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87-579: The Centaurus Cluster (A3526) is a cluster of hundreds of galaxies , located approximately 170 million light-years away in the Centaurus constellation. The brightest member galaxy is the elliptical galaxy NGC 4696 (~11m). The Centaurus cluster shares its supercluster, the Hydra–Centaurus Supercluster , with IC4329 Cluster and Hydra Cluster . The cluster consists of two different sub-groups of galaxies with different velocities. Cen 30

174-664: A Taylor series expansion: z = R ( t 0 ) R ( t e ) − 1 ≈ R ( t 0 ) R ( t 0 ) ( 1 + ( t e − t 0 ) H ( t 0 ) ) − 1 ≈ ( t 0 − t e ) H ( t 0 ) , {\displaystyle z={\frac {R(t_{0})}{R(t_{e})}}-1\approx {\frac {R(t_{0})}{R(t_{0})\left(1+(t_{e}-t_{0})H(t_{0})\right)}}-1\approx (t_{0}-t_{e})H(t_{0}),} If

261-495: A frequency (SI unit: s ), leading the reciprocal of H 0 to be known as the Hubble time (14.4 billion years). The Hubble constant can also be stated as a relative rate of expansion. In this form H 0  = 7%/ Gyr , meaning that at the current rate of expansion it takes one billion years for an unbound structure to grow by 7%. Although widely attributed to Edwin Hubble ,

348-464: A "Hubble diagram" in which the velocity (assumed approximately proportional to the redshift) of an object is plotted with respect to its distance from the observer. A straight line of positive slope on this diagram is the visual depiction of Hubble's law. After Hubble's discovery was published, Albert Einstein abandoned his work on the cosmological constant , a term he had inserted into his equations of general relativity to coerce them into producing

435-496: A concentration of mass equivalent to tens of thousands of galaxies. The Great Attractor, discovered in 1986, lies at a distance of between 150 million and 250 million light-years in the direction of the Hydra and Centaurus constellations . In 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. In 1987, astronomer R. Brent Tully of

522-447: A discussion of the significance of this): r HS = c H 0   . {\displaystyle r_{\text{HS}}={\frac {c}{H_{0}}}\ .} Since the Hubble "constant" is a constant only in space, not in time, the radius of the Hubble sphere may increase or decrease over various time intervals. The subscript '0' indicates the value of the Hubble constant today. Current evidence suggests that

609-524: A galaxy is at distance D , and this distance changes with time at a rate d t D . We call this rate of recession the "recession velocity" v r : v r = d t D = d t R R D . {\displaystyle v_{\text{r}}=d_{t}D={\frac {d_{t}R}{R}}D.} We now define the Hubble constant as H ≡ d t R R , {\displaystyle H\equiv {\frac {d_{t}R}{R}},} and discover

696-411: A galaxy is typically determined by measuring redshift , a shift in the light it emits toward the red end of the visible light spectrum . The discovery of Hubble's law is attributed to work published by Edwin Hubble in 1929. Hubble's law is considered the first observational basis for the expansion of the universe , and today it serves as one of the pieces of evidence most often cited in support of

783-541: 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. Because 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

870-400: A higher-dimensional analogue of the 2D surface of a sphere that is finite in area but has no edge. It is plausible that the galaxies within the observable universe represent only a minuscule fraction of the galaxies in the universe. According to the theory of cosmic inflation initially introduced by Alan Guth and D. Kazanas , if it is assumed that inflation began about 10 seconds after

957-410: A linear relation of redshift to recessional velocity applies, but more generally the redshift-distance law is nonlinear, meaning the co-relation must be derived specifically for each given model and epoch. The redshift z is often described as a redshift velocity , which is the recessional velocity that would produce the same redshift if it were caused by a linear Doppler effect (which, however,

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1044-406: A low-impact journal. In the 1931 high-impact English translation of this article, a critical equation was changed by omitting reference to what is now known as the Hubble constant." It is now known that the alterations in the translated paper were carried out by Lemaître himself. Before the advent of modern cosmology, there was considerable talk about the size and shape of the universe . In 1920,

