Observable universe

Observable universe
Visualization of the 93 billion light year – or 28 billion parsec – three-dimensional observable universe. The scale is such that the fine grains of light 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 in the image.

In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light (or other signals) from those objects has had time to reach us since the beginning of the cosmological expansion. Assuming the universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction—that is, the observable universe is a spherical volume (a ball) centered on the observer, regardless of the shape of the universe as a whole. Every location in the universe has its own observable universe which may or may not overlap with the one centered on the Earth.

The word observable used in this sense does not depend on whether modern technology actually permits detection of radiation from an object in this region (or indeed on whether there is any radiation to detect). It simply indicates that it is possible in principle for light or other signals from the object to reach an observer on Earth. In practice, we can see light only from as far back as the time of photon decoupling in the recombination epoch, which is when particles were first able to emit photons that were not quickly re-absorbed by other particles, before which the Universe was filled with a plasma opaque to photons. The collection of points in space at just the right distance so that photons emitted at the time of photon decoupling would be reaching us today form the surface of last scattering, and the photons emitted at the surface of last scattering are the ones we detect today as the cosmic microwave background radiation (CMBR). However, it may be possible in the future to observe the still older neutrino background, or even more distant events via gravitational waves (which also move at the speed of light). Sometimes a distinction is made between the visible universe, which includes only signals emitted since recombination, and the observable universe, which includes signals since the beginning of the cosmological expansion (the Big Bang in traditional cosmology, the end of the inflationary epoch in modern cosmology). The current comoving distance to the particles which emitted the CMBR, representing the radius of the visible universe, is calculated to be about 14.0 billion parsecs (about 45.7 billion light years), while the current comoving distance to the edge of the observable universe is calculated to be 14.3 billion parsecs (about 46.6 billion light years),[1] about 2% larger.

The age of the universe is about 13.75 billion years, but due to the expansion of space we are observing objects that were originally much closer but are now considerably farther away (as defined in terms of cosmological proper distance, which is equal to the comoving distance at the present time) than a static 13.75 billion light-years distance.[2] The diameter of the observable universe is estimated to be about 28 billion parsecs (93 billion light-years),[3] putting the edge of the observable universe at about 46–47 billion light-years away.[4][5]


The universe versus the observable universe

Some parts of the universe may simply be too far away for the light emitted from there at any moment since the Big Bang to have had enough time to reach Earth at present, so these portions of the universe would currently lie outside the observable universe. In the future the light from distant galaxies will have had more time to travel, so some regions not currently observable will become observable in the future. However, due to Hubble's law regions sufficiently distant from us are expanding away from us much faster than the speed of light (special relativity prevents nearby objects in the same local region from moving faster than the speed of light with respect to each other, but there is no such constraint for distant objects when the space between them is expanding; see uses of the proper distance for a discussion), and the expansion rate appears to be accelerating due to dark energy. 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 our observable universe at any time in the infinite future, because light emitted by objects outside that limit can never reach points that are expanding away from us at less than the speed of light (a subtlety here is that because the Hubble parameter is decreasing with time, there can be cases where a galaxy that is receding from us just a bit faster than light does manage to emit a signal which reaches us eventually[5][6]). This future visibility limit is calculated to be at a comoving distance of 19 billion parsecs (62 billion light years), which implies the number of galaxies that we can ever theoretically observe in the infinite future (leaving aside the issue that some may be impossible to observe in practice due to redshift, as discussed in the following paragraph) is only larger than the number currently observable by a factor of 2.36.[1][7]

Though 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.[8][9][10] An additional subtlety is that a 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 past history (say, a signal sent from the galaxy only 500 million years after the Big Bang), but because of the universe's expansion, there may be some later age at which a signal sent from the same galaxy will never be able to reach us 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),[11] even though it remains at the same comoving distance (comoving distance is defined to be constant with time, unlike proper distance which is used to define recession velocity due to the expansion of space) which is less than the comoving radius of the observable universe. This fact can be used to define a type of cosmic event horizon whose distance from us 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 would eventually be able to reach us in the future if the event was less than 16 billion light years away, but the signal would never reach us if the event was more than 16 billion light years away.[5]

