\n\n## Observations\nAnother indicator of large-scale structure is the '[Lyman-alpha forest](https://bliptext.com/articles/lyman-alpha-forest)'.\nThis is a collection of [absorption lines](https://bliptext.com/articles/spectral-line) that appear in the spectra of light from [quasars](https://bliptext.com/articles/quasar), which are interpreted as indicating the existence of huge thin sheets of intergalactic (mostly [hydrogen](https://bliptext.com/articles/hydrogen)) gas.\nThese sheets appear to collapse into filaments, which can feed galaxies as they grow where filaments either cross or are dense.\nAn early direct evidence for this cosmic web of gas was the 2019 detection, by astronomers from the RIKEN Cluster for Pioneering Research in Japan and Durham University in the U.K., of light from the brightest part of this web, surrounding and illuminated by a cluster of forming galaxies, acting as cosmic flashlights for intercluster medium hydrogen fluorescence via Lyman-alpha emissions.\nIn 2021, an international team, headed by Roland Bacon from the Centre de Recherche Astrophysique de Lyon (France), reported the first observation of diffuse extended Lyman-alpha emission from redshift 3.1 to 4.5 that traced several cosmic web filaments on scales of 2.5−4 [cMpc](https://bliptext.com/articles/cmpc) (comoving mega-parsecs), in filamentary environments outside massive structures typical of web nodes.\nSome caution is required in describing structures on a cosmic scale because they are often different from how they appear.\n[Gravitational lensing](https://bliptext.com/articles/gravitational-lens) can make an image appear to originate in a different direction from its real source, when foreground objects curve surrounding spacetime (as predicted by [general relativity](https://bliptext.com/articles/general-relativity)) and deflect passing light rays.\nRather usefully, strong gravitational lensing can sometimes magnify distant galaxies, making them easier to detect.\n[Weak lensing](https://bliptext.com/articles/weak-gravitational-lensing) by the intervening universe in general also subtly changes the observed large-scale structure.\nThe large-scale structure of the universe also looks different if only redshift is used to measure distances to galaxies.\nFor example, galaxies behind a galaxy cluster are attracted to it and fall towards it, and so are [blueshifted](https://bliptext.com/articles/blueshift) (compared to how they would be if there were no cluster).\nOn the near side, objects are redshifted.\nThus, the environment of the cluster looks somewhat pinched if using redshifts to measure distance.\nThe opposite effect is observed on galaxies already within a cluster: the galaxies have some random motion around the cluster center, and when these random motions are converted to redshifts, the cluster appears elongated.\nThis creates a \"[finger of God](https://bliptext.com/articles/redshift-space-distortions)\"—the illusion of a long chain of galaxies pointed at Earth.\n# [\"Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. The image is derived from the 2MASS Extended Source Catalog (XSC)—more than 1.5 million galaxies, and the Point Source Catalog (PSC)—nearly 0.5 billion Milky Way stars. The galaxies are color-coded by 'redshift' obtained from the UGC, CfA, Tully NBGC, LCRS, 2dF, 6dFGS, and SDSS surveys (and from various observations compiled by the NASA Extragalactic Database), or photo-metrically deduced from the K band (2.2 μm). Blue are the nearest sources (z < 0.01); green are at moderate distances (0.01 < z < 0.04) and red are the most distant sources that 2MASS resolves (0.04 < z < 0.1). The map is projected with an equal area Aitoff in the Galactic system (Milky Way at center).\" | upload.wikimedia.org/wikipedia/commons/7/7d/2MASS///LSS///chart-NEW///Nasa.jpg]\n# [Constellations grouped in galactic quadrants (N/S, 1–4) and their approximate divisions vis-a-vis celestial quadrants (NQ/SQ) | upload.wikimedia.org/wikipedia/commons/7/74/Galactic%2Bcelestial///quads.jpg]\n\n## Cosmography of Earth's cosmic neighborhood\nAt the centre of the [Hydra–Centaurus Supercluster](https://bliptext.com/articles/hydra-centaurus-supercluster), a gravitational anomaly called the [Great Attractor](https://bliptext.com/articles/great-attractor) affects the motion of galaxies over a region hundreds of millions of light-years across.\nThese galaxies are all [redshifted](https://bliptext.com/articles/redshift), in accordance with [Hubble's law](https://bliptext.com/articles/hubble-s-law).\nThis indicates that they are receding from us and from each other, but the variations in their redshift are sufficient to reveal the existence of a concentration of mass equivalent to tens of thousands of galaxies.\nThe Great Attractor, discovered in 1986, lies at a distance of between 150 million and 250 million light-years in the direction of the [Hydra](https://bliptext.com/articles/hydra-constellation) and [Centaurus](https://bliptext.com/articles/centaurus) [constellations](https://bliptext.com/articles/constellation).\nIn its vicinity there is a preponderance of large old galaxies, many of which are colliding with their neighbours, or radiating large amounts of radio waves.