1131-487: A phenomenon that has been referred to as the End of Greatness . The organization of structure arguably begins at the stellar level, though most cosmologists rarely address astrophysics on that scale. Stars are organized into galaxies , which in turn form galaxy groups , galaxy clusters , superclusters , sheets, walls and filaments , which are separated by immense voids , creating a vast foam-like structure sometimes called

1218-405: A smaller velocity than earlier ones. Redshift can be measured by determining the wavelength of a known transition, such as hydrogen α-lines for distant quasars, and finding the fractional shift compared to a stationary reference. Thus, redshift is a quantity unambiguously acquired from observation. Care is required, however, in translating these to recessional velocities: for small redshift values,

1305-436: A supernova brightness, which provides information about its distance, and the redshift z = ∆ λ / λ of its spectrum of radiation. Hubble correlated brightness and parameter z . Combining his measurements of galaxy distances with Vesto Slipher and Milton Humason 's measurements of the redshifts associated with the galaxies, Hubble discovered a rough proportionality between redshift of an object and its distance. Though there

1392-559: Is 4.8% of the total critical density or 4.08 × 10  kg/m . To convert this density to mass we must multiply by volume, a value based on the radius of the "observable universe". Since the universe has been expanding for 13.8 billion years, the comoving distance (radius) is now about 46.6 billion light-years. Thus, volume ( ⁠ 4 / 3 ⁠ πr ) equals 3.58 × 10  m and the mass of ordinary matter equals density ( 4.08 × 10  kg/m ) times volume ( 3.58 × 10  m ) or 1.46 × 10  kg . Sky surveys and mappings of

1479-464: Is also the density for which the expansion of the universe is poised between continued expansion and collapse. From the Friedmann equations , the value for ρ c {\displaystyle \rho _{\text{c}}} critical density, is: where G is the gravitational constant and H = H 0 is the present value of the Hubble constant . The value for H 0 , as given by

1566-421: Is anything to be detected. It refers to the physical limit created by the speed of light itself. No signal can travel faster than light, hence there is a maximum distance, called the particle horizon , beyond which nothing can be detected, as the signals could not have reached us yet. Sometimes astrophysicists distinguish between the observable universe and the visible universe. The former includes signals since

1653-422: Is apparent. The superclusters and filaments seen in smaller surveys are randomized to the extent that the smooth distribution of the universe is visually apparent. It was not until the redshift surveys of the 1990s were completed that this scale could accurately be observed. Another indicator of large-scale structure is the ' Lyman-alpha forest '. This is a collection of absorption lines that appear in

1740-414: Is considered a fundamental relation between recessional velocity and distance. However, the relation between recessional velocity and redshift depends on the cosmological model adopted and is not established except for small redshifts. For distances D larger than the radius of the Hubble sphere r HS , objects recede at a rate faster than the speed of light ( See Uses of the proper distance for

1827-438: Is constant at any given moment in time, the Hubble parameter H , of which the Hubble constant is the current value, varies with time, so the term constant is sometimes thought of as somewhat of a misnomer. A decade before Hubble made his observations, a number of physicists and mathematicians had established a consistent theory of an expanding universe by using Einstein field equations of general relativity . Applying

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1914-414: Is exactly equal to the reachable limit (16 billion light-years) added to the current visibility limit (46 billion light-years). Both popular and professional research articles in cosmology often use the term "universe" to mean "observable universe". This 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 from

2001-563: Is not the case, as the velocities involved are too large to use a non-relativistic formula for Doppler shift). This redshift velocity can easily exceed the speed of light. In other words, to determine the redshift velocity v rs , the relation: v rs ≡ c z , {\displaystyle v_{\text{rs}}\equiv cz,} is used. That is, there is no fundamental difference between redshift velocity and redshift: they are rigidly proportional, and not related by any theoretical reasoning. The motivation behind

2088-475: Is required in describing structures on a cosmic scale because they are often different from how they appear. Gravitational lensing 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 ) and deflect passing light rays. Rather usefully, strong gravitational lensing can sometimes magnify distant galaxies, making them easier to detect. Weak lensing by