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 experimentation about any part of the universe that is causally disconnected from us, 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, though some models propose it could be finite but unbounded, like a higher-dimensional analogue of the 2D surface of a sphere which is finite in area but has no edge. It is plausible that the galaxies within our observable universe represent only a minuscule fraction of the galaxies in the universe. According to the theory of cosmic inflation and its founder, Alan Guth, if it is assumed that inflation began about 10−37 seconds after the Big Bang, then with the plausible assumption that the size of the universe at this time 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 1023 times larger than the size of the observable universe.[12]

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 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. A 2004 paper[13] claims to establish a lower bound of 24 gigaparsecs (78 billion light-years) on the diameter of the whole universe, meaning the smallest possible diameter for the whole universe would be only slightly smaller than the observable universe (and this is only a lower bound, so the whole universe could be much larger, even infinite). This value is based on matching-circle analysis of the WMAP data; this approach has been disputed.[14]


The comoving distance from Earth to the edge of the observable universe is about 14 billion parsecs (46 billion, or 4.6 × 1010, light years) in any direction. The observable universe is thus a sphere with a diameter of about 28 billion parsecs (93 billion, or 9.3 × 1010, light years). Assuming that space is roughly flat, this size corresponds to a comoving volume of about 3.5 × 1080 cubic meters. This is equivalent to a volume of about 410 nonillion cubic light-years (4.1 × 1032 cubic light years).

The figures quoted above are distances now (in cosmological time), not distances at 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,[15][16] which occurred around 13.7 billion (1.37×1010) 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 us.[citation needed] To 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 which is used to model the expanding universe, if at the present time we receive light with a redshift of z, then the scale factor at the time the light was originally emitted is given by the equation \! a(t) = \frac{1}{1 + z}.[17][18] WMAP seven-year results give the redshift of photon decoupling as z=1090.89[15] which implies that the scale factor at the time of photon decoupling would be \! \frac{1}{1091.89}. So if the matter that originally emitted the oldest CMBR photons has a present distance of 46 billion light years, then at the time of decoupling when the photons were originally emitted, the distance would have been only about 42 million light-years away.


Many secondary sources have reported a wide variety of incorrect figures for the size of the visible universe. Some of these figures are listed below, with brief descriptions of possible reasons for misconceptions about them.

  • 13.7 billion light-years. The age of the universe is estimated to be 13.7 billion years. While it is commonly understood that nothing can accelerate to velocities equal to or greater than that of light, it is a common misconception that the radius of the observable universe must therefore amount to only 13.7 billion light-years. This reasoning makes sense only if the universe is the flat spacetime of special relativity; in the real universe, spacetime is highly curved on cosmological scales[citation needed], which means that 3-space (which is roughly flat) is expanding, as evidenced by Hubble's law. Distances obtained as the speed of light multiplied by a cosmological time interval have no direct physical significance.[19]
  • 15.8 billion light-years. This is obtained in the same way as the 13.7 billion light year figure, but starting from an incorrect age of the universe which was reported in the popular press in mid-2006.[20][21][22] For an analysis of this claim and the paper that prompted it, see.[23]
  • 27.4 billion light-years. This is a diameter obtained from the (incorrect) radius of 13.7 billion light-years.
  • 78 billion light-years. This is a lower bound for the diameter of the whole universe (not just the observable part), if we postulate that the universe is finite in size due to its having a nontrivial topology (as discussed in articles here and here), with this lower bound based on the estimated current distance between points that we can see on opposite sides of the cosmic microwave background radiation (CMBR). If the whole universe is smaller than this sphere, then light has had time to circumnavigate it since the big bang, producing multiple images of distant points in the CMBR, which would show up as patterns of repeating circles.[24] Cornish et al. looked for such an effect at scales of up to 24 gigaparsecs (78 billion light years) and failed to find it, and suggested that if they could extend their search to all possible orientations, they would then "be able to exclude the possibility that we live in a universe smaller than 24 Gpc in diameter". The authors also estimated that with "lower noise and higher resolution CMB maps (from WMAP's extended mission and from Planck), we will be able to search for smaller circles and extend the limit to ~28 Gpc."[13] This estimate of the maximum diameter of the CMBR sphere that will be visible in planned experiments corresponds to a radius of 14 gigaparsecs, or around 46 billion light years, about the same as the figure for the radius of the observable universe given in the opening section.
  • 156 billion light-years. This figure was obtained by doubling 78 billion light-years on the assumption that it is a radius.[25] Since 78 billion light-years is already a diameter (the original paper by Cornish et al. says 'By extending the search to all possible orientations, we will be able to exclude the possibility that we live in a universe smaller than 24 Gpc in diameter', and 24 Gpc is 78 billion light years),[13] the doubled figure is incorrect. This figure was very widely reported.[25][26][27] A press release from Montana State University – Bozeman, where Cornish works as an astrophysicist, noted the error when discussing a story that had appeared in Discover magazine, saying "Discover mistakenly reported that the universe was 156 billion light-years wide, thinking that 78 billion was the radius of the universe instead of its diameter."[28]
  • 180 billion light-years. This estimate accompanied the age estimate of 15.8 billion years in some sources;[29] it was obtained by adding 15% to the figure of 156 billion light years.

