A waste of time
This is where I tell you that everything earlier in this series may have been a colossal waste of time. I spent all this time arguing for the possibility of an infinite past, but who cares about the possibility? In our actual reality, it appears that the universe in fact does not have an infinite past. This is based on our knowledge of physics, and experimental confirmations of Big Bang theory. It's based on science.
In my mind, the scientific confirmation of a beginning trumps all the philosophical arguments in the world. If we had evidence that the universe has no beginning, Christians like William Lane Craig (WLC) would not give up Christianity. They'd just find a new argument for why a lack of a beginning is consistent with Christianity. The only reason WLC can't think of an argument now is that he doesn't have the motivation.
That goes for atheists too! A few atheists will respond to the cosmological argument by saying that the universe does not have a beginning. But if science all but proved that the universe has a beginning, would these people just concede the cosmological argument? Nah, they'd find a new reason why it's wrong.
On the basis of scientific evidence, I am totally willing to concede that the universe in fact has a beginning. That particular premise of the cosmological argument is true. I spent all that time arguing about infinities not because I believe the universe has no beginning, but because I genuinely believe WLC has infinities all wrong.
I also genuinely believe WLC has the cosmology all wrong. WLC greatly exaggerates the scientific case for a beginning. I agree that cosmology argues for a universe with a beginning, but it is by no means assured. It's arguable, though I would not argue it, that the question of a beginning is still completely up for grabs.
The known and unknown
WLC describes the case for the beginning of the universe using the words of physicists:
The universe began from a state of infinite density. . . . Space and time were created in that event and so was all the matter in the universe. It is not meaningful to ask what happened before the Big Bang; it is like asking what is north of the North Pole. Similarly, it is not sensible to ask where the Big Bang took place. The point-universe was not an object isolated in space; it was the entire universe, and so the answer can only be that the Big Bang happened everywhere.That is indeed one of the standard narratives cosmologists use to explain the beginning of the universe to popular audiences. But I have the vague impression that this narrative was popular a generation ago (the quote comes from 1976), but contemporary cosmologists realize that there are alternative narratives which are about as likely to be true.
My field is condensed matter, not cosmology, so I get these impressions from colloquia and buzz among physicists. If I were willing to put in a lot of effort, I would look through the recent papers on ArXiV and see if there are any discussing alternative narratives for the beginning of the universe. But that would just lead to he said she said, a battle of the experts. Instead, let me explain a bit why contemporary cosmologists are unsure about the beginning of the universe, and why you should be too.
The Big Bang theory is the theory that the universe evolved from a very hot and dense state to a sparse and cool state. It's a prediction of Einstein's theory of General Relativity, which is a theory of gravity. According to General Relativity, the universe can't really have constant and uniform density, because all that matter will attract. Instead, the density will change over time in the manner described by General Relativity.
The experimental evidence for the Big Bang theory is many-fold, and stands independently from all the evidence for General Relativity. There is the redshift of far away galaxies, indicating that they are moving away from us. There is the cosmic microwave background radiation, which is thermal radiation from when the universe became cool enough that it became transparent. The observed proportions of elements agrees with the prediction from Big Bang theory. There are numerous other lines of evidence that I don't have time to name.
But note that all the evidence I cited only shows that the universe expanded from a hot and dense state. It does not actually tell us what happened at t=0. As we get to t=0, there are more and more disputes over what happened.
There's Inflationary theory, which says that between 10-36 to 10-33 seconds after t=0, there was a period of really fast expansion. During that time the universe grew in size by about 78 orders of magnitude. The primary evidence for inflation comes from agreement with tiny variations in the cosmic microwave background radiation. I would say inflation is part of standard cosmology, but it certainly has its detractors, and it could still turn out to be wrong. The cause of inflation is unknown and disputed. The theories I've heard usually posit a new quantum field.
And then there's the big dispute--quantum gravity. The only reason we believe that the expansion continues all the way to t=0 is that General Relativity predicts it, but General Relativity is not a complete theory. It's a classical theory, yet to be converted to a quantum theory. Very early in the universe,* the universe was dense enough that quantum effects would definitely come into play. We're not just lacking evidence regarding the very early universe, we're lacking reliable theoretical predictions. We need a theory of quantum gravity to make predictions. The major candidate theory is String Theory, which is famously controversial. There are also a lot of other candidates like Quantum Loop gravity, and lots of other stuff I don't understand.
*I don't know exactly how early, because I couldn't find a reference. Sorry.
Besides the beginning-as-north-pole narrative, there are actually a lot of possibilities for the history of the universe. WLC mentions one, the oscillating universe, which he quickly dismisses.
As the late Professor Tinsley of Yale explains, in oscillating models "even though the mathematics say that the universe oscillates, there is no known physics to reverse the collapse and bounce back to a new expansion. The physics seems to say that those models start from the Big Bang, expand, collapse, then end." In order for the oscillating model to be correct, it would seem that the known laws of physics would have to be revised.This may be correct, but it's misleading. The known laws of physics have to be revised regardless, because we need a theory of quantum gravity to make any predictions. There is no known physics to reverse a collapse, but the same could be said of any early universe scenario. There is no known physics to explain inflation, even though we think inflation exists. All that's known is that classical physics predicts a beginning (I believe this is also in dispute), and that classical physics doesn't apply.
I believe the oscillating universe hypothesis has fallen out of favor, but a lot of new speculations have come into favor as well. Based on Inflationary theory, there are a lot of speculations about universes budding from each other. Perhaps there are certain conditions under which a small volume of an older universe will suddenly inflate and become its own universe. Or perhaps inflation is the normal state of things, but every so often a tiny bit of this infinitely inflating universe stops inflating and becomes a slowly expanding universe. I've also heard speculations coming from string theory. Maybe the Big Bang was a collision of branes that exist in a realm of higher dimensions. So on and so forth.
I call them speculations, because that's what they are. Probably none of them are correct. It's probably something we haven't even thought of. The point is that the Big Bang as the beginning isn't the only possibility, nor is it the only possibility we can imagine.
WLC also uses the Second Law of thermodynamics as another scientific argument, but this is getting too long, so I'll save it for next time.
"A few things wrong about the cosmological argument"
1. Actual and potential infinities
2. Actual infinities in physics
3. What is real?
4. The "absurdity" of Hilbert's Hotel
5. Interlude: God is infinite
6. Forming Infinity, one by one
7. Uncertain beginnings
8. Entropy: The unsolved problem
9. Kalam as an inductive argument
10. Getting from First Cause to God