The observable universe is a spherical region of the Universe comprising all matter that can be observed from Earth at the present time because electromagnetic radiation from these objects has had time to reach Earth since the beginning of the cosmological expansion.

There are at least 2 trillion galaxies in the observable universe.

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 has a spherical volume (a ball) centered on the observer.

Every location in the Universe has its own observable universe, which may or may not overlap with the one centered on Earth.

The word observable in this sense does not refer to the capability of modern technology to detect light or other information from an object, or whether there is anything to be detected.

It refers to the physical limit created by the speed of light itself. Because no signals can travel faster than light, any object farther away from us than light could travel in the age of the Universe (estimated as of 2015 around 13.799±0.021 billion years) simply cannot be detected, as they have not reached us yet.

Sometimes astrophysicists distinguish 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 physical cosmology, the end of the inflationary epoch in modern cosmology).

According to calculations, the current comoving distance—proper distance, which takes into account that the universe has expanded since the light was emitted—to particles from which the cosmic microwave background radiation (CMBR) was emitted, which represent the radius of the visible universe, is about 14.0 billion parsecs (about 45.7 billion light-years), while the comoving distance to the edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light-years), about 2% larger.

The radius of the observable universe is therefore estimated to be about 46.5 billion light-years and its diameter about 28.5 gigaparsecs (93 billion light-years, 8.8×1023 kilometers or 5.5×1023 miles).

The total mass of ordinary matter in the universe can be calculated using the critical density and the diameter of the observable universe to be about 1.5 × 1053 kg. In November 2018, astronomers reported that the extragalactic background light (EBL) amounted to 4 × 1084 photons.

Since the expansion of the universe is known to accelerate and will become exponential in the future, the light emitted from all distant objects, past some time depending on their current redshift, will never reach the Earth.

In the future, all currently observable objects will slowly freeze in time while emitting progressively redder and fainter light. For instance, objects with the current redshift z from 5 to 10 will remain observable for no more than 4–6 billion years.

In addition, light emitted by objects currently situated beyond a certain comoving distance (currently about 19 billion parsecs) will never reach Earth.

The universe versus the observable universe

Some parts of the universe are too far away for the light emitted since the Big Bang to have had enough time to reach Earth, and so lie outside the observable universe.

In the future, light from distant galaxies will have had more time to travel, so additional regions will become observable.

However, due to Hubble’s law, regions sufficiently distant from the Earth are expanding away from it 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 furthermore 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 would never reach the Earth.

This future visibility limit is calculated at a comoving distance of 19 billion parsecs (62 billion light-years), assuming the universe will keep expanding forever, 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 redshifted 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, but because of the universe’s expansion, there may be some later age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future (so, for example, we might never see what the galaxy looked like 10 billion years after the Big Bang), even though it remains at the same comoving distance, 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 the Earth changes over time.

For example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening at present can eventually reach the Earth in the future if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is 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 the Earth, although many credible theories require a total universe much larger than the observable universe.

No evidence exists to suggest that the boundary of the observable universe constitutes a boundary on the universe as a whole, nor do any of the mainstream cosmological models propose that the universe has any physical boundary in the first place, though some models propose it could be finite but unbounded, like a higher-dimensional analog of the 2D surface of a sphere that 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 initially introduced by its founder, Alan Guth (and by D. Kazanas), 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 before the inflation occurred 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 3×1023 times the radius of the observable universe.

There are also lower estimates claiming that the entire universe is in excess of 250 times larger than the observable universe and also higher estimates implying that the universe is at least 101010122 times larger than 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.

Bielewicz et al. claim to establish a lower bound of 27.9 gigaparsecs (91 billion light-years) on the diameter of the last scattering surface (since this is only a lower bound, the paper leaves open the possibility that the whole universe is much larger, even infinite).

This value is based on matching-circle analysis of the WMAP 7 year data. This approach has been disputed.

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*This article was originally published at en.wikipedia.org.