Large-scale structure

Sky surveys and mappings of 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 as a hierarchical model with organization up to the scale of superclusters and filaments. Larger than this, there seems to be no continued structure, a phenomenon which has been referred to as the End of Greatness.

Walls, filaments and voids

DTFE reconstruction of the inner parts of the 2dF Galaxy Redshift Survey

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 clusters and superclusters that are separated by immense voids, creating a vast foam-like structure sometimes called 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, 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 wide, but only 15 million light-years thick. The 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. In April 2003, another large-scale structure was discovered, the Sloan Great Wall. In August 2007, a possible supervoid was detected in the constellation Eridanus.[30] It coincides with the 'WMAP 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.

In more recent studies the universe appears as a collection of giant bubble-like voids separated by sheets and filaments of galaxies, with the superclusters appearing as occasional relatively dense nodes. This network is clearly visible in the 2dF Galaxy Redshift Survey. In the figure a 3-D reconstruction of the inner parts of the survey is shown, revealing an impressive view on the cosmic structures in the nearby universe. Several superclusters stand out, such as the Sloan Great Wall, the largest structure in the universe known to date.

End of Greatness

The End of Greatness is an observational scale discovered at roughly 100 Mpc (roughly 300 million lightyears) where the lumpiness seen in the large-scale structure of the universe is homogenized and isotropized as per the Cosmological Principle. 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.[31]


"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 um). 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)." [32]

Another indicator of large-scale structure is the 'Lyman alpha forest'. This is a collection of absorption lines which appear in the spectral lines of light from quasars, which are interpreted as indicating the existence of huge thin sheets of intergalactic (mostly hydrogen) gas. These sheets appear to be associated with the formation of new galaxies.

Some caution is required in describing structures on a cosmic scale because things are not always as they appear to be. Bending of light by gravitation (gravitational lensing) can result in images which appear to originate in a different direction from their real source. This is caused by foreground objects (such as galaxies) curving the space around themselves (as predicted by general relativity), deflecting light rays that pass nearby. Rather usefully, strong gravitational lensing can sometimes magnify distant galaxies, making them easier to detect. Weak lensing (gravitational shear) by the intervening universe in general also subtly changes the observed large-scale structure. In 2004, measurements of this subtle shear show considerable promise as a test of cosmological models.

The large-scale structure of the universe also looks different if one only uses redshift to measure distances to galaxies. For example, galaxies behind a galaxy cluster will be attracted to it, and so fall towards it, and so be slightly blueshifted (compared to how they would be if there were no cluster); on the near side, things are slightly redshifted. Thus, the environment of the cluster looks a bit squashed if using redshifts to measure distance. An opposite effect works on the galaxies already within the cluster: the galaxies have some random motion around the cluster centre, and when these random motions are converted to redshifts, the cluster will appear elongated. This creates what is known as a finger of God: the illusion of a long chain of galaxies pointed at the Earth.

Cosmography of our cosmic neighborhood

At the centre of the Hydra supercluster there is a gravitational anomaly, known as the Great Attractor, which 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, indicating 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.

The Great Attractor, discovered in 1986, lies at a distance of between 150 million and 250 million light-years (250 million is the most recent estimate), 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, and/or radiating large amounts of radio waves.