\nIn 1987, [astronomer](https://bliptext.com/articles/astronomer) [R. Brent Tully](https://bliptext.com/articles/r-brent-tully) of the [University of Hawaii](https://bliptext.com/articles/university-of-hawaii)'s Institute of Astronomy identified what he called the [Pisces–Cetus Supercluster Complex](https://bliptext.com/articles/pisces-cetus-supercluster-complex), a structure one billion [light-years](https://bliptext.com/articles/light-year) long and 150 million [light-years](https://bliptext.com/articles/light-year) across in which, he claimed, the Local Supercluster is embedded.\n\n# Most distant objects\nThe most distant [astronomical object](https://bliptext.com/articles/astronomical-object) identified (as of August of 2024) is a galaxy classified as [JADES-GS-z14-0](https://bliptext.com/articles/jades-gs-z14-0).\nIn 2009, a [gamma ray burst](https://bliptext.com/articles/gamma-ray-burst), [GRB 090423](https://bliptext.com/articles/grb-090423), was found to have a [redshift](https://bliptext.com/articles/redshift) of 8.2, which indicates that the collapsing star that caused it exploded when the universe was only 630 million years old.\nThe burst happened approximately 13 billion years ago, so a distance of about 13 billion light-years was widely quoted in the media, or sometimes a more precise figure of 13.035 billion light-years.\nThis would be the \"light travel distance\" (see [Distance measures (cosmology)](https://bliptext.com/articles/distance-measures-cosmology)) rather than the \"[proper distance](https://bliptext.com/articles/comoving-and-proper-distances)\" used in both [Hubble's law](https://bliptext.com/articles/hubble-s-law) and in defining the size of the observable universe.\nCosmologist [Ned Wright](https://bliptext.com/articles/edward-l-wright) argues against using this measure.\nThe proper distance for a redshift of 8.2 would be about 9.2 [Gpc](https://bliptext.com/articles/megaparsecs), or about 30 billion light-years.\n\n# Horizons\nThe limit of observability in the universe is set by cosmological horizons which limit—based on various physical constraints—the extent to which information can be obtained about various events in the universe.\nThe most famous horizon is the [particle horizon](https://bliptext.com/articles/particle-horizon) which sets a limit on the precise distance that can be seen due to the finite [age of the universe](https://bliptext.com/articles/age-of-the-universe).\nAdditional horizons are associated with the possible future extent of observations, larger than the particle horizon owing to the [expansion of space](https://bliptext.com/articles/expansion-of-space), an \"optical horizon\" at the [surface of last scattering](https://bliptext.com/articles/cosmic-microwave-background), and associated horizons with the [surface of last scattering](https://bliptext.com/articles/cosmic-microwave-background) for [neutrinos](https://bliptext.com/articles/cosmic-neutrino-background) and [gravitational waves](https://bliptext.com/articles/gravitational-wave-background).\n\n# Gallery","current_revision":"190f2b2b-f581-40d7-a48c-c66b46dfa524","created_by":"kACnuQmH5k9Nr5VNVEpVeg7XpvUmMctD","created_at":"2025-01-25T18:54:56.615Z","updated_at":"2025-10-16T20:42:02.248Z","search_vector":"'observ':1 'univers':2","search_content":" \n \n**Bodystyle:** width:25em; \n**Title:** Observable universe \n**Label 1:** Diameter \n**Data 1:** 8.8 m or 880 [Ym](https://bliptext.com/articles/yotta) (28.5 Gpc or 93 Gly) \n**Label 2:** Circumference \n**Data 2:** 2.764 m or 2.764 [Rm](https://bliptext.com/articles/ronna) (89.6 Gpc or 292.2 Gly) \n**Label 3:** Volume \n**Data 3:** 3.566 m3 \n**Label 4:** Mass (ordinary matter) \n**Data 4:** 1.5 kg \n**Label 5:** Density (of total energy) \n**Data 5:** 9.9 kg/m3 (equivalent to 6 [protons](https://bliptext.com/articles/proton) per cubic meter of space) \n**Label 6:** Age \n**Data 6:** 13.787 billion years \n**Label 7:** Average temperature \n**Data 7:** 2.725 [K](https://bliptext.com/articles/kelvin) \n**Label 8:** Contents \n**Data 8:** [Ordinary (baryonic)](https://bliptext.com/articles/baryon) [matter](https://bliptext.com/articles/matter) (4.9%) · Dark [matter](https://bliptext.com/articles/matter) (26.8%) · [Dark energy](https://bliptext.com/articles/dark-energy) (68.3%) \n \n\n# \nThe **observable [universe](https://bliptext.com/articles/universe)** is a [spherical](https://bliptext.com/articles/ball-mathematics) region of the [universe](https://bliptext.com/articles/universe) consisting of all [matter](https://bliptext.com/articles/matter) that can be [observed](https://bliptext.com/articles/observation) from [Earth](https://bliptext.com/articles/earth) or its space-based telescopes and exploratory probes at the present time; the [electromagnetic radiation](https://bliptext.com/articles/electromagnetic-radiation) from these [objects](https://bliptext.com/articles/astronomical-object) has had time to reach the [Solar System](https://bliptext.com/articles/solar-system) and [Earth](https://bliptext.com/articles/earth) since the beginning of the [cosmological expansion](https://bliptext.com/articles/metric-expansion-of-space).\nAssuming the universe is [isotropic](https://bliptext.com/articles/isotropy), the distance to the edge of the observable universe is roughly [the same](https://bliptext.com/articles/equidistant) in every direction.\nThat is, the observable universe is a [spherical](https://bliptext.com/articles/sphere) region centered on the observer.\nEvery location in the universe has its own observable universe, which may or may not overlap with the one centered on Earth.