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Hubble volume
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In 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. Mathematically this comoving radius is , where is the speed of light and is the Hubble constant. More generally, the term "Hubble volume" can be applied to any region of space with a volume of order .
The term "Hubble volume" is also frequently (but mistakenly) used as a synonym for the observable universe.
Expansion of the universe changed
The distance is known as the "Hubble length".
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Encyclopedia
In 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. Mathematically this comoving radius is , where is the speed of light and is the Hubble constant. More generally, the term "Hubble volume" can be applied to any region of space with a volume of order .
The term "Hubble volume" is also frequently (but mistakenly) used as a synonym for the observable universe.
Expansion of the universe changed
The distance is known as the "Hubble length". It is equal to 13.8 billion light years in the standard cosmological model, similar to but somewhat larger than times the age of the universe. This is because gives the age of the universe by a backward extrapolation which assumes that the recession speed of each galaxy has been constant since the big bang. In fact, recession speeds initially decelerate due to gravity, and are now accelerating due to dark energy, so that is only an approximation to the true age. The surprising accuracy of this approximation formed the basis for an April fool paper posted on arXiv.
Hubble limit
The boundary of the Hubble volume is known as the "Hubble limit". Per Hubble's law, objects at the Hubble limit have an average comoving speed of c relative to an observer on the Earth. This is significant, because, in a universe in which the Hubble parameter was constant, light emitted at the present time by objects outside the Hubble limit could never be seen by an observer on the Earth. However, the Hubble "constant" is not constant. In a decelerating Friedmann universe, the Hubble sphere expands faster than than the Universe and its boundary overtakes light emitted by receding galaxies. In an accelerating universe, the Hubble sphere expands more slowly than the Universe, and bodies move out of the Hubble sphere. So the Hubble limit need not define the cosmological event horizon (that is, the boundary separating events visible at some time or other and those that are never visible), because (depending upon the cosmological model) light emitted at earlier times by objects outside the Hubble sphere still may eventually arrive inside the sphere and be seen by us. If, as is inferred from current observations, the expansion of the universe is in fact accelerating, then at a later time, some objects within the Hubble limit no longer will be observed (by us) as they are today.
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