Hubble volume
However, the term is also frequently (but mistakenly) used as a synonym for the observable universe; the latter is larger than the Hubble volume.
is 14.4 billion light years in the standard cosmological model, equivalent to
The Hubble time is the reciprocal of the Hubble constant,[5] and is slightly larger than the age of the universe (13.8 billion years) as it is the age the universe would have had if expansion was linear.
For example, in a decelerating Friedmann universe the Hubble sphere expands with time, and its boundary overtakes light emitted by more distant galaxies so that light emitted at earlier times by objects outside the Hubble volume still may eventually arrive inside the sphere and be seen by us.
[3] Similarly, in an accelerating universe with a decreasing Hubble constant, the Hubble volume expands with time and can overtake light from sources previously receding relative to us.
In a universe with an increasing Hubble constant, the Hubble horizon will contract, and its boundary overtakes light emitted by nearer galaxies so that light emitted at earlier times by objects inside the Hubble sphere will eventually recede outside the sphere and will never be seen by us.
[1] If the shrinkage of the Hubble volume does not stop due to some yet unknown phenomenon (one suggestion is the "early phase transition"), the Hubble volume will become nearly a point (due to the uncertainty principle pure singularities are impossible; also a proportion of their self-interactions are energetic enough to produce escaping particles via quantum tunneling), meeting the criteria of big bang.
[citation needed] The justification of this view is that no subluminal Hubble volume will exist and pointwise superluminal expansion (the generalization of the Big Bang theory) will prevail everywhere or at least in a vast region of the universe.
[citation needed] Observations indicate that the expansion of the universe is accelerating,[8] and the Hubble constant is thought to be decreasing.
A fairly counter-intuitive result is that photons we observe from the first ~5 billion years of the universe come from regions that are, and always have been, receding from us at superluminal speeds.
