Observable universe
Encyclopedia
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 photon
s 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 wave
s (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 parsec
s (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), 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-year
s distance. The diameter
of the observable universe is estimated to be about 28 billion parsecs (93 billion light-year
s), putting the edge of the observable universe at about 46–47 billion light-years away.
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). 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.
Though in principle more galaxies will become observable in the future, in practice an increasing number of galaxies will become extremely redshift
ed due to ongoing expansion, so much so that they will seem to disappear from view and become invisible. 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), 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.
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.
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 claims to establish a lower bound of 24 gigaparsec
s (78 billion
light-year
s) 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.
from Earth to the edge of the observable universe is about 14 billion
parsec
s (46 billion, or 4.6 × 1010, light years) in any direction. The observable universe is thus a sphere with a diameter
of about 29 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, which occurred around 13.7 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 us. 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
. WMAP seven-year results give the redshift of photon decoupling as z=1090.89 which implies that the scale factor at the time of photon decoupling would be 
. 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.
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 supercluster
s and filament
s. Larger than this, there seems to be no continued structure, a phenomenon which 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. Star
s are organized into galaxies
, which in turn form clusters and supercluster
s that are separated by immense void
s, 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-year
s 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 redshift
s.
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. 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 supercluster
s 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.
is homogenized and isotropized as per the Cosmological Principle
. The supercluster
s 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 survey
s 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 which appear in the spectral line
s of light from quasar
s, 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.
, which affects the motion of galaxies over a region hundreds of millions of light-years across. These galaxies are all redshift
ed, 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
constellation
s. 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.
s (30 sextillion to a septillion stars), organized in more than 80 billion galaxies
, which themselves form clusters and supercluster
s.
Two approximate calculations give the number of atom
s 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, although the figure has not been updated since 2005 and a number of new estimates of the Hubble parameter have been made since then. The present value of the Hubble parameter
is important because it is related to the value of the critical density at the present, 
, by the equation
where G is the gravitational constant
. WMAP seven-year results from 2010 estimate the value of the
at 70.4 (km/s)/Mpc 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
, so if we make the simplifying assumption that all the atoms are hydrogen atom
s (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.
, 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. 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.
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
, (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. 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.
calculated the mass of an observable steady-state universe using the formula:
which can also be stated as
or approximately 8 × 1052 kg.
Here H = Hubble constant, ρ = Hoyle's value for the density, G = gravitational constant
and c = speed of light
.
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. 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), 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, 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. The galaxy's light now observable on Earth would have begun to emanate from its source about 750 million years after the Big Bang
.
s 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, so its distance at the present epoch defines the size of the observable universe. 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
that has passed since the Big Bang
, times the speed of light
. The quantity 
is given by,
where
is the scale factor of the Friedmann–Lemaître–Robertson–Walker metric, and we have taken the Big Bang to be at 
. In other words, the particle horizon recedes constantly as time passes, and the observed fraction of the universe always increases. Since proper distance at a given time is just comoving distance times the scale factor (with comoving distance normally defined to be equal to proper distance at the present time, so 
at present), the proper distance to the particle horizon at time 
is given by
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. At present, this cosmic event horizon is thought to be at a comoving distance of about 16 billion light years. In general, the proper distance to the event horizon at time
is given by
where
is the time-coordinate of the end of the universe, which would be infinite in the case of a universe that expands forever.
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
cosmology
Cosmology
Cosmology is the discipline that deals with the nature of the Universe as a whole. Cosmologists seek to understand the origin, evolution, structure, and ultimate fate of the Universe at large, as well as the natural laws that keep it in order...
, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
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
Cosmological Principle
In modern physical cosmology, the cosmological principle is the working assumption that observers on Earth do not occupy an unusual or privileged location within the universe as a whole, judged as observers of the physical phenomena produced by uniform and universal laws of physics...