2175-402: Is some reference time. If light is emitted from a galaxy at time t e and received by us at t 0 , it is redshifted due to the expansion of the universe, and this redshift z is simply: z = R ( t 0 ) R ( t e ) − 1. {\displaystyle z={\frac {R(t_{0})}{R(t_{\text{e}})}}-1.} Suppose

2262-489: Is the main subgroup containing NGC 4696 . Cen 45 which is centered on NGC 4709 , is moving at 1500 km/s relative to Cen 30, and is believed to be merging with the main cluster. This galaxy-cluster-related article is a stub . You can help Misplaced Pages by expanding it . Groups and clusters of galaxies The word observable in this sense does not refer to the capability of modern technology to detect light or other information from an object, or whether there

2349-522: Is unknown, and it may be infinite in extent. Some parts of the universe are too far away for the light emitted since the Big Bang to have had enough time to reach Earth or space-based instruments, and therefore lie outside the observable universe. In the future, light from distant galaxies will have had more time to travel, so one might expect that additional regions will become observable. Regions distant from observers (such as us) are expanding away faster than

2436-550: The Big Bang and Steady State theories of cosmology. In 1927, two years before Hubble published his own article, the Belgian priest and astronomer Georges Lemaître was the first to publish research deriving what is now known as Hubble's law. According to the Canadian astronomer Sidney van den Bergh , "the 1927 discovery of the expansion of the universe by Lemaître was published in French in

2523-439: The Big Bang model. The motion of astronomical objects due solely to this expansion is known as the Hubble flow . It is described by the equation v = H 0 D , with H 0 the constant of proportionality—the Hubble constant —between the "proper distance" D to a galaxy (which can change over time, unlike the comoving distance ) and its speed of separation v , i.e. the derivative of proper distance with respect to

2610-526: The Shapley–Curtis debate took place between Harlow Shapley and Heber D. Curtis over this issue. Shapley argued for a small universe the size of the Milky Way galaxy, and Curtis argued that the universe was much larger. The issue was resolved in the coming decade with Hubble's improved observations. Edwin Hubble did most of his professional astronomical observing work at Mount Wilson Observatory , home to

2697-607: The University of Hawaii 's Institute of Astronomy identified what he called the Pisces–Cetus Supercluster Complex , a structure one billion light-years long and 150 million light-years across in which, he claimed, the Local Supercluster is embedded. The most distant astronomical object identified (as of August of 2024) is a galaxy classified as JADES-GS-z14-0 . In 2009, a gamma ray burst , GRB 090423 ,

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2784-405: The cosmic time coordinate. (See Comoving and proper distances § Uses of the proper distance for discussion of the subtleties of this definition of velocity. ) The Hubble constant is most frequently quoted in km / s / Mpc , which gives the speed of a galaxy 1 megaparsec (3.09 × 10  km) away as 70 km/s . Simplifying the units of the generalized form reveals that H 0 specifies

2871-417: The grains of beach sand on planet Earth . Other estimates are in the hundreds of billions rather than trillions. The estimated total number of stars in an inflationary universe (observed and unobserved) is 10 . Assuming the mass of ordinary matter is about 1.45 × 10  kg as discussed above, and assuming all atoms are hydrogen atoms (which are about 74% of all atoms in the Milky Way by mass),

2958-415: The intergalactic medium (IGM). However, it excludes dark matter and dark energy . This quoted value for the mass of ordinary matter in the universe can be estimated based on critical density. The calculations are for the observable universe only as the volume of the whole is unknown and may be infinite. Critical density is the energy density for which the universe is flat. If there is no dark energy, it

3045-404: The proportionality constant of Hubble's law. Georges Lemaître independently found a similar solution in his 1927 paper discussed in the following section. The Friedmann equations are derived by inserting the metric for a homogeneous and isotropic universe into Einstein's field equations for a fluid with a given density and pressure . This idea of an expanding spacetime would eventually lead to