In 1987 Astronomer R. Brent Tully of 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 was embedded.[33][34]

Matter content

The observable universe contains about 3 to 100 × 1022 stars (30 sextillion to a septillion stars),[35][36][37][38] organized in more than 80 billion galaxies, which themselves form clusters and superclusters.[39]

Two approximate calculations give the number of atoms in the observable universe to be close to 1080.

1. Observations of the cosmic microwave background from the Wilkinson Microwave Anisotropy Probe suggest that the spatial curvature of the universe is very close to zero, which in current cosmological models implies that the value of the density parameter must be very close to a certain critical value. A NASA page gives this density, which includes dark energy, dark matter and ordinary matter all lumped together, as 9.9×10−27 kg/m3,[40] although the figure has not been updated since 2005[41] and a number of new estimates of the Hubble parameter have been made since then. The present value of the Hubble parameter H0 is important because it is related to the value of the critical density at the present, ρc, by the equation[42]

\rho_c = \frac{3H_0^2}{8 \pi G}

where G is the gravitational constant. WMAP seven-year results from 2010 estimate the value of the H0 at 70.4 (km/s)/Mpc[15] or 2.28×10−18 s−1, which gives a critical density of 9.30×10−27 kg/m3.

Analysis of the WMAP results suggests that only about 4.6% of the critical density is in the form of normal atoms, while 23% is thought to be made of cold dark matter and 73% is thought to be dark energy,[15] so if we make the simplifying assumption that all the atoms are hydrogen atoms (which in reality make up about 74% of all atoms in our galaxy by mass, see Abundance of the chemical elements) which each have a mass of about 1.67×10−27kg, this implies about 0.26 atoms/m3. Multiplying this by the volume of the visible universe (with a radius of 14 billion parsecs, the volume would be about 3.38×1080 m3) gives an estimate of about 8.8×1079 atoms in the visible universe, while multiplying it by the volume of the observable universe (with a radius of 14.3 billion parsecs, the volume would be about 3.60×1080 m3) gives an estimate of about 9.4×1079 atoms in the observable universe.

2. A typical star has a mass of about 2×1030 kg, which is about 1×1057 atoms of hydrogen per star. A typical galaxy has about 400 billion stars so that means each galaxy has 1×1057 × 4×1011 = 4×1068 hydrogen atoms. There are possibly 80 billion galaxies in the universe, so that means that there are about 4×1068 × 8×1010 = 3×1079 hydrogen atoms in the observable universe. But this is definitely a lower limit calculation, and it ignores many possible atom sources such as intergalactic gas.[43]


Some care is required in defining what is meant by the total mass of the observable universe. In relativity, mass and energy are equivalent, and energy can take on a variety of forms, including energy that is associated with the curvature of spacetime itself, not with its contents such as atoms and photons. Defining the total energy of a large region of curved spacetime is problematic because there is no single agreed-upon way to define the energy due to gravity (the energy associated with spacetime curvature); for example, when photons are redshifted due to the expansion of the universe, they lose energy, and some physicists would say the energy has been converted to gravitational energy while others would say the energy has simply been lost.[44] One can, however, derive an order-of-magnitude estimate of the mass due to sources other than gravity, namely visible matter, dark matter and dark energy, based on the volume of the observable universe and the mean density.[45]

Estimation based on critical density

As noted in the previous section, since the universe seems to be close to spatially flat, this suggests the density is close to the critical density, estimated above at 9.30×10−27 kg/m3. Multiplying this by (A) the estimated volume of the visible universe (3.38×1080 m3) gives a total mass for the visible universe of 3.14×1054 kg, while multiplying by (B) the estimated volume of the observable universe (3.60×1080 m3) gives a total mass for the observable universe of 3.35×1054 kg. The WMAP 7-year results estimate that 4.56% of the universe's mass is made up of normal atoms,[15] so this would give an estimate (A) of 1.43×1053 kg, or (B) 1.53×1053 kg, for all the atoms in the observable universe. The fraction of these atoms that make up stars is probably less than 10%.[46]

Estimation based on the measured stellar density

One way to calculate the mass of the visible matter which makes up the observable universe is to assume a mean stellar mass and to multiply that by an estimate of the number of stars in the observable universe, as seen in the paper 'On the Expansion of the Universe' from the Mathematical Thinking in Physics section of a former NASA educational site, the Glenn Learning Technologies Project. The paper derives its estimate of the number of stars in the Universe from its value for the volume of the "observable universe"

