Did the Universe Have a Beginning?
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.
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
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
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