3132-466: The " proper distance " used in both Hubble's law and in defining the size of the observable universe. Cosmologist Ned Wright argues against using this measure. The proper distance for a redshift of 8.2 would be about 9.2 Gpc , or about 30 billion light-years. The 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

3219-478: The "cosmic web". Prior to 1989, it was commonly assumed that virialized galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction. However, since the early 1980s, more and more structures have been discovered. In 1983, Adrian Webster identified the Webster LQG , a large quasar group consisting of 5 quasars. The discovery

3306-608: The "redshift velocity" terminology is that the redshift velocity agrees with the velocity from a low-velocity simplification of the so-called Fizeau–Doppler formula z = λ o λ e − 1 = 1 + v c 1 − v c − 1 ≈ v c . {\displaystyle z={\frac {\lambda _{\text{o}}}{\lambda _{\text{e}}}}-1={\sqrt {\frac {1+{\frac {v}{c}}}{1-{\frac {v}{c}}}}}-1\approx {\frac {v}{c}}.} Here, λ o , λ e are

3393-557: 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 × 10 light-years—at least 3 × 10 times the radius of the observable universe. If the universe is finite but unbounded, it is also possible that the universe is smaller than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated

3480-449: 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. The 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. Simple observation will show the future visibility limit (62 billion light-years)

3567-426: The Earth, although many credible theories require a total universe much larger than the observable universe. No 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. However, some models propose it could be finite but unbounded, like

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3654-660: The European Space Agency's Planck Telescope, is H 0 = 67.15 kilometres per second per megaparsec. This gives a critical density of 0.85 × 10  kg/m , or about 5 hydrogen atoms per cubic metre. This density includes four significant types of energy/mass: ordinary matter (4.8%), neutrinos (0.1%), cold dark matter (26.8%), and dark energy (68.3%). Although neutrinos are Standard Model particles, they are listed separately because they are ultra-relativistic and hence behave like radiation rather than like matter. The density of ordinary matter, as measured by Planck,

3741-601: The Giant Void mentioned above. Another large-scale structure is the SSA22 Protocluster , a collection of galaxies and enormous gas bubbles that measures about 200 million light-years across. In 2011, a large quasar group was discovered, U1.11 , measuring about 2.5 billion light-years across. On January 11, 2013, another large quasar group, the Huge-LQG , was discovered, which was measured to be four billion light-years across,

3828-419: The Hubble constant. Hubble inferred the recession velocity of the objects from their redshifts , many of which were earlier measured and related to velocity by Vesto Slipher in 1917. Combining Slipher's velocities with Henrietta Swan Leavitt 's intergalactic distance calculations and methodology allowed Hubble to better calculate an expansion rate for the universe. Though the Hubble constant H 0

3915-463: The Hubble law: v r = H D . {\displaystyle v_{\text{r}}=HD.} From this perspective, Hubble's law is a fundamental relation between (i) the recessional velocity associated with the expansion of the universe and (ii) the distance to an object; the connection between redshift and distance is a crutch used to connect Hubble's law with observations. This law can be related to redshift z approximately by making

4002-675: 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. In 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 (comoving mega-parsecs), in filamentary environments outside massive structures typical of web nodes. Some caution

4089-466: The centre of the Hydra–Centaurus Supercluster , a gravitational anomaly called the Great Attractor affects the motion of galaxies over a region hundreds of millions of light-years across. These galaxies are all redshifted , in accordance with Hubble's law . This indicates that they are receding from us and from each other, but the variations in their redshift are sufficient to reveal the existence of

4176-479: The constellation Boötes from observations captured by the Sloan Digital Sky Survey . The End of Greatness is an observational scale discovered at roughly 100  Mpc (roughly 300 million light-years) where the lumpiness seen in the large-scale structure of the universe is homogenized and isotropized in accordance with the cosmological principle . At this scale, no pseudo-random fractalness

4263-650: The distance is not too large, all other complications of the model become small corrections, and the time interval is simply the distance divided by the speed of light: z ≈ ( t 0 − t e ) H ( t 0 ) ≈ D c H ( t 0 ) , {\displaystyle z\approx (t_{0}-t_{\text{e}})H(t_{0})\approx {\frac {D}{c}}H(t_{0}),} or c z ≈ D H ( t 0 ) = v r . {\displaystyle cz\approx DH(t_{0})=v_{r}.} According to this approach,

