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The Hubble volume, or Hubble sphere, is the volume of the universe bounded by the Hubble limit.
The term "Hubble volume" is also frequently used as a synonym for the observable universe.^[1]

In cosmology, the Hubble volume, or Hubble sphere, is the spherical region of space centered on the Earth with a comoving radius of c / H₀, where c is the speed of light and H₀ is the Hubble constant. More generally, the term "Hubble volume" can be applied to any region of space with a volume of order (c / H₀)³.

The distance c / H₀ 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 c times the age of the universe. This is because 1 / H₀ 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 1 / H₀ 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^[2].

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. In other words, the Hubble limit would define the cosmological event horizon for us. If, as is inferred from current observations, the expansion of the universe is in fact accelerating, then even some objects within the Hubble limit will never be observed (by us) as they are today.

• Hubble's Law

• The Hubble Volume Simulations

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