Accelerating expansion
In 1998 two studies were undertaken to determine the expansion rate of the universe.
They compared the distances of supernovae with their recessional speed. Supernovae are a class of brilliant exploding stars that can be seen even in distant galaxies. It was found that considering the redshift of their light; the distant supernovae were fainter than would be expected.
This would happen only if space were expanding at an accelerating rate thereby increasing the redshift effect over time so that light from distant supernovae reaches us fainter than would be expected if the redshift effect and, therefore, the rate of expansion of the universe was constant.
Given the supporting evidence for the accelerating inflationary big bang model, we can be reasonably confident that the universe is about 13-14 billion years old
Therefore, the universe must have begun to exist 13-14 billion years ago.
Multiverse
However, some have postulated the multiverse hypothesis, whereby our universe is just one part of a subset of a hypothetical collection of potentially infinite number of universes. The idea is that then our universe always existed in its current form or its previous incarnations, and, therefore, the universe never began to exist.
The major candidates for multiverses are set out below:
Eternal inflationary multiverse
The inflationary multiverse is based on the aforesaid process of cosmological inflation whereby the extremely rapid expansion or inflation is driven by repulsive gravity in a false vacuum that creates regions of space far larger than the observable universe. If the transition to non-inflation in the true vacuum state occurs at different times and places, there will be regions where inflation is still occurring leading to distinct post inflationary regions of which our universe is only one.
If inflation is past-eternal, then it could be argued that our universe always existed.
Whilst this model is based on generally accepted process of cosmological inflation, there nevertheless remains the problem of:
- infinite regress: “What caused the original ‘big bang’ of the multiverse?”
- the violation of the Borde-Guth-Vilenkin incompleteness theorem
Borde-Guth-Vilenkin incompleteness theorem[1]
Borde, Guth and Vilenkin showed that any universe or multiverse which is on average expanding cannot be past-eternal.
Since eternal inflation causes eternal expansion, any multiverse based on eternal inflation cannot be past-eternal. It may be eternal in the future, but it cannot be eternal in the past.
Therefore, any inflationary multiverse must have begun to exist.
The assumption of an inflationary multiverse only pushes back the beginning of our universe.