\frac{4}{3} \pi {S_\mathrm{horizon}}^3 = 9 \times 10^{30}\ \mathrm{ly}^3

Note however that this volume is not derived from the 46 billion light year radius given by most authors, but rather from the Hubble volume which is the volume of a sphere with radius equal to the Hubble length (the distance at which galaxies would currently be receding from us at the speed of light), which the paper gives as 13 billion light years. In any case, the paper combines this volume with an estimate of the average stellar density calculated from observations by the Hubble Space Telescope

\frac{5 \times 10^{21}\ \textrm{stars}}{4 \times 10^{30} \ \textrm{ly}^3} = 10^{-9} \ \textrm{stars}/\textrm{ly}^3 = 1\ \textrm{star} \ \textrm{per}\ \textrm{billion}\ \textrm{ly}^3 , (or 1 star per cube, 1,000 ly to a side (x,y,z))

yielding an estimate of the number of stars in the observable universe of 9 × 1021 stars (9 sextillion (short scale) stars).

Taking the mass of Sol (2 × 1030 kg) as the mean stellar mass (on the basis that the large population of dwarf stars balances out the population of stars whose mass is greater than Sol) and rounding the estimate of the number of stars up to 1022 yields a total mass for all the stars in the observable universe of 3 × 1052 kg.[47] However, aside from the issue that the calculation is based on the Hubble volume, as noted above the WMAP results in combination with the Lambda-CDM model predict that less than 5% of the total mass of the observable universe is made up of baryonic matter (atoms), the rest being made up of dark matter and dark energy, and it is also estimated that less than 10% of baryonic matter consists of stars.

Estimation based on steady-state universe

Sir Fred Hoyle calculated the mass of an observable steady-state universe using the formula:[48]

\frac{4}{3}\cdot \pi \cdot \rho \cdot \left(\frac{c}{H}\right)^3

which can also be stated as

\frac{c^3}{2GH} \

or approximately 8 × 1052 kg.

Here H = Hubble constant, ρ = Hoyle's value for the density, G = gravitational constant and c = speed of light.

Most distant objects

The most distant astronomical object yet announced as of January 2011 is a galaxy candidate classified UDFj-39546284. In 2009, a gamma ray burst, GRB 090423, 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.[49] The burst happened approximately 13 billion years ago,[50] 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),[49] though this would be the "light travel distance" (see Distance measures (cosmology)) rather than the "proper distance" used in both Hubble's law and in defining the size of the observable universe (cosmologist Ned Wright argues against the common use of light travel distance in astronomical press releases on this page, and at the bottom of the page offers online calculators that can be used to calculate the current proper distance to a distant object in a flat universe based on either the redshift z or the light travel time). The proper distance for a redshift of 8.2 would be about 9.2 Gpc,[51] or about 30 billion light years. Another record-holder for most distant object is a galaxy observed through and located beyond Abell 2218, also with a light travel distance of approximately 13 billion light years from Earth, with observations from the Hubble telescope indicating a redshift between 6.6 and 7.1, and observations from Keck telescopes indicating a redshift towards the upper end of this range, around 7.[52] The galaxy's light now observable on Earth would have begun to emanate from its source about 750 million years after the Big Bang.[53]

Particle horizon

The particle horizon is the maximum distance from which particles could have traveled to the observer in the age of the universe. It represents the boundary between the observable and the unobservable regions of the universe,[54] so its distance at the present epoch defines the size of the observable universe.[55] The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model being discussed.

In terms of comoving distance, the particle horizon is equal to the conformal time η0 that has passed since the Big Bang, times the speed of light c. The quantity η0 is given by,

