Scientists Arvin Borde, Alan Guth and Alexander Vilenkin, in their 2003 paper “Inflationary Spacetimes Are Incomplete in Past Directions”, ruled out past-eternal inflationary models of the universe. Does this prove that
1. the universe had an absolute beginning,
2. that it must have had a cause and
3. that the cause was God?
No, no and no.
These three scientists in particular, by virtue of their joint paper, are name-checked more often than any others by apologists not just arguing for an absolute beginning to the universe but claiming that this has been established beyond doubt. Apologists up to and including William Lane Craig do this to support the cosmological argument for God, which requires such a beginning to be indisputable.
So what does the paper actually say? Feel free to read it via the link above (it’s dense but short), but the thrust is in the title: spacetime can’t have been inflating infinitely into the past.
That seems obvious since you’d think you’d eventually reach a singularity if you worked backwards, but models have been proposed wherein the farther back you go the slower the expansion is. Some reasoned that perhaps the universe has spent eternity inflating extremely slowly from a size barely larger than a singularity, speeding up as it went along.
Borde, Guth and Vilenkin examined this idea and found, essentially, that it wouldn’t work in the real world. In doing so they pretty much dismissed every model of an expanding universe (or multiverse) that doesn’t involve a proper singularity and Big Bang. Therefore, according to apologists, the universe definitely had an absolute beginning, which must have had a cause, and that cause was God.
That’s going way too far because, for a start, the paper doesn’t take a position on whether the now-confirmed Big Bang was an absolute beginning. There are many more universe/multiverse models wherein the Big Bang was merely an event in an ongoing sequence – where the matter in the singularity came from somewhere, not nowhere. Borde et al only intended to rule out a family of models that clearly don’t work.
Get that? Borde, Guth and Vilenkin did NOT rule out an eternal universe, even if the result of their paper is correct. They merely ruled out one kind of eternal universe, the kind where the Big Bang never happened. The fact that there was a Big Bang does not mean there was nothing before the Big Bang.
The Big Bang as absolute zero, or an absolute beginning in general, is a poor platform for apologetics in any case. The idea that whatever begins to exist has a cause is not based on anything which physically began to exist in the same way the universe supposedly did, completely ex nihilo (literally “from nothing”). We’ve never seen anything like that happen, so:
1. it’s curious that so many people assume the universe came about in this way,
2. there’s no basis for assigning a cause if it did and
3. even if it had a cause, it’s a huge leap to declare it any kind of god, let alone someone’s specific personal deity.
“These three scientists in particular, by virtue of their joint paper, are name-checked more often than any others by apologists not just arguing for an absolute beginning to the universe but claiming that this has been established beyond doubt.”