In case you're not familiar with the connection between these two people, here's a quick summary:
Craig is a theologian who has long propagated the Kalam Cosmological Argument (KCA), which states:
- Everything that begins to exist has a cause to its existence
- The universe began to exist
- Ergo, the universe has a cause to its existence
But in any case, Craig often appeals to modern physics to support the second premise. That's where Alexander Vilenkin comes in. While plenty of physicists have debated whether the universe is eternal or has a beginning, Vilenkin – along with Arvind Borde and Alan Guth – developed a theorem (called the BGV Theorem for short) that, so Craig will tell you, proves that the universe began to exist. This is pivotally important not just to Craig's KCA, but to theism in general. Because, in the words of Stephen Hawking,
So long as the universe had a beginning, we could suppose it had a creator. But if the universe is really self-contained, having no boundary or edge, it would have neither beginning nor end: it would simply be. What place, then, for a creator?
The BGV Theorm and the Beginning of the Universe
Does the BGV Theorem really prove that the universe began to exist? And if it does, does it imply that universe requires an external cause? That's certainly what William Lane Craig, and many other theists, would have you believe. But I want to take a look at the original literature and see for myself.
First, let's take a look at the paper generally cited by Craig, entitled Inflationary Spacetimes are not Past-Complete (full PDF available here). The abstract is as follows:
Many inflating spacetimes are likely to violate the weak energy condition, a key assumption of singularity theorems. Here we offer a simple kinematical argument, requiring no energy condition, that a cosmological model which is inflating -- or just expanding sufficiently fast -- must be incomplete in null and timelike past directions. Specifically, we obtain a bound on the integral of the Hubble parameter over a past-directed timelike or null geodesic. Thus inflationary models require physics other than inflation to describe the past boundary of the inflating region of spacetime.The key phrase here is the last sentence:
Thus inflationary models require physics other than inflation to describe the past boundary of the inflating region of spacetime.I'm going to lay the cards on the table: what the BGV Theorem is saying is not "the universe had a beginning", but that inflationary models cannot go infinitely into the past, and require physics other than inflationary models to describe the boundary condition. This paper is a direct response to physicists who attempt to use inflationary models to describe an eternal universe. In case that's not completely clear, the authors elaborate in the paper itself (emphasis mine):
What can lie beyond this boundary? Several possibilities have been discussed, one being that the boundary of the inflating region corresponds to the beginning of the Universe in a quantum nucleation event . The boundary is then a closed spacelike hypersurface which can be determined from the appropriate instanton.
Whatever the possibilities for the boundary, it is clear that unless the averaged expansion condition can somehow be avoided for all past-directed geodesics, inflation alone is not sufficient to provide a complete description of the Universe, and some new physics is necessary in order to determine the correct conditions at the boundary . This is the chief result of our paper.How has Craig made the leap from "inflation alone is not sufficient to provide a complete description of the universe" to suggesting that the BGV Theorem has proved "the universe began to exist"? Even Borde, Guth and Vilenkin clearly suggest that a "beginning" is merely one possibility that might correspond to the boundary condition.
In Many Worlds in One, Vilenkin talks a bit about a quantum tunneling model that constitutes these "new physics". He compares and contrasts his approach with that of Stephen Hawking, known as the Hartle-Hawking No Boundary Proposal. There are many other options, and I believe Sean Carroll makes a valid point by stating:
The definition of “singularity in the past” is not really the same as “had a beginning” — it means that some geodesics must eventually come to an end. (Others might not.) Most importantly, I don’t think that any result dealing with classical spacetimes can teach us anything definitive about the beginning of the universe. The moment of the Big Bang is, if anything is, a place where quantum gravity is supremely important. The Borde-Guth-Vilenkin results are simply not about quantum gravity.The point ought to be clear: the BGV Theorem does not say that the universe began to exist; it says that inflationary models are past-incomplete, and require new physics to describe the boundary condition.
For the next part, I'm going to jump right into Craig's review of Many Worlds in One, as well as address the second question I mentioned: if the universe has a beginning, does that mean it requires an external cause? I'll let the cat out of the bag: Vilenkin's answer is "no", but Craig (predictably) doesn't like it. You can find the book here, and read Craig's review here.