, 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
Ball (mathematics)
In mathematics, a ball is the space inside a sphere. It may be a closed ball or an open 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
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...
actually permits detection of radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
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
Recombination (cosmology)
In cosmology, recombination refers to the epoch at which charged electrons and protons first became bound to form electrically neutral hydrogen atoms.Note that the term recombination is a misnomer, considering that it represents the first time that electrically neutral hydrogen formed. After the...
epoch
Epoch (astronomy)
In astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as celestial coordinates, or elliptical orbital elements of a celestial body, where these are subject to perturbations and vary with time...
, which is when particles were first able to emit photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s that were not quickly re-absorbed by other particles, before which the Universe was filled with a plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
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
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....
(CMBR). However, it may be possible in the future to observe the still older neutrino background, or even more distant events via gravitational wave
Gravitational wave
In physics, gravitational waves are theoretical ripples in the curvature of spacetime which propagates as a wave, traveling outward from the source. Predicted to exist by Albert Einstein in 1916 on the basis of his theory of general relativity, gravitational waves theoretically transport energy as...
s (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
Inflationary epoch
In physical cosmology the inflationary epoch was the period in the evolution of the early universe when, according to inflation theory, the universe underwent an extremely rapid exponential expansion...
in modern cosmology). The current comoving distance
Comoving distance
In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects...
to the particles which emitted the CMBR, representing the radius of the visible universe, is calculated to be about 14.0 billion parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s (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), about 2% larger.
The age of the universe
Age of the universe
The age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model...
is about 13.75 billion years, but due to the expansion of space
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting 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
Comoving distance
In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects...
at the present time) than a static 13.75 billion light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s distance. The diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
of the observable universe is estimated to be about 28 billion parsecs (93 billion light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s), putting the edge of the observable universe at about 46–47 billion light-years away.
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 lawHubble's law
Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...
regions sufficiently distant from us are expanding away from us much faster than the speed of light (special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
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
Accelerating universe
The accelerating universe is the observation that the universe appears to be expanding at an increasing rate, which in formal terms means that the cosmic scale factor a has a positive second derivative, implying that the velocity at which a given galaxy is receding from us should be continually...
due to dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...
. Assuming dark energy remains constant (an unchanging cosmological constant
Cosmological constant
In physical cosmology, the cosmological constant was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe...
), 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). This future visibility limit is calculated to be at a comoving distance
Comoving distance
In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects...
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.
Though in principle more galaxies will become observable in the future, in practice an increasing number of galaxies will become extremely redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
ed due to ongoing expansion, so much so that they will seem to disappear from view and become invisible. 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), 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
Event horizon
In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms it is defined as "the point of no return" i.e. the point at which the gravitational pull becomes so great as to make escape impossible. The most common case...
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.
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
Causality (physics)
Causality is the relationship between causes and effects. It is considered to be fundamental to all natural science, especially physics. Causality is also a topic studied from the perspectives of philosophy and statistics....
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
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
within our observable universe represent only a minuscule fraction of the galaxies in the universe. According to the theory of cosmic inflation
Cosmic inflation
In physical cosmology, cosmic inflation, cosmological inflation or just inflation is the theorized extremely rapid exponential expansion of the early universe by a factor of at least 1078 in volume, driven by a negative-pressure vacuum energy density. The inflationary epoch comprises the first part...
and its founder, Alan Guth
Alan Guth
Alan Harvey Guth is an American theoretical physicist and cosmologist. Guth has researched elementary particle theory...
, 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.
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 claims to establish a lower bound of 24 gigaparsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s (78 billion
1000000000 (number)
1,000,000,000 is the natural number following 999,999,999 and preceding 1,000,000,001.In scientific notation, it is written as 109....
light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s) 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.
Size
The comoving distanceComoving distance
In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects...
from Earth to the edge of the observable universe is about 14 billion
1000000000 (number)
1,000,000,000 is the natural number following 999,999,999 and preceding 1,000,000,001.In scientific notation, it is written as 109....
parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s (46 billion, or 4.6 × 1010, light years) in any direction. The observable universe is thus a sphere with a diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
of about 29 billion parsecs (93 billion, or 9.3 × 1010, light years). Assuming that space is roughly flat
Euclidean space
In mathematics, Euclidean space is the Euclidean plane and three-dimensional space of Euclidean geometry, as well as the generalizations of these notions to higher dimensions...
