FML, and the Big Bounce

The day started with a realization by Price-Whelan (Princeton) and me that, in our project The Joker, because of how we do our sampling, we have everything we need at the end of the sampling to compute precisely the fully marginalized likelihood of the input model. That's useful, because we are not just making posteriors, we are also making decisions (about, say, what to put in a table or what to follow up). Of course (and as my loyal reader knows), I don't think it is ever a good idea to compute the FML!

At lunch, Paul Steinhardt (Princeton) gave a great black-board talk about the idea that the Universe might have started in a bounce from a previously collapsing universe. His main point (from my perspective; he also has particle-physics objectives) is that the work that inflation does with a quantum mechanism might be possible to achieve with a classical mechanism, if you could design the bounce right. I like that, of course, because I am skeptical that the original fluctuations are fundamentally quantum in nature. I have many things to say here, but I'll just say a few random thoughts: One is that the strongest argument for inflation is the causality argument, and that can be achieved with other space-time histories, like a bounce. That is, the causality (and related problems) are fundamentally about the geometry of the space and the horizon as a function of time, and there are multiple possible universe-histories that would address the problem. So that's a good idea. Another random thought is that there is no way to make the bounce happen (people think) without violating the null-energy condition. That's bad, but so are various things about inflation! A third thought is that the pre-universe (the collapsing one) probably has to be filled with something very special, like a few scalar fields. That's odd, but so is the inflaton! And those fields could be classical. I walked into this talk full of skepticism, and ended up thinking it's a pretty good program to be pursuing.