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Concept
Hubble volume
Summary
In cosmology, a Hubble volume (named for the astronomer Edwin Hubble) or Hubble sphere, Hubble bubble, subluminal sphere, causal sphere and sphere of causality is a spherical region of the observable universe surrounding an observer beyond which objects recede from that observer at a rate greater than the speed of light due to the expansion of the universe. The Hubble volume is approximately equal to 1031 cubic light years (or about 1079 cubic meters).
The proper radius of a Hubble sphere (known as the Hubble radius or the Hubble length) is , where is the speed of light and is the Hubble constant. The surface of a Hubble sphere is called the microphysical horizon, the Hubble surface, or the Hubble limit.
More generally, the term Hubble volume can be applied to any region of space with a volume of order . However, the term is also frequently (but mistakenly) used as a synonym for the observable universe; the latter is larger than the Hubble volume.
The center of the Hubble volume and observable universe is arbitrary in relation to the overall universe; instead it is centered around its origin (impersonal or personal "observer").
The Hubble length is 14.4 billion light years in the standard cosmological model, somewhat larger than times the age of the universe, 13.8 billion years.
For objects at the Hubble limit, the space between us and the object of interest has an average expansion speed of c. So, in a universe with constant Hubble parameter, light emitted at the present time by objects outside the Hubble limit would never be seen by an observer on Earth. That is, the Hubble limit would coincide with a cosmological event horizon (a boundary separating events visible at some time and those that are never visible). See Hubble horizon for more details.
However, the Hubble parameter is not constant in various cosmological models so that the Hubble limit does not, in general, coincide with a cosmological event horizon.
Official source
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