, 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, which occurred around 13.7 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 us. 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
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
of z, then the scale factor at the time the light was originally emitted is given by the equation
. WMAP seven-year results give the redshift of photon decoupling as z=1090.89 which implies that the scale factor at the time of photon decoupling would be . 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.Misconceptions
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 universeAge of the universeThe age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model...
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, spacetimeSpacetimeIn physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...
is highly curved on cosmological scales, which means that 3-spaceComoving distanceIn standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects...
(which is roughly flat) is expandingMetric expansion of spaceThe metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...
, as evidenced by Hubble's lawHubble's lawHubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...
. Distances obtained as the speed of light multiplied by a cosmological time interval have no direct physical significance. - 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. For an analysis of this claim and the paper that prompted it, see.
- 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 topologyTopologyTopology is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing...
(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 radiationCosmic microwave background radiationIn cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....
(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. 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 WMAPWilkinson Microwave Anisotropy ProbeThe Wilkinson Microwave Anisotropy Probe — also known as the Microwave Anisotropy Probe , and Explorer 80 — is a spacecraft which measures differences in the temperature of the Big Bang's remnant radiant heat — the Cosmic Microwave Background Radiation — across the full sky. Headed by Professor...
's extended mission and from Planck), we will be able to search for smaller circles and extend the limit to ~28 Gpc." 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. 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), the doubled figure is incorrect. This figure was very widely reported. 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 magazineDiscover (magazine)Discover is an American science magazine that publishes articles about science for a general audience. The monthly magazine was launched in October 1980 by Time Inc. It was sold to Family Media, the owners of Health, in 1987. Walt Disney Company bought the magazine when Family Media went out of...
, 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." - 180 billion light-years. This estimate accompanied the age estimate of 15.8 billion years in some sources; it was obtained by adding 15% to the figure of 156 billion light years.
Large-scale structure
Sky surveysRedshift survey
In astronomy, a redshift survey, or galaxy survey, is a survey of a section of the sky to measure the redshift of astronomical objects. Using Hubble's law, the redshift can be used to calculate the distance of an object from Earth. By combining redshift with angular position data, a redshift...
and mappings of the various wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
bands of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
(in particular 21-cm emission
Hydrogen line
The hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 MHz, which is equivalent to the vacuum...
) have yielded much information on the content and character of the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
's structure. The organization of structure appears to follow as a hierarchical
Hierarchy
A hierarchy is an arrangement of items in which the items are represented as being "above," "below," or "at the same level as" one another...
model with organization up to the scale
Scale (spatial)
Spatial scale provides a "shorthand" form for discussing relative lengths, areas, distances and sizes. A microclimate, for instance, is one which might occur in a mountain valley or near a lakeshore, whereas a megatrend is one which involves the whole planet....
of supercluster
Supercluster
Superclusters are large groups of smaller galaxy groups and clusters and are among the largest known structures of the cosmos. They are so large that they are not gravitationally bound and, consequently, partake in the Hubble expansion.-Existence:...
s and filament
Galaxy filament
In physical cosmology, galaxy filaments, also called supercluster complexes or great walls, are, so far, the largest known cosmic structures in the universe. They are massive, thread-like structures with a typical length of 50 to 80 megaparsecs h-1 that form the boundaries between large voids in...
s. 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
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
on that scale. Star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s are organized into galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
, which in turn form clusters and supercluster
Supercluster
Superclusters are large groups of smaller galaxy groups and clusters and are among the largest known structures of the cosmos. They are so large that they are not gravitationally bound and, consequently, partake in the Hubble expansion.-Existence:...