\nThe word observable in this sense does not refer to the capability of modern technology to detect [light](https://bliptext.com/articles/light) or other information from an object, or whether there is anything to be detected.\nIt refers to the physical limit created by the [speed of light](https://bliptext.com/articles/speed-of-light) itself.\nNo signal can travel faster than light, hence there is a maximum distance, called the [particle horizon](https://bliptext.com/articles/particle-horizon), beyond which nothing can be detected, as the signals could not have reached us yet.\nSometimes astrophysicists distinguish between the observable universe and the visible universe.\nThe former includes signals since the end of the [inflationary epoch](https://bliptext.com/articles/inflationary-epoch), while the latter includes only signals emitted since [recombination](https://bliptext.com/articles/recombination-cosmology).\nAccording to calculations, the current [comoving distance](https://bliptext.com/articles/comoving-distance) to particles from which the [cosmic microwave background radiation](https://bliptext.com/articles/cosmic-microwave-background-radiation) (CMBR) was emitted, which represents the radius of the visible universe, is about 14.0 billion [parsecs](https://bliptext.com/articles/parsec) (about 45.7 billion light-years).\nThe comoving distance to the edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light-years), about 2% larger.\nThe [radius](https://bliptext.com/articles/radius) of the observable universe is therefore estimated to be about 46.5 billion light-years.\nUsing the [critical density](https://bliptext.com/articles/friedmann-equations) and the diameter of the observable universe, the total mass of ordinary matter in the universe can be calculated to be about 1.5 kg. In November 2018, astronomers reported that [extragalactic background light](https://bliptext.com/articles/extragalactic-background-light) (EBL) amounted to 4 photons.\nAs the universe's expansion is accelerating, all currently observable objects, outside the local [supercluster](https://bliptext.com/articles/supercluster), will eventually appear to freeze in time, while emitting progressively redder and fainter light.\nFor instance, objects with the current [redshift](https://bliptext.com/articles/redshift) [z](https://bliptext.com/articles/redshift) from 5 to 10 will only be observable up to an age of 4–6 billion years.\nIn addition, light emitted by objects currently situated beyond a certain comoving distance (currently about 19 Gpc) will never reach Earth.\n\n# Overview\nThe universe's size is unknown, and it may be infinite in extent.\nSome parts of the universe are too far away for the light emitted since the [Big Bang](https://bliptext.com/articles/big-bang) to have had enough time to reach Earth or space-based instruments, and therefore lie outside the observable universe.\nIn the future, light from distant galaxies will have had more time to travel, so one might expect that additional regions will become observable.\nRegions distant from observers (such as us) are expanding away faster than the speed of light, at rates estimated by [Hubble's law](https://bliptext.com/articles/hubble-s-law).\nThe [expansion rate appears to be accelerating](https://bliptext.com/articles/accelerating-expansion-of-the-universe), which [dark energy](https://bliptext.com/articles/dark-energy) was proposed to explain.\nAssuming dark energy remains constant (an unchanging [cosmological constant](https://bliptext.com/articles/cosmological-constant)) so that the expansion rate of the universe continues to accelerate, there is a \"future visibility limit\" beyond which objects will never enter the observable universe at any time in the future because light emitted by objects outside that limit could never reach the Earth.\nNote that, because the [Hubble parameter](https://bliptext.com/articles/hubble-s-law) is decreasing with time, there can be cases where a galaxy that is receding from Earth only slightly faster than light emits a signal that eventually reaches Earth.\nThis future visibility limit is calculated at a [comoving distance](https://bliptext.com/articles/comoving-distance) of 19 billion parsecs (62 billion light-years), assuming the universe will keep expanding forever, which implies the number of galaxies that can ever be theoretically observed in the infinite future is only larger than the number currently observable by a factor of 2.36 (ignoring redshift effects).\nIn principle, more galaxies will become observable in the future; in practice, an increasing number of galaxies will become extremely [redshifted](https://bliptext.com/articles/redshift) due to ongoing expansion, so much so that they will seem to disappear from view and become invisible.\nA galaxy at a given comoving distance is defined to lie within the \"observable universe\" if we can receive signals emitted by the galaxy at any age in its history, say, a signal sent from the galaxy only 500 million years after the Big Bang.\nBecause of the universe's expansion, there may be some later age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future, so, for example, we might never see what the galaxy looked like 10 billion years after the Big Bang, even though it remains at the same comoving distance less than that of the observable universe.\nThis can be used to define a type of cosmic [event horizon](https://bliptext.com/articles/event-horizon) whose distance from the Earth changes over time.