4350-586: 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 , which is used to model the expanding universe, if we receive light with a redshift of z , then the scale factor at the time the light was originally emitted is given by a ( t ) = 1 1 + z {\displaystyle a(t)={\frac {1}{1+z}}} . WMAP nine-year results combined with other measurements give

4437-545: The edge of the observable universe is about 14.26 giga parsecs (46.5 billion light-years or 4.40 × 10  m) in any direction. The observable universe is thus a sphere with a diameter of about 28.5 gigaparsecs (93 billion light-years or 8.8 × 10  m). Assuming that space is roughly flat (in the sense of being a Euclidean space ), this size corresponds to a comoving volume of about 1.22 × 10  Gpc ( 4.22 × 10  Gly or 3.57 × 10  m ). These are distances now (in cosmological time ), not distances at

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4524-517: The edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light-years), about 2% larger. The radius of the observable universe is therefore estimated to be about 46.5 billion light-years. Using the critical density and the diameter of the observable universe, the total mass of ordinary matter in the universe can be calculated to be about 1.5 × 10  kg . In November 2018, astronomers reported that extragalactic background light (EBL) amounted to 4 × 10 photons. As

4611-635: The edge of the observable universe is the age of the universe times the speed of light , 13.8 billion light years. This is the distance that a photon emitted shortly after the Big Bang, such as one from the cosmic microwave background , has traveled to reach observers on Earth. Because spacetime is curved, corresponding to the expansion of space , this distance does not correspond to the true distance at any moment in time. The observable universe contains as many as an estimated 2 trillion galaxies and, overall, as many as an estimated 10 stars – more stars (and, potentially, Earth-like planets) than all

4698-400: The end of the inflationary epoch , while the latter includes only signals emitted since recombination . According to calculations, the current comoving distance to particles from which the cosmic microwave background radiation (CMBR) was emitted, which represents the radius of the visible universe, is about 14.0 billion parsecs (about 45.7 billion light-years). The comoving distance to

4785-415: The environment of the cluster looks somewhat pinched if using redshifts to measure distance. The 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. This creates a " finger of God "—the illusion of a long chain of galaxies pointed at Earth. At

4872-462: 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. The result is approximately 10 hydrogen atoms, also known as the Eddington number . The mass of the observable universe is often quoted as 10  kg. In this context, mass refers to ordinary (baryonic) matter and includes the interstellar medium (ISM) and

4959-467: The expansion of the universe is accelerating ( see Accelerating universe ), meaning that for any given galaxy, the recession velocity dD/dt is increasing over time as the galaxy moves to greater and greater distances; however, the Hubble parameter is actually thought to be decreasing with time, meaning that if we were to look at some fixed distance D and watch a series of different galaxies pass that distance, later galaxies would pass that distance at

5046-425: The future because light emitted by objects outside that limit could never reach the Earth. Note that, because the Hubble parameter 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. This future visibility limit is calculated at a comoving distance of 19 billion parsecs (62 billion light-years), assuming

5133-429: 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. This can be used to define a type of cosmic event horizon whose distance from the Earth changes over time. For 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

5220-433: The intervening universe in general also subtly changes the observed large-scale structure. The large-scale structure of the universe also looks different if only redshift is used to measure distances to galaxies. For example, galaxies behind a galaxy cluster are attracted to it and fall towards it, and so are blueshifted (compared to how they would be if there were no cluster). On the near side, objects are redshifted. Thus,

5307-672: The largest known structure in the universe at that time. In November 2013, astronomers discovered the Hercules–Corona Borealis Great Wall , an even bigger structure twice as large as the former. It was defined by the mapping of gamma-ray bursts . In 2021, the American Astronomical Society announced the detection of 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

