The next topic from the Carroll-Craig debate which I wish to discuss is what Science has to say about whether or not there was a beginning. Was there a first moment of time, before which the universe did not exist? What does Modern Cosmology have to say about this question?
I think that Modern Cosmology gives a fairly clear answer: probably, but not almost certainly. But, rather than try to argue only for one particular conclusion, I will instead try to provide the evidence in both directions, on which my opinions are based.
The reason why I say probably is that, given our current best theories of the universe, there are some decent reasons to think that the universe had some type of beginning at the so-called "Big Bang". However, once you get to an early enough moment of time, we don't really understand anything anymore, so really anything might have happened. That is why the term "Big Bang Model" refers to the (very well-confirmed) theory of the expansion of the universe after the Big Bang, rather than to the Big Bang singularity itself.
Given our current best understanding of particle physics, we think we can describe fairly well the history of the universe starting at around seconds after the Big Bang. We're certainly on-base in the period from about 10 seconds to 20 minutes, since this is when Big Bang nucleosythesis occurred (creating the first atomic nuclei), and we can check that the current abundances of H, He, and Li atoms are in agreement with what our theory of nucleosynthesis predicts.
Inflation (which would have happened at a much earlier time) is somewhat less certain, but it makes pretty good predictions so almost everyone believes in it these days. The recent BICEP2 results
indicate that the energy scale of inflation was just a couple orders of magnitude below the Planck scale seem to have been contaminated by too much dust to be reliable, although most models of inflation still place it at a ridiculously high energy scale. This is a much higher energy scale than anything else we can measure in physics, although it is comparable to the GUT scale (where most particle physicists, but not I, believe that the forces probably unify into one force). During the inflation era, the universe grew in an extremely rapid way, stretching out and diluting any information about what the universe was like before inflation.
The Planck era was approximately the first "seconds after" the "Big Bang". This is the era where strong quantum gravity effects become important. In other words, the quantum uncertainty in concepts of "space" and "time" become so large that our classical concepts break down. That's why I put scare-quotes around things in this paragraph—we no longer know what on earth (or in the heavens) we are talking about. This is the point when everything is pretty much up for grabs.
So, even if we can say there appears to have been a beginning based on an extrapolation of the Big Bang Model to early times, there are also reasons why we can't be completely sure, so long as we don't completely understand quantum spacetime (or the initial conditions for inflation). Certainly the universe as we know it began, but we cannot completely eliminate the possibility of a pre-Big-Bang stage.
Nevertheless, in the next few posts I will discuss the limited evidence which we do have, especially those points which were mentioned in the debate. In particular I will cover singularity theorems, the BGV theorem, the 2nd Law of Thermodynamics, the quantum eternity theorem. Oh, and the Hartle-Hawking no-boundary proposal. That too.
[Updated description of BICEP2 results]