\nFor example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening at present can eventually reach the Earth if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is further away.\nThe space before this cosmic event horizon can be called \"reachable universe\", that is all galaxies closer than that could be reached if we left for them today, at the speed of light; all galaxies beyond that are unreachable.\nSimple observation will show the future visibility limit (62 billion light-years) is exactly equal to the reachable limit (16 billion light-years) added to the current visibility limit (46 billion light-years).\n# [Observable Universe as a function of time and distance, in context of the expanding Universe | upload.wikimedia.org/wikipedia/commons/f/f2/Home///in///Relation///to///Everything-Observable///Universe.png]\n# [The reachable Universe as a function of time and distance, in context of the expanding Universe. | upload.wikimedia.org/wikipedia/commons/8/88/Home///in///Relation///to///Everything-Reachable///Universe.png]\n\n# \"The universe\" versus \"the observable universe\"\nBoth popular and professional research articles in cosmology often use the term \"universe\" to mean \"observable universe\".\nThis can be justified on the grounds that we can never know anything by direct observation about any part of the universe that is [causally disconnected](https://bliptext.com/articles/causality-physics) from the Earth, although many credible theories require a total universe much larger than the observable universe.\nNo evidence exists to suggest that the boundary of the observable universe constitutes a boundary on the universe as a whole, nor do any of the mainstream cosmological models propose that the universe has any physical boundary in the first place.\nHowever, some models propose it could be finite but unbounded, like a higher-dimensional analogue of the 2D surface of a sphere that is finite in area but has no edge.\nIt is plausible that the [galaxies](https://bliptext.com/articles/galaxy) within the observable universe represent only a minuscule fraction of the [galaxies](https://bliptext.com/articles/galaxy) in the universe.\nAccording to the theory of [cosmic inflation](https://bliptext.com/articles/cosmic-inflation) initially introduced by [Alan Guth](https://bliptext.com/articles/alan-guth) and [D. Kazanas](https://bliptext.com/articles/d-kazanas), if it is assumed that inflation began about 10−37 seconds after the Big Bang and that the pre-inflation size of the universe was approximately equal to the speed of light times its age, that would suggest that at present the entire universe's size is at least 1.5 light-years—at least 3 times the radius of the observable universe.\nIf the universe is finite but unbounded, it is also possible that the universe is smaller than the observable universe.\nIn this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated the universe.\nIt is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different.\nBielewicz et al. claim to establish a lower bound of 27.9 gigaparsecs (91 billion light-years) on the diameter of the last scattering surface.\nThis value is based on matching-circle analysis of the [WMAP](https://bliptext.com/articles/wmap) 7-year data.\nThis approach has been disputed.\n\n# Size\nThe [comoving distance](https://bliptext.com/articles/comoving-and-proper-distances) from Earth to the edge of the observable universe is about 14.26 giga[parsecs](https://bliptext.com/articles/parsec) (46.5 [billion](https://bliptext.com/articles/1000000000-number) [light-years](https://bliptext.com/articles/light-year) or 14.26 Gpc) in any direction.\nThe observable universe is thus a sphere with a [diameter](https://bliptext.com/articles/diameter) of about 28.5 gigaparsecs (93 billion light-years or 28.5 Gpc).\nAssuming that space is roughly [flat](https://bliptext.com/articles/shape-of-the-universe) (in the sense of being a [Euclidean space](https://bliptext.com/articles/euclidean-space)), this size corresponds to a comoving volume of about 1.22 Gpc3 (4.22 Gly3 or 3.57 m3).\nThese are distances now (in [cosmological time](https://bliptext.com/articles/cosmological-time)), not distances at the time the light was emitted.\nFor example, the cosmic microwave background radiation that we see right now was emitted at the [time of photon decoupling](https://bliptext.com/articles/recombination-cosmology), estimated to have occurred about 380,000 years after the Big Bang, which occurred around 13.8 billion years ago.\nThis radiation was emitted by matter that has, in the intervening time, mostly condensed into galaxies, and those galaxies are now calculated to be about 46 billion light-years from Earth.\nTo estimate the distance to that matter at the time the light was emitted, we may first note that according to the [Friedmann–Lemaître–Robertson–Walker metric](https://bliptext.com/articles/friedmann-lema-tre-robertson-walker-metric), which is used to model the expanding universe, if we receive light with a [redshift](https://bliptext.com/articles/redshift) of z, then the [scale factor](https://bliptext.com/articles/scale-factor-cosmology) at the time the light was originally emitted is given by \".