s that are separated by immense void
Void (astronomy)
In astronomy, voids are the empty spaces between filaments, the largest-scale structures in the Universe, that contain very few, or no, galaxies. They were first discovered in 1978 during a pioneering study by Stephen Gregory and Laird A. Thompson at the Kitt Peak National Observatory...
s, 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
Redshift survey
In astronomy, a redshift survey, or galaxy survey, is a survey of a section of the sky to measure the redshift of astronomical objects. Using Hubble's law, the redshift can be used to calculate the distance of an object from Earth. By combining redshift with angular position data, a redshift...
data, in 1989 Margaret Geller
Margaret Geller
Margaret Joan Geller is an American astronomer and professor. She is a Senior Astronomer at the Smithsonian Astrophysical Observatory, and has written numerous articles and produced several award-winning scientific short films....
and John Huchra
John Huchra
John Peter Huchra [pronounced HUCK-rah] was an American astronomer and professor. He was the Vice Provost for Research Policy at Harvard University and a Professor of Astronomy at the Harvard-Smithsonian Center for Astrophysics. He was also a former chair of the United States National Committee...
discovered the "Great Wall
Great Wall (astronomy)
The Great Wall , sometimes specifically referred to as the CfA2 Great Wall, is one of the largest known super-structures in the Universe...
", a sheet of galaxies more than 500 million light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s 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 redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
s.
In April 2003, another large-scale structure was discovered, the Sloan Great Wall
Sloan Great Wall
The Sloan Great Wall is a cosmic structure formed by a giant wall of galaxies , and to the present day it is the largest known structure in the universe. Its discovery was announced on October 20, 2003 by J. Richard Gott III of Princeton University and Mario Jurić and their colleagues, based on...
. In August 2007, a possible supervoid was detected in the constellation Eridanus. It coincides with the 'WMAP Cold Spot
WMAP cold spot
The CMB Cold Spot or WMAP Cold Spot is a region of the sky seen in microwaves which analysis found to be unusually large and cold relative to the expected properties of the cosmic microwave background radiation...
', 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
Void (astronomy)
In astronomy, voids are the empty spaces between filaments, the largest-scale structures in the Universe, that contain very few, or no, galaxies. They were first discovered in 1978 during a pioneering study by Stephen Gregory and Laird A. Thompson at the Kitt Peak National Observatory...
separated by sheets and filaments of galaxies
Galaxy filament
In physical cosmology, galaxy filaments, also called supercluster complexes or great walls, are, so far, the largest known cosmic structures in the universe. They are massive, thread-like structures with a typical length of 50 to 80 megaparsecs h-1 that form the boundaries between large voids in...
, with the supercluster
Supercluster
Superclusters are large groups of smaller galaxy groups and clusters and are among the largest known structures of the cosmos. They are so large that they are not gravitationally bound and, consequently, partake in the Hubble expansion.-Existence:...
s appearing as occasional relatively dense nodes. This network is clearly visible in the 2dF Galaxy Redshift Survey
2dF Galaxy Redshift Survey
In astronomy, the 2dF Galaxy Redshift Survey , 2dF or 2dFGRS is a redshift survey conducted by the Anglo-Australian Observatory with the 3.9m Anglo-Australian Telescope between 1997 and 11 April 2002. The data from this survey were made public on 30 June 2003...
. 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
Sloan Great Wall
The Sloan Great Wall is a cosmic structure formed by a giant wall of galaxies , and to the present day it is the largest known structure in the universe. Its discovery was announced on October 20, 2003 by J. Richard Gott III of Princeton University and Mario Jurić and their colleagues, based on...
, 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 universeUniverse
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
is homogenized and isotropized as per the Cosmological Principle
Cosmological Principle
In modern physical cosmology, the cosmological principle is the working assumption that observers on Earth do not occupy an unusual or privileged location within the universe as a whole, judged as observers of the physical phenomena produced by uniform and universal laws of physics...