\eta_0 = \int_{0}^{t_0} \frac{dt'}{a(t')}

where a(t) is the scale factor of the Friedmann–Lemaître–Robertson–Walker metric, and we have taken the Big Bang to be at t = 0. In other words, the particle horizon recedes constantly as time passes, and the observed fraction of the universe always increases.[54][56] Since proper distance at a given time is just comoving distance times the scale factor[2] (with comoving distance normally defined to be equal to proper distance at the present time, so a(t) = 1 at present), the proper distance to the particle horizon at time t0 is given by[57]

d_p(t_0) = a(t_0) \int_{0}^{t_0} \frac{dt'}{a(t')}

The particle horizon differs from the cosmic event horizon in that the particle horizon represents the largest comoving distance from which light could have reached the observer by a specific time, while the event horizon is the largest comoving distance from which light emitted now can ever reach the observer in the future.[58] At present, this cosmic event horizon is thought to be at a comoving distance of about 16 billion light years.[5] In general, the proper distance to the event horizon at time t0 is given by[57]

d_e(t_0) = a(t_0) \int_{t_0}^{t_{max}} \frac{dt'}{a(t')}

where tmax is the time-coordinate of the end of the universe, which would be infinite in the case of a universe that expands forever.

A diagram of our location in the observable universe. (Click here for larger image.)

See also


  1. ^ a b Gott III, J. Richard; Mario Jurić, David Schlegel, Fiona Hoyle, Michael Vogeley, Max Tegmark, Neta Bahcall, Jon Brinkmann (2005). "A Map of the Universe". The Astrophysics Journal 624 (2): 463. arXiv:astro-ph/0310571. Bibcode 2005ApJ...624..463G. doi:10.1086/428890. http://www.astro.princeton.edu/universe/ms.pdf. 
  2. ^ a b Davis, Tamara M.; Charles H. Lineweaver (2004). "Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe". Publications of the Astronomical Society of Australia 21 (1): 97. arXiv:astro-ph/0310808. Bibcode 2004PASA...21...97D. doi:10.1071/AS03040. 
  3. ^ Itzhak Bars; John Terning (November 2009). Extra Dimensions in Space and Time. Springer. pp. 27–. ISBN 9780387776378. http://books.google.com/books?id=fFSMatekilIC&pg=PA27. Retrieved 1 May 2011. 
  4. ^ Frequently Asked Questions in Cosmology. Astro.ucla.edu. Retrieved on 2011-05-01.
  5. ^ a b c d Lineweaver, Charles; Tamara M. Davis (2005). "Misconceptions about the Big Bang". Scientific American. http://space.mit.edu/~kcooksey/teaching/AY5/MisconceptionsabouttheBigBang_ScientificAmerican.pdf. Retrieved 2008-11-06. 
  6. ^ Is the universe expanding faster than the speed of light? (see the last two paragraphs)
  7. ^ The comoving distance of the future visibility limit is calculated on p. 8 of Gott et al.'s A Map of the Universe to be 4.50 times the Hubble radius, given as 4.220 billion parsecs (13.76 billion light years), whereas the current comoving radius of the observable universe is calculated on p. 7 to be 3.38 times the Hubble radius. The number of galaxies in a sphere of a given comoving radius is proportional to the cube of the radius, so as shown on p. 8 the ratio between the number of galaxies observable in the future visibility limit to the number of galaxies observable today would be (4.50/3.38)3 = 2.36.
  8. ^ Krauss, Lawrence M.; Robert J. Scherrer (2007). "The Return of a Static Universe and the End of Cosmology". General Relativity and Gravitation 39 (10): 1545–1550. arXiv:0704.0221. Bibcode 2007GReGr..39.1545K. doi:10.1007/s10714-007-0472-9. 
  9. ^ Using Tiny Particles To Answer Giant Questions. Science Friday, 3 Apr 2009. According to the transcript, Brian Greene makes the comment "And actually, in the far future, everything we now see, except for our local galaxy and a region of galaxies will have disappeared. The entire universe will disappear before our very eyes, and it's one of my arguments for actually funding cosmology. We've got to do it while we have a chance."
  10. ^ See also Faster than light#Universal_expansion and Future of an expanding universe#Galaxies outside the Local Supercluster are no longer detectable.
  11. ^ Loeb, Abraham (2002). "The Long-Term Future of Extragalactic Astronomy". Physical Review D 65 (4). arXiv:astro-ph/0107568. Bibcode 2002PhRvD..65d7301L. doi:10.1103/PhysRevD.65.047301. 
  12. ^ Alan H. Guth (17 March 1998). The inflationary universe: the quest for a new theory of cosmic origins. Basic Books. pp. 186–. ISBN 9780201328400. http://books.google.com/books?id=P2V1RbwvE1EC&pg=PA186. Retrieved 1 May 2011. 
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Further reading

External links

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