My interest in this book has to do with one of my favorite arguments in philosophy, the Kalam cosmological argument for a cause of the universe. The argument can be formulated in different ways but it typically goes like this:
- Everything that begins to exist has a cause.
- The universe began to exist.
- Therefore, the universe has a cause.
This argument is often used in support of theism. Not all theists like the argument, but personally I think it's a good one.
Vilenkin is not a theist, and he does not defend this argument. As far as I know, he is an agnostic. But he developed a theorem with two other physicists named Arvind Borde and Alan Guth—it is thus called the Borde-Guth-Vilenkin theorem—and the theorem is often cited in support of the argument's second premise. Vilenkin talks about the theorem in chapter 16 of his book, titled "Did the Universe Have a Beginning?" I had heard some of these words quoted in debates before, but it was interesting to read them in context, especially since I didn't realize they were taken from his book.
Here are some fun excerpts:
The theorem we proved in that paper is amazingly simple. Its proof does not go beyond high school mathematics, but its implications for the beginning of the universe are very profound. (p. 174)A remarkable thing about this theorem is its sweeping generality. We made no assumptions about the material content of the universe. 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. The only assumption we made was that the expansion rate of the universe never gets below some nonzero value, no matter how small. This assumption should certainly be satisfied in the inflating false vacuum. The conclusion is that past-eternal inflation without a beginning is impossible. (p. 175)What about a cyclic universe? It has alternating periods of expansion and contraction. Can this help the universe to escape from the clutches of the theorem? The answer turns out to be no. An essential feature of the cyclic scenario, which allows it to avoid the heat-death problem, is that the volume of the universe increases in every cycle, so on average the universe is expanding. . . . Hence, the same conclusions still apply. (pp. 175–6)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. There is no escape: they have to face the problem of a cosmic beginning. (p. 176)
The argument involved in the theorem seems simple enough. It only takes Vilenkin a couple of pages to explain it. But I have to admit it's a bit over my head, so I'm sitting with it for a while to try to make sure I understand it. I'll probably do a second post about that, and then I'll do a third post related to Vilenkin's own views about how the universe came from nothing. I'm also curious to see if the theorem has ever been refuted, since it's been about two decades since he wrote this book.