. The supercluster
Supercluster
Superclusters are large groups of smaller galaxy groups and clusters and are among the largest known structures of the cosmos. They are so large that they are not gravitationally bound and, consequently, partake in the Hubble expansion.-Existence:...
s and filaments
Galaxy filament
In physical cosmology, galaxy filaments, also called supercluster complexes or great walls, are, so far, the largest known cosmic structures in the universe. They are massive, thread-like structures with a typical length of 50 to 80 megaparsecs h-1 that form the boundaries between large voids in...
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 survey
Redshift survey
In astronomy, a redshift survey, or galaxy survey, is a survey of a section of the sky to measure the redshift of astronomical objects. Using Hubble's law, the redshift can be used to calculate the distance of an object from Earth. By combining redshift with angular position data, a redshift...
s of the 1990s were completed that this scale could accurately be observed.
Observations
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
s of light from quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s, which are interpreted as indicating the existence of huge thin sheets of intergalactic (mostly hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
) 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 lens
A gravitational lens refers to a distribution of matter between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer...
(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
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
), deflecting light rays that pass nearby. Rather usefully, strong gravitational lensing can sometimes magnify distant galaxies, making them easier to detect. Weak lensing
Weak gravitational lensing
While the presence of any mass bends the path of light passing near it, this effect rarely produces the giant arcs and multiple images associated with strong gravitational lensing. Most lines of sight in the universe are thoroughly in the weak lensing regime, in which the deflection is impossible...
(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
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
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
Fingers of God
Fingers of God is an effect in observational cosmology that causes clusters of galaxies to be elongated in redshift space, with an axis of elongation pointed toward the observer. It is caused by a Doppler shift associated with the peculiar velocities of galaxies in a cluster...
: 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 AttractorGreat Attractor
The Great Attractor is a gravity anomaly in intergalactic space within the range of the Centaurus Supercluster that reveals the existence of a localised concentration of mass equivalent to tens of thousands of Milky Ways, observable by its effect on the motion of galaxies and their associated...
, which affects the motion of galaxies over a region hundreds of millions of light-years across. These galaxies are all redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
ed, in accordance with Hubble's law
Hubble's law
Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...
, 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
Hydra (constellation)
Hydra is the largest of the 88 modern constellations, measuring 1303 square degrees. It has a long history, having been included among the 48 constellations listed by the 2nd century astronomer Ptolemy. It is commonly represented as a water snake...
and Centaurus
Centaurus
Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations.-Stars:...
constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
s. 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
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
R. Brent Tully
R. Brent Tully
R. Brent Tully is an astronomer at the Institute for Astronomy in Honolulu, Hawaii.Tully's specialty is astrophysics of galaxies. He, along with J. Richard Fisher, proposed the now-famous Tully-Fisher relation in a paper, A New Method of Determining Distances to Galaxies, published in Astronomy...
of the University of Hawaii
University of Hawaii
The University of Hawaii System, formally the University of Hawaii and popularly known as UH, is a public, co-educational college and university system that confers associate, bachelor, master, and doctoral degrees through three university campuses, seven community college campuses, an employment...
’s Institute of Astronomy identified what he called the Pisces-Cetus Supercluster Complex
Pisces-Cetus Supercluster Complex
The Pisces-Cetus Supercluster Complex is a complex of galaxy superclusters or galaxy filament that includes the Virgo Supercluster .-Discovery:...
, a structure one billion light years long and 150 million light years across in which, he claimed, the Local Supercluster was embedded.
Matter content
The observable universe contains about 3 to 100 × 1022 starStar
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s (30 sextillion to a septillion stars), organized in more than 80 billion galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
, which themselves form clusters and supercluster
Supercluster
Superclusters are large groups of smaller galaxy groups and clusters and are among the largest known structures of the cosmos. They are so large that they are not gravitationally bound and, consequently, partake in the Hubble expansion.-Existence:...
s.
Two approximate calculations give the number of atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s in the observable universe to be close to 1080.
