Evidence · Case 09 · Atheism
Borde, Guth, and Vilenkin, “Inflationary Spacetimes Are Incomplete in Past Directions”
One exhibit from the free evidence repository at apologetics.pro.
Standard cosmology already describes a universe of finite age. The Borde, Guth, and Vilenkin theorem strengthens this. On general grounds it shows that any universe expanding on average cannot be extended infinitely into the past, so it must have a past boundary. Vilenkin himself has put the point plainly, that a universe expanding on average cannot have a past that is eternal. The honest complication is that the theorem shows past incompleteness for spacetimes that expand on average; it does not by itself rule out an earlier phase that was not expanding, and Alan Guth, one of its authors, has said he suspects the universe is probably eternal. Vilenkin reads the result as pointing to a beginning; the theorem itself leaves that door open.
Sources
Objections & responses
- The BGV theorem shows that classical spacetime is past-incomplete, not that the universe began. Guth himself thinks the universe is probably eternal, and the theorem doesn’t apply where quantum gravity takes over.
That is the correct technical scope, in Guth’s own words: the theorem shows that inflating spacetimes cannot be extended into the infinite past under classical assumptions, and it says nothing about a quantum-gravity regime that might precede them. Vilenkin’s stronger reading, that the theorem combined with the failure of the main eternal models makes a beginning probable, is Vilenkin’s, and should be attributed to him rather than to the theorem. The claim survives in the form the leading eternal-universe models face a theorem-level obstacle, which is what the exhibit shows.