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5394-439: The most general principles to the nature of the universe yielded a dynamic solution that conflicted with the then-prevalent notion of a static universe . In 1912, Vesto M. Slipher measured the first Doppler shift of a " spiral nebula " (the obsolete term for spiral galaxies) and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at

5481-593: The notion of the universe expanding at a calculable rate was first derived from general relativity equations in 1922 by Alexander Friedmann . Friedmann published a set of equations, now known as the Friedmann equations , showing that the universe might be expanding, and presenting the expansion speed if that were the case. Before Hubble, German astronomer Carl Wilhelm Wirtz had, in two publications dating 1922 and 1924, already deduced with his own data that galaxies that appeared smaller and dimmer had larger redshifts and thus that more distant galaxies recede faster from

5568-515: The observational basis for modern cosmology. The cosmological constant has regained attention in recent decades as a hypothetical explanation for dark energy . The discovery of the linear relationship between redshift and distance, coupled with a supposed linear relation between recessional velocity and redshift, yields a straightforward mathematical expression for Hubble's law as follows: v = H 0 D {\displaystyle v=H_{0}\,D} where Hubble's law

5655-417: The observed and emitted wavelengths respectively. The "redshift velocity" v rs is not so simply related to real velocity at larger velocities, however, and this terminology leads to confusion if interpreted as a real velocity. Next, the connection between redshift or redshift velocity and recessional velocity is discussed. Suppose R ( t ) is called the scale factor of the universe, and increases as

5742-449: The observer. Then Georges Lemaître , in a 1927 article, independently derived that the universe might be expanding, observed the proportionality between recessional velocity of, and distance to, distant bodies, and suggested an estimated value for the proportionality constant; this constant, when Edwin Hubble confirmed the existence of cosmic expansion and determined a more accurate value for it two years later, came to be known by his name as

5829-534: The position of galaxies in three dimensions, which involves combining location information about the galaxies with distance information from redshifts . Two years later, astronomers Roger G. Clowes and Luis E. Campusano discovered the Clowes–Campusano LQG , a 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. In April 2003, another large-scale structure

5916-421: The redshift of photon decoupling as z  =  1 091 .64 ± 0.47 , which implies that the scale factor at the time of photon decoupling would be 1 ⁄ 1092.64 . So if the matter that originally emitted the oldest CMBR photons 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. The light-travel distance to

6003-552: The spectra of light from quasars , which are interpreted as indicating the existence of huge thin sheets of intergalactic (mostly hydrogen ) gas. These sheets appear to collapse into filaments, which can feed galaxies as they grow where filaments either cross or are dense. An 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

6090-414: The speed of light, at rates estimated by Hubble's law . The expansion rate appears to be accelerating , which dark energy was proposed to explain. Assuming dark energy remains constant (an unchanging 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

6177-400: The static solution he previously considered the correct state of the universe. The Einstein equations in their simplest form model either an expanding or contracting universe, so Einstein introduced the constant to counter expansion or contraction and lead to a static and flat universe. After Hubble's discovery that the universe was, in fact, expanding, Einstein called his faulty assumption that

6264-476: The surface of last scattering for neutrinos and gravitational waves . Hubble%27s law#Interpretation Hubble's law , also known as the Hubble–Lemaître law , is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving away. For this purpose, the recessional velocity of

6351-424: The time it was highly controversial whether or not these nebulae were "island universes" outside the Milky Way galaxy. In 1922, Alexander Friedmann derived his Friedmann equations from Einstein field equations , showing that the universe might expand at a rate calculable by the equations. The parameter used by Friedmann is known today as the scale factor and can be considered as a scale invariant form of

6438-476: The time the light was emitted. For example, the cosmic microwave background radiation that we see right now was emitted at the time of photon decoupling , estimated to have occurred about 380,000 years after the Big Bang, which occurred around 13.8 billion years ago. This 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. To estimate

6525-552: The universe expands in a manner that depends upon the cosmological model selected. Its meaning is that all measured proper distances D ( t ) between co-moving points increase proportionally to R . (The co-moving points are not moving relative to their local environments.) In other words: D ( t ) D ( t 0 ) = R ( t ) R ( t 0 ) , {\displaystyle {\frac {D(t)}{D(t_{0})}}={\frac {R(t)}{R(t_{0})}},} where t 0