1. Observations of the cosmic microwave background from the Wilkinson Microwave Anisotropy Probe
Wilkinson Microwave Anisotropy Probe
The Wilkinson Microwave Anisotropy Probe — also known as the Microwave Anisotropy Probe , and Explorer 80 — is a spacecraft which measures differences in the temperature of the Big Bang's remnant radiant heat — the Cosmic Microwave Background Radiation — across the full sky. Headed by Professor...
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, although the figure has not been updated since 2005 and a number of new estimates of the Hubble parameter have been made since then. The present value of the Hubble parameter
is important because it is related to the value of the critical density at the present, , by the equationwhere G is the gravitational constant
Gravitational constant
The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...
. WMAP seven-year results from 2010 estimate the value of the
at 70.4 (km/s)/Mpc 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
Cold dark matter
Cold dark matter is the improvement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation and whose constituent particles move slowly...
and 73% is thought to be dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...
, so if we make the simplifying assumption that all the atoms are hydrogen atom
Hydrogen atom
A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively-charged proton and a single negatively-charged electron bound to the nucleus by the Coulomb force...
s (which in reality make up about 74% of all atoms in our galaxy by mass, see Abundance of the chemical elements
Abundance of the chemical elements
The abundance of a chemical element measures how relatively common the element is, or how much of the element is present in a given environment by comparison to all other 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
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
has a mass of about 2×1030 kg, which is about 1×1057 atoms of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
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.
Mass
Some care is required in defining what is meant by the total mass of the observable universe. In relativity, mass and energy are equivalentMass-energy equivalence
In physics, mass–energy equivalence is the concept that the mass of a body is a measure of its energy content. In this concept, mass is a property of all energy, and energy is a property of all mass, and the two properties are connected by a constant...
, 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. One can, however, derive an order-of-magnitude estimate of the mass due to sources other than gravity, namely visible matter, dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
and dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...
, based on the volume of the observable universe and the mean density.
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, 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%.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"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
, (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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
(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. 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
Lambda-CDM model
ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter, which is also known as the cold dark matter model with dark energy...
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 HoyleFred Hoyle
Sir Fred Hoyle FRS was an English astronomer and mathematician noted primarily for his contribution to the theory of stellar nucleosynthesis and his often controversial stance on other cosmological and scientific matters—in particular his rejection of the "Big Bang" theory, a term originally...
calculated the mass of an observable steady-state universe using the formula:
which can also be stated as
or approximately 8 × 1052 kg.
Here H = Hubble constant, ρ = Hoyle's value for the density, G = gravitational constant
Gravitational constant
The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...
and c = speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
.
Most distant objects
The most distant astronomical objectAstronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...
yet announced as of January 2011 is a galaxy candidate classified UDFj-39546284
UDFj-39546284
UDFj-39546284 is a compact galaxy of blue stars that existed as we see it 13.2 billion years ago, around 480 million years after the Big Bang. It is the oldest galaxy found and exceeds the previous distance record holder by roughly 150 million years. It could remain so until the anticipated launch...
. In 2009, a gamma ray burst
Gamma ray burst
Gamma-ray bursts are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical...
, GRB 090423
GRB 090423
GRB 090423 is a gamma-ray burst detected by the Swift Gamma-Ray Burst Mission on April 23, 2009 at 07:55:19 UTC. The afterglow of GRB 090423 was detected in the infrared, and allowed astronomers to determine that the redshift of GRB 090423 is z = 8.2, which makes GRB 090423 the second...
, was found to have a redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
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), though this would be the "light travel distance" (see Distance measures (cosmology)
Distance measures (cosmology)
Distance measures are used in physical cosmology to give a natural notion of the distance between two objects or events in the universe. They are often used to tie some observable quantity to another quantity that is not directly...
) rather than the "proper distance" used in both Hubble's law
Hubble's law
Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...
and in defining the size of the observable universe (cosmologist Ned Wright
Edward L. Wright
Edward L. Wright is an American astrophysicist and cosmologist, well known for his achievements in the COBE project and as a strong Big Bang proponent in web tutorials on cosmology and theory of relativity....