\"\n[WMAP nine-year results](https://bliptext.com/articles/wilkinson-microwave-anisotropy-probe) combined with other measurements give the redshift of photon [decoupling](https://bliptext.com/articles/decoupling-cosmology) as z = 1,091.64, which implies that the scale factor at the time of photon [decoupling](https://bliptext.com/articles/decoupling-cosmology) would be 1/undefined.\nSo if the matter that originally emitted the oldest CMBR [photons](https://bliptext.com/articles/photon) has a present distance of 46 billion light-years, then the distance would have been only about 42 million light-years at the time of decoupling.\nThe [light-travel distance](https://bliptext.com/articles/light-travel-distance) to the edge of the observable universe is the [age of the universe](https://bliptext.com/articles/age-of-the-universe) times the [speed of light](https://bliptext.com/articles/speed-of-light), 13.8 billion light years.\nThis is the distance that a photon emitted shortly after the Big Bang, such as one from the [cosmic microwave background](https://bliptext.com/articles/cosmic-microwave-background), has traveled to reach observers on Earth.\nBecause [spacetime](https://bliptext.com/articles/spacetime) is curved, corresponding to the [expansion of space](https://bliptext.com/articles/expansion-of-the-universe), this distance does not correspond to the true distance at any moment in time.\n# [Hubble Ultra-Deep Field image of a region of the observable universe (equivalent sky area size shown in bottom left corner), near the constellation Fornax. Each spot is a galaxy, consisting of billions of stars. The light from the smallest, most redshifted galaxies originated nearly 13.8 billion years ago. | upload.wikimedia.org/wikipedia/commons/e/e4/HubbleUltraDeepFieldwithScaleComparison.jpg]\n\n# Matter and mass\n\n## Number of galaxies and stars\nThe observable universe contains as many as an estimated 2 trillion galaxies and, overall, as many as an estimated 1024 stars – more stars (and, potentially, [Earth](https://bliptext.com/articles/earth)-like planets) than all the [grains of beach sand](https://bliptext.com/articles/sand) on planet [Earth](https://bliptext.com/articles/earth).\nOther estimates are in the hundreds of billions rather than trillions.\nThe estimated total number of stars in an [inflationary universe](https://bliptext.com/articles/inflation-cosmology) (observed and unobserved) is 10100.\n\n## Matter content—number of atoms\nAssuming the mass of ordinary matter is about 1.45 kg as discussed above, and assuming all atoms are [hydrogen atoms](https://bliptext.com/articles/hydrogen-atom) (which are about 74% of all atoms in the Milky Way by mass), the estimated total number of atoms in the observable universe is obtained by dividing the mass of ordinary matter by the mass of a hydrogen atom.\nThe result is approximately 1080 hydrogen atoms, also known as the [Eddington number](https://bliptext.com/articles/eddington-number).\n\n## Mass of ordinary matter\nThe mass of the observable universe is often quoted as 1053 kg. In this context, mass refers to ordinary (baryonic) matter and includes the [interstellar medium](https://bliptext.com/articles/interstellar-medium) (ISM) and the [intergalactic medium](https://bliptext.com/articles/intergalactic-medium) (IGM).\nHowever, it excludes [dark matter](https://bliptext.com/articles/dark-matter) and [dark energy](https://bliptext.com/articles/dark-energy).\nThis quoted value for the mass of ordinary matter in the universe can be estimated based on critical density.\nThe calculations are for the observable universe only as the volume of the whole is unknown and may be infinite.\n\n## Estimates based on critical density\nCritical density is the energy density for which the universe is flat.\nIf there is no dark energy, it is also the [density](https://bliptext.com/articles/density) for which the expansion of the universe is poised between continued expansion and collapse.\nFrom the [Friedmann equations](https://bliptext.com/articles/friedmann-equations), the value for critical density, is:\nwhere G is the [gravitational constant](https://bliptext.com/articles/gravitational-constant) and H = H0 is the present value of the [Hubble constant](https://bliptext.com/articles/hubble-constant).\nThe value for H0, as given by the European Space Agency's Planck Telescope, is H0 = 67.15 kilometres per second per megaparsec.\nThis gives a critical density of 0.85 kg/m3, or about 5 hydrogen atoms per cubic metre.\nThis density includes four significant types of energy/mass: ordinary matter (4.8%), neutrinos (0.1%), [cold dark matter](https://bliptext.com/articles/cold-dark-matter) (26.8%), and [dark energy](https://bliptext.com/articles/dark-energy) (68.3%).\nAlthough neutrinos are [Standard Model](https://bliptext.com/articles/standard-model) particles, they are listed separately because they are [ultra-relativistic](https://bliptext.com/articles/scale-factor-cosmology) and hence [behave](https://bliptext.com/articles/equation-of-state-cosmology) like radiation rather than like matter.\nThe density of ordinary matter, as measured by Planck, is 4.8% of the total critical density or 4.08 kg/m3.\nTo convert this density to mass we must multiply by volume, a value based on the radius of the \"observable universe\".\nSince the universe has been expanding for 13.8 billion years, the [comoving distance](https://bliptext.com/articles/comoving-and-proper-distances) (radius) is now about 46.6 billion light-years.\nThus, volume (4⁄3πr3) equals 3.58 m3 and the mass of ordinary matter equals density (4.08 kg/m3) times volume (3.58 m3) or 1.46 kg.