Hubble volume

Visualization of the three-dimensional large-scale structure of the universe in the Hubble volume. The scale is such that the fine grains of light represent collections of large numbers of superclusters. The Virgo Supercluster - home of our own galaxy - is at the center of our Hubble volume, but is too small to be seen in the image.

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, due to the expansion of the Universe.

The comoving radius of the Hubble sphere is $c/H_0$, where $c$ is the speed of light and $H_0$ is the Hubble constant. More generally, the term "Hubble volume" can be applied to any region of space with a volume of order $(c/H_0)^3$.

The term "Hubble volume" is also frequently (but mistakenly) used as a synonym for the observable universe; the latter is larger than the Hubble volume.

Expansion of the universe changed

The distance $c/H_0$ is known as the "Hubble length". It is equal to 13.9 billion light years in the standard cosmological model, similar to but somewhat larger than $c$ times the age of the universe. This is because $1/H_0$ 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 theory, recession speeds initially decelerate due to gravity, and are now accelerating due to dark energy, so that $1/H_0$ is only an approximation to the true age.

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 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.

See also

• Hubble's Law

External links

• The Hubble Volume Simulations