My first argument was based on the origin of the universe. And I argued first that the universe
began to exist, and here he says, well but look there are different multiverse scenarios,
various models of the universe. I talked about those in my opening speech and explained
that the Borde–Guth–Vilenkin theorem applies to those and shows the beginning of the universe.
In 2003, Arvind Borde, Alan Guth, and Alexander Vilenkin were able to prove that
any universe which is on average in a state of cosmic expansion throughout its history
cannot be infinite in the past, but must have a past space-time boundary.
What makes their proof so powerful is that it holds regardless of the physical
description of the very early universe. Because we don't yet have a quantum theory of gravity,
we are not able to provide a physical description of the first split second of the early universe.
But the Borde–Guth–Vilenkin theorem is independent of any such physical description of that moment.
Their theorem implies that the quantum vacuum state of the early universe,
which some popularizers have misleadingly and inaccurately characterized as
nothing cannot be eternal in the past but must have had an absolute beginning.
Even if our universe is just a tiny part of a so-called multiverse composed of many universes,
their theorem requires that the multiverse itself must have a beginning.
Now of course, highly speculative scenarios like loop quantum gravity models, string models,
even closed time-like curves have been proposed to try to avoid this absolute beginning. However
these models are fraught with problems, and the bottom line is that none of these theories, even
if true, succeed in restoring an eternal past at most they just pushed the beginning back a step.
And then he says but Vilenkin says that you can avoid the Borde–Guth–Vilenkin theorem by
positing a contraction prior to this one. [Millican] In response to the question,
does your theorem prove that the universe must have had a beginning Alex Vilenkin answers no,
but it proves that the expansion of the universe must have had a beginning. You can evade the
theorem by postulating that the universe was contracting prior to some time. [Craig] Now
this is a statement from a letter of Vilenkin to Victor Stenger, which is very often quoted out
of context by atheists. Let me read you the full context: Vilenkin says you can evade the theorem
by postulating that the universe was contracting prior to some time. This sounds as if there is
nothing wrong with having a contraction prior to expansion, but the problem is that a contracting
universe is highly unstable. Small perturbations would cause it to develop all sorts of messy
singularities, so it would never make it to the expanding phase. So he says if someone asked me
whether or not the theorem I proved with Borde and Guth implies that the universe had a beginning, I
would say that the short answer is yes. If you are willing to get into subtleties, then the answer is
no, but that is to say you've got the problem with the messy singularities that prevent re-expansion.
So in fact the Borde–Guth–Vilenkin theorem does imply an absolute beginning of the universe.
Dr. Millican says but we need a quantum theory of gravity to describe the early universe. the
Borde–Guth–Vilenkin theorem is independent of that. Vilenkin says the remarkable thing
about this theorem is it's sweeping generality. We did not even assume that gravity is described by
Einstein's equations. So if Einstein's gravity requires some modification, our conclusion
will still hold so it isn't affected by having a quantum gravity description. Here's Vilenkin's
conclusion. It is said that an argument is what convinces reasonable men, and a proof is what
it takes to convince even an unreasonable man. With the proof now in place, cosmologists can
no longer hide behind the possibility of a past eternal universe. They have to face the problem
of a cosmic beginning. [Kuhn] So okay we want to find out about the beginning of the universe how
it began, and we have these three properties we have the composition of the universe,
we have the geometry of the universe, and we have the in homogeneity as well as the homogeneity of
the universe and understanding those three. Now each of the three the mechanisms to explain that,
are they indeed consistent with at least this universe having a beginning? [Guth] Yeah,
all three of these depend crucially on the idea that the universe started out incredibly small
and has been expanding dramatically since then, and that's what results in the cooling which
allows these funny processes that create a net baryon number to happen in the early universe,
but then clearly they turned off. In the present universe, the baryons just sit there and are
stable, and that's true of the other properties as well. So yeah it certainly looks like the universe
that we observe around us, the universe as we know it, definitely had a beginning.
That doesn't mean that that beginning was necessarily the ultimate beginning of all of
reality. There may have been some prehistory to what we're here calling the beginning,
but the universe as we know it certainly began we think about 13.7 billion years ago. [Kuhn] And the
inflation theory which explains some of those properties so wondrously would engender the
possibility of there being other beginnings, but I think you and some others have shown
that ultimately there had to be some original, the mother of all beginnings. [Guth] Yes that's
right. Those issues are still a little unclear. I wouldn't say that those things are shown
beyond doubt, but with reasonable assumptions one could show that even in the context of inflation
with many bubbles forming, it would still be somewhere an ultimate beginning.