\n\n# Large-scale structure\n[Sky surveys](https://bliptext.com/articles/redshift-survey) and mappings of the various [wavelength](https://bliptext.com/articles/wavelength) bands of [electromagnetic radiation](https://bliptext.com/articles/electromagnetic-radiation) (in particular [21-cm emission](https://bliptext.com/articles/hydrogen-line)) have yielded much information on the content and character of the [universe](https://bliptext.com/articles/universe)'s structure.\nThe organization of structure appears to follow a [hierarchical](https://bliptext.com/articles/hierarchy) model with organization up to the [scale](https://bliptext.com/articles/scale-spatial) of [superclusters](https://bliptext.com/articles/supercluster) and [filaments](https://bliptext.com/articles/galaxy-filament).\nLarger than this (at scales between 30 and 200 megaparsecs), there seems to be no continued structure, a phenomenon that has been referred to as the End of Greatness.\n# [Galaxy clusters, like RXC J0142.9+4438, are the nodes of the cosmic web that permeates the entire Universe. | upload.wikimedia.org/wikipedia/commons/1/16/Galactic///treasure///chest///RXC///J0142.9%2B4438.jpg]\n\n## Walls, filaments, nodes, and voids\nThe organization of structure arguably begins at the stellar level, though most cosmologists rarely address [astrophysics](https://bliptext.com/articles/astrophysics) on that scale.\n[Stars](https://bliptext.com/articles/star) are organized into [galaxies](https://bliptext.com/articles/galaxy), which in turn form [galaxy groups](https://bliptext.com/articles/galaxy-group), [galaxy clusters](https://bliptext.com/articles/galaxy-cluster), [superclusters](https://bliptext.com/articles/supercluster), sheets, [walls and filaments](https://bliptext.com/articles/galaxy-filament), which are separated by immense [voids](https://bliptext.com/articles/void-astronomy), creating a vast foam-like structure sometimes called the \"cosmic web\".\nPrior to 1989, it was commonly assumed that [virialized](https://bliptext.com/articles/virial-theorem) galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction.\nHowever, since the early 1980s, more and more structures have been discovered.\nIn 1983, Adrian Webster identified the [Webster LQG](https://bliptext.com/articles/webster-lqg), a [large quasar group](https://bliptext.com/articles/large-quasar-group) consisting of 5 quasars.\nThe discovery was the first identification of a large-scale structure, and has expanded the information about the known grouping of matter in the universe.\nIn 1987, [Robert Brent Tully](https://bliptext.com/articles/r-brent-tully) identified the [Pisces–Cetus Supercluster Complex](https://bliptext.com/articles/pisces-cetus-supercluster-complex), the galaxy filament in which the Milky Way resides.\nIt is about 1 billion light-years across.\nThat same year, an unusually large region with a much lower than average distribution of galaxies was discovered, the [Giant Void](https://bliptext.com/articles/giant-void), which measures 1.3 billion light-years across.\nBased on [redshift survey](https://bliptext.com/articles/redshift-survey) data, in 1989 [Margaret Geller](https://bliptext.com/articles/margaret-geller) and [John Huchra](https://bliptext.com/articles/john-huchra) discovered the \"[Great Wall](https://bliptext.com/articles/cfa2-great-wall)\", a sheet of galaxies more than 500 million [light-years](https://bliptext.com/articles/light-year) long and 200 million [light-years](https://bliptext.com/articles/light-year) wide, but only 15 million [light-years](https://bliptext.com/articles/light-year) thick.\nThe existence of this structure escaped notice for so long because it requires locating the position of galaxies in three dimensions, which involves combining location information about the galaxies with distance information from [redshifts](https://bliptext.com/articles/redshift).\nTwo years later, astronomers Roger G. Clowes and Luis E. Campusano discovered the [Clowes–Campusano LQG](https://bliptext.com/articles/clowes-campusano-lqg), a [large quasar group](https://bliptext.com/articles/large-quasar-group) measuring two billion light-years at its widest point, which was the largest known structure in the universe at the time of its announcement.\nIn April 2003, another large-scale structure was discovered, the [Sloan Great Wall](https://bliptext.com/articles/sloan-great-wall).\nIn August 2007, a possible supervoid was detected in the constellation [Eridanus](https://bliptext.com/articles/eridanus-constellation).\nIt coincides with the '[CMB cold spot](https://bliptext.com/articles/cmb-cold-spot)', a cold region in the microwave sky that is highly improbable under the currently favored cosmological model.\nThis supervoid could cause the cold spot, but to do so it would have to be improbably big, possibly a billion light-years across, almost as big as the Giant Void mentioned above.\nAnother large-scale structure is the [SSA22 Protocluster](https://bliptext.com/articles/ssa22-protocluster), a collection of galaxies and enormous gas bubbles that measures about 200 million light-years across.\nIn 2011, a large quasar group was discovered, [U1.11](https://bliptext.com/articles/u1-11), measuring about 2.5 billion light-years across.\nOn January 11, 2013, another large quasar group, the [Huge-LQG](https://bliptext.com/articles/huge-lqg), was discovered, which was measured to be four billion light-years across, the largest known structure in the universe at that time.