6612-481: The universe is static his "greatest mistake". On its own, general relativity could predict the expansion of the universe, which (through observations such as the bending of light by large masses , or the precession of the orbit of Mercury ) could be experimentally observed and compared to his theoretical calculations using particular solutions of the equations he had originally formulated. In 1931, Einstein went to Mount Wilson Observatory to thank Hubble for providing

6699-509: 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). In principle, more galaxies will become observable in the future; in practice, an increasing number of galaxies will become extremely redshifted due to ongoing expansion, so much so that they will seem to disappear from view and become invisible. A galaxy at

6786-520: The universe's expansion is accelerating, all currently observable objects, outside the local supercluster , will eventually appear to freeze in time, while emitting progressively redder and fainter light. For instance, objects with the current redshift z from 5 to 10 will only be observable up to an age of 4–6 billion years. In addition, light emitted by objects currently situated beyond a certain comoving distance (currently about 19 gigaparsecs (62 Gly)) will never reach Earth. The universe's size

6873-533: The universe. It 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. Bielewicz et al. claim to establish a lower bound of 27.9 gigaparsecs (91 billion light-years) on the diameter of the last scattering surface. This value is based on matching-circle analysis of the WMAP 7-year data. This approach has been disputed. The comoving distance from Earth to

6960-403: The universe. The most famous horizon is the particle horizon which sets a limit on the precise distance that can be seen due to the finite age of the universe . Additional horizons are associated with the possible future extent of observations, larger than the particle horizon owing to the expansion of space , an "optical horizon" at the surface of last scattering , and associated horizons with

7047-431: The various wavelength bands of electromagnetic radiation (in particular 21-cm emission ) have yielded much information on the content and character of the universe 's structure. The organization of structure appears to follow a hierarchical model with organization up to the scale of superclusters and filaments . Larger than this (at scales between 30 and 200 megaparsecs), there seems to be no continued structure,

7134-511: The world's most powerful telescope at the time. His observations of Cepheid variable stars in "spiral nebulae" enabled him to calculate the distances to these objects. Surprisingly, these objects were discovered to be at distances which placed them well outside the Milky Way. They continued to be called nebulae , and it was only gradually that the term galaxies replaced it. The parameters that appear in Hubble's law, velocities and distances, are not directly measured. In reality we determine, say,

7221-530: Was considerable scatter (now known to be caused by peculiar velocities —the 'Hubble flow' is used to refer to the region of space far enough out that the recession velocity is larger than local peculiar velocities), Hubble was able to plot a trend line from the 46 galaxies he studied and obtain a value for the Hubble constant of 500 (km/s)/Mpc (much higher than the currently accepted value due to errors in his distance calibrations; see cosmic distance ladder for details). Hubble's law can be easily depicted in

7308-463: Was discovered, the Giant Void , which measures 1.3 billion light-years across. Based on redshift survey data, in 1989 Margaret Geller and John Huchra discovered the " Great Wall ", a sheet of galaxies more than 500 million light-years long and 200 million light-years wide, but only 15 million light-years thick. The existence of this structure escaped notice for so long because it requires locating

7395-478: Was discovered, the Sloan Great Wall . In August 2007, a possible supervoid was detected in the constellation Eridanus . It coincides with the ' CMB cold spot ', a cold region in the microwave sky that is highly improbable under the currently favored cosmological model. This 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

7482-450: Was found to have a redshift of 8.2, which indicates that the collapsing star that caused it exploded when the universe was only 630 million years old. The 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. This would be the "light travel distance" (see Distance measures (cosmology) ) rather than

7569-417: Was the first identification of a large-scale structure, and has expanded the information about the known grouping of matter in the universe. In 1987, Robert Brent Tully identified the Pisces–Cetus Supercluster Complex , the galaxy filament in which the Milky Way resides. It is about 1 billion light-years across. That same year, an unusually large region with a much lower than average distribution of galaxies

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