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, or about 30 billion light years. Another record-holder for most distant object is a galaxy observed through and located beyond Abell 2218
Abell 2218
Abell 2218 is a cluster of galaxies about 2 billion light-years away in the constellation Draco.Acting as a powerful lens, it magnifies and distorts all galaxies lying behind the cluster core into long arcs. The lensed galaxies are all stretched along the cluster's center and some of them are...
, 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. The galaxy's light now observable on Earth would have begun to emanate from its source about 750 million years after the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
.
Particle horizon
The particle horizon is the maximum distance from which particleElementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
s could have traveled to the observer
Observation
Observation is either an activity of a living being, such as a human, consisting of receiving knowledge of the outside world through the senses, or the recording of data using scientific instruments. The term may also refer to any data collected during this activity...
in the age of the universe
Age of the universe
The age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model...
. It represents the boundary between the observable and the unobservable regions of the universe, so its distance at the present epoch defines the size of the observable universe. The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model being discussed.
In terms of comoving distance
Comoving distance
In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects...
, the particle horizon is equal to the conformal time
that has passed since the Big BangBig Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
, times the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
. The quantity is given by,where
is the scale factor of the Friedmann–Lemaître–Robertson–Walker metric, and we have taken the Big Bang to be at . In other words, the particle horizon recedes constantly as time passes, and the observed fraction of the universe always increases. Since proper distance at a given time is just comoving distance times the scale factor (with comoving distance normally defined to be equal to proper distance at the present time, so at present), the proper distance to the particle horizon at time is given byThe particle horizon differs from the cosmic event horizon
Event horizon
In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms it is defined as "the point of no return" i.e. the point at which the gravitational pull becomes so great as to make escape impossible. The most common case...
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. At present, this cosmic event horizon is thought to be at a comoving distance of about 16 billion light years. In general, the proper distance to the event horizon at time
is given bywhere
is the time-coordinate of the end of the universe, which would be infinite in the case of a universe that expands forever.See also
- Causality (physics)Causality (physics)Causality is the relationship between causes and effects. It is considered to be fundamental to all natural science, especially physics. Causality is also a topic studied from the perspectives of philosophy and statistics....
- Dark flowDark flowDark flow is an astrophysical term describing a peculiar velocity of galaxy clusters. The actual measured velocity is the sum of the velocity predicted by Hubble's Law plus a small and unexplained velocity flowing in a common direction....
- Event horizon of the universe
- Hubble volumeHubble volumeIn cosmology, the Hubble volume, or Hubble sphere, is the region of the Universe surrounding an observer beyond which objects recede from the observer at a rate greater than the speed of light, due to the expansion of the Universe....
- MultiverseMultiverseThe multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...
- OmniverseOmniverseThe Omniverse is the conceptual ensemble of all possible universes, with all possible laws of physics.In this physical cosmology context, the limitation of the definition of "universe" that it has only one set of "physical laws and constants that govern them," is expanded to include multiple sets...
External links
- "Millennium Simulation" of structure forming Max Planck Institute of Astrophysics, Garching, Germany
- The Sloan Great Wall: Largest Known Structure? on APOD
- Cosmology FAQ
- Forming Galaxies Captured In The Young Universe By Hubble, VLT & Spitzer
- NASA featured Images and Galleries
- Star Survey reaches 70 sextillion
- Animation of the cosmic light horizon
- Inflation and the Cosmic Microwave Background by Charles Lineweaver
- Logarithmic Maps of the Universe
- List of publications of the 2dF Galaxy Redshift Survey
- List of publications of the 6dF Galaxy Redshift and peculiar velocity survey
- The Universe Within 14 Billion Light Years—NASA Atlas of the Universe (note—this map only gives a rough cosmographical estimate of the expected distribution of superclusters within the observable universe; very little actual mapping has been done beyond a distance of one billion light years):
- Video: "The Known Universe", from the American Museum of Natural History
- NASA/IPAC Extragalactic Database