\nIn November 2013, astronomers discovered the [Hercules–Corona Borealis Great Wall](https://bliptext.com/articles/hercules-corona-borealis-great-wall), an even bigger structure twice as large as the former.\nIt was defined by the mapping of [gamma-ray bursts](https://bliptext.com/articles/gamma-ray-burst).\nIn 2021, the [American Astronomical Society](https://bliptext.com/articles/american-astronomical-society) announced the detection of the [Giant Arc](https://bliptext.com/articles/the-giant-arc); a crescent-shaped string of galaxies that span 3.3 billion light years in length, located 9.2 billion light years from Earth in the constellation [Boötes](https://bliptext.com/articles/bo-tes) from observations captured by the [Sloan Digital Sky Survey](https://bliptext.com/articles/sloan-digital-sky-survey).\n# [Map of the cosmic web generated from a slime mould-inspired algorithm | upload.wikimedia.org/wikipedia/commons/2/26/Map///of///the///Cosmic///Web///Generated///from///Slime///Mould///Algorithm.jpg]\n# [Computer simulated image of an area of space more than 50 million light-years across, presenting a possible large-scale distribution of light sources in the universe—precise relative contributions of galaxies and quasars are unclear. | upload.wikimedia.org/wikipedia/commons/6/6d/Large-scale///structure///of///light///distribution///in///the///universe.jpg]\n\n## End of Greatness\nThe End of Greatness is an observational scale discovered at roughly 100 [Mpc](https://bliptext.com/articles/megaparsec) (roughly 300 million light-years) where the lumpiness seen in the large-scale structure of the [universe](https://bliptext.com/articles/universe) is [homogenized](https://bliptext.com/articles/homogeneous) and [isotropized](https://bliptext.com/articles/isotropic) in accordance with the [cosmological principle](https://bliptext.com/articles/cosmological-principle).\nAt this scale, no pseudo-random [fractalness](https://bliptext.com/articles/fractal) is apparent.\nThe [superclusters](https://bliptext.com/articles/supercluster) and [filaments](https://bliptext.com/articles/galaxy-filament) seen in smaller surveys are [randomized](https://bliptext.com/articles/random) to the extent that the smooth distribution of the universe is visually apparent.\nIt was not until the [redshift surveys](https://bliptext.com/articles/redshift-survey) of the 1990s were completed that this scale could accurately be observed.\n\n## Observations\nAnother indicator of large-scale structure is the '[Lyman-alpha forest](https://bliptext.com/articles/lyman-alpha-forest)'.\nThis is a collection of [absorption lines](https://bliptext.com/articles/spectral-line) that appear in the spectra of light from [quasars](https://bliptext.com/articles/quasar), which are interpreted as indicating the existence of huge thin sheets of intergalactic (mostly [hydrogen](https://bliptext.com/articles/hydrogen)) gas.\nThese sheets appear to collapse into filaments, which can feed galaxies as they grow where filaments either cross or are dense.\nAn early direct evidence for this cosmic web of gas was the 2019 detection, by astronomers from the RIKEN Cluster for Pioneering Research in Japan and Durham University in the U.K., of light from the brightest part of this web, surrounding and illuminated by a cluster of forming galaxies, acting as cosmic flashlights for intercluster medium hydrogen fluorescence via Lyman-alpha emissions.\nIn 2021, an international team, headed by Roland Bacon from the Centre de Recherche Astrophysique de Lyon (France), reported the first observation of diffuse extended Lyman-alpha emission from redshift 3.1 to 4.5 that traced several cosmic web filaments on scales of 2.5−4 [cMpc](https://bliptext.com/articles/cmpc) (comoving mega-parsecs), in filamentary environments outside massive structures typical of web nodes.\nSome caution is required in describing structures on a cosmic scale because they are often different from how they appear.\n[Gravitational lensing](https://bliptext.com/articles/gravitational-lens) can make an image appear to originate in a different direction from its real source, when foreground objects curve surrounding spacetime (as predicted by [general relativity](https://bliptext.com/articles/general-relativity)) and deflect passing light rays.\nRather usefully, strong gravitational lensing can sometimes magnify distant galaxies, making them easier to detect.\n[Weak lensing](https://bliptext.com/articles/weak-gravitational-lensing) by the intervening universe in general also subtly changes the observed large-scale structure.\nThe large-scale structure of the universe also looks different if only redshift is used to measure distances to galaxies.\nFor example, galaxies behind a galaxy cluster are attracted to it and fall towards it, and so are [blueshifted](https://bliptext.com/articles/blueshift) (compared to how they would be if there were no cluster).\nOn the near side, objects are redshifted.\nThus, the environment of the cluster looks somewhat pinched if using redshifts to measure distance.\nThe opposite effect is observed on galaxies already within a cluster: the galaxies have some random motion around the cluster center, and when these random motions are converted to redshifts, the cluster appears elongated.