Did the Universe Have a Beginning?

Finally, I would like to discuss the central topic of this session, the question of whether or not the universe had a beginning.

The name eternal inflation, as I pointed out earlier, could be phrased more accurately as future-eternal inflation. Everything that has been said so far implies only that inflation, once started, continues indefinitely into the future. It is more difficult to determine what can be said about the distant past.

For the explicit constructions of eternally inflating models, the answer is clear. Such models start with a state in which there are no pocket universes at all, just pure repulsive-gravity material filling space. So there is definitely a beginning to the models that we know how to construct.

In 1993 Borde and Vilenkinproved a theorem which showed under fairly plausible assumptions that every eternally inflating model would have to start with an initial singularity, and hence must have a beginning. In 1997, however, theynoted that one of their assumed conditions, although valid at the classical level, was violated by quantum fluctuations that could be significant in eternally inflating models. They concluded that their earlier proof would not apply to such cases, so the door was open for the construction of models without a beginning. They noted, however, that no such models had been found.

At the present time, I think it is fair to say that it is an open question whether or not eternally inflating universes can avoid having a beginning. In my own opinion, it looks like eternally inflating models necessarily have a beginning. I believe this for two reasons. The first is the fact that, as hard as physicists have worked to try to construct an alternative, so far all the models that we construct have a beginning; they are eternal into the future, but not into the past. The second reason is that the technical assumption questioned in the 1997 Borde-Vilenkin paper does not seem important enough to me to change the conclusion, even though it does undercut the proof. Specifically, we could imagine approximating the laws of physics in a way that would make them consistent with the assumptions of the earlier Borde-Vilenkin paper, and eternally inflating models would still exist. Although those modifications would be unrealistic, they would not drastically change the behavior of eternally inflating models, so it seems unlikely that they would change the answer to the question of whether these models require a beginning.

So, as is often the case when one attempts to discuss scientifically a deep question, the answer is inconclusive. It looks to me that probably the universe had a beginning, but I would not want to place a large bet on the issue.

Acknowledgments

This work is supported in part by funds provided by the U.S. Department of Energy (D.O.E.) under cooperative research agreement #DF-FC02-94ER40818.

Contributed by: Dr. Alan Guth