\nThis creates a \"[finger of God](https://bliptext.com/articles/redshift-space-distortions)\"—the illusion of a long chain of galaxies pointed at Earth.\n# [\"Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. The image is derived from the 2MASS Extended Source Catalog (XSC)—more than 1.5 million galaxies, and the Point Source Catalog (PSC)—nearly 0.5 billion Milky Way stars. The galaxies are color-coded by 'redshift' obtained from the UGC, CfA, Tully NBGC, LCRS, 2dF, 6dFGS, and SDSS surveys (and from various observations compiled by the NASA Extragalactic Database), or photo-metrically deduced from the K band (2.2 μm). Blue are the nearest sources (z < 0.01); green are at moderate distances (0.01 < z < 0.04) and red are the most distant sources that 2MASS resolves (0.04 < z < 0.1). The map is projected with an equal area Aitoff in the Galactic system (Milky Way at center).\" | upload.wikimedia.org/wikipedia/commons/7/7d/2MASS///LSS///chart-NEW///Nasa.jpg]\n# [Constellations grouped in galactic quadrants (N/S, 1–4) and their approximate divisions vis-a-vis celestial quadrants (NQ/SQ) | upload.wikimedia.org/wikipedia/commons/7/74/Galactic%2Bcelestial///quads.jpg]\n\n## Cosmography of Earth's cosmic neighborhood\nAt the centre of the [Hydra–Centaurus Supercluster](https://bliptext.com/articles/hydra-centaurus-supercluster), a gravitational anomaly called the [Great Attractor](https://bliptext.com/articles/great-attractor) affects the motion of galaxies over a region hundreds of millions of light-years across.\nThese galaxies are all [redshifted](https://bliptext.com/articles/redshift), in accordance with [Hubble's law](https://bliptext.com/articles/hubble-s-law).\nThis indicates that they are receding from us and from each other, but the variations in their redshift are sufficient to reveal the existence of a concentration of mass equivalent to tens of thousands of galaxies.\nThe Great Attractor, discovered in 1986, lies at a distance of between 150 million and 250 million light-years in the direction of the [Hydra](https://bliptext.com/articles/hydra-constellation) and [Centaurus](https://bliptext.com/articles/centaurus) [constellations](https://bliptext.com/articles/constellation).\nIn its vicinity there is a preponderance of large old galaxies, many of which are colliding with their neighbours, or radiating large amounts of radio waves.\nIn 1987, [astronomer](https://bliptext.com/articles/astronomer) [R. Brent Tully](https://bliptext.com/articles/r-brent-tully) of the [University of Hawaii](https://bliptext.com/articles/university-of-hawaii)'s Institute of Astronomy identified what he called the [Pisces–Cetus Supercluster Complex](https://bliptext.com/articles/pisces-cetus-supercluster-complex), a structure one billion [light-years](https://bliptext.com/articles/light-year) long and 150 million [light-years](https://bliptext.com/articles/light-year) across in which, he claimed, the Local Supercluster is embedded.\n\n# Most distant objects\nThe most distant [astronomical object](https://bliptext.com/articles/astronomical-object) identified (as of August of 2024) is a galaxy classified as [JADES-GS-z14-0](https://bliptext.com/articles/jades-gs-z14-0).\nIn 2009, a [gamma ray burst](https://bliptext.com/articles/gamma-ray-burst), [GRB 090423](https://bliptext.com/articles/grb-090423), was found to have a [redshift](https://bliptext.com/articles/redshift) of 8.2, which indicates that the collapsing star that caused it exploded when the universe was only 630 million years old.\nThe burst happened approximately 13 billion years ago, so a distance of about 13 billion light-years was widely quoted in the media, or sometimes a more precise figure of 13.035 billion light-years.\nThis would be the \"light travel distance\" (see [Distance measures (cosmology)](https://bliptext.com/articles/distance-measures-cosmology)) rather than the \"[proper distance](https://bliptext.com/articles/comoving-and-proper-distances)\" used in both [Hubble's law](https://bliptext.com/articles/hubble-s-law) and in defining the size of the observable universe.\nCosmologist [Ned Wright](https://bliptext.com/articles/edward-l-wright) argues against using this measure.\nThe proper distance for a redshift of 8.2 would be about 9.2 [Gpc](https://bliptext.com/articles/megaparsecs), or about 30 billion light-years.\n\n# Horizons\nThe limit of observability in the universe is set by cosmological horizons which limit—based on various physical constraints—the extent to which information can be obtained about various events in the universe.\nThe most famous horizon is the [particle horizon](https://bliptext.com/articles/particle-horizon) which sets a limit on the precise distance that can be seen due to the finite [age of the universe](https://bliptext.com/articles/age-of-the-universe).\nAdditional horizons are associated with the possible future extent of observations, larger than the particle horizon owing to the [expansion of space](https://bliptext.com/articles/expansion-of-space), an \"optical horizon\" at the [surface of last scattering](https://bliptext.com/articles/cosmic-microwave-background), and associated horizons with the [surface of last scattering](https://bliptext.com/articles/cosmic-microwave-background) for [neutrinos](https://bliptext.com/articles/cosmic-neutrino-background) and [gravitational waves](https://bliptext.com/articles/gravitational-wave-background).\n\n# Gallery","revisionCount":1},"hasMore":false,"nextCursor":null}

History