Cosmology Meets Astronomy in Philadelphia

After spending a splendid Spring break back in Syracuse, hanging out with friends between working on a paper, a review article and two grant proposals, on Friday I drove back to Philadelphia, where in the evening I was giving a talk to the Delaware Valley Amateur Astronomers (DVAA). The talk was a slightly more technical version of the public lecture - Modern Cosmology and the Building Blocks of the Universe - that I have given in many different places over the last few years, and at this point I'm quite comfortable with it. However, I am more nervous giving talks to amateur astronomers than to any other group. This is because, for a cosmologist, I know remarkably little about astronomy. It isn't that I'm a complete ignoramus, but just that my knowledge is relatively rudimentary, and amounts to less than the content of an undergraduate astronomy degree, although I do know some extra things about a few advanced topics. My training is in mathematics and physics, and I've basically picked up the astronomy I know through necessity either because of research topics or teaching. Nevertheless I bravely soldiered on, and the DVAA crowd proved to be quite delightful. My talk begins with a brief tour of the major pillars of the big bang theory - the Hubble expansion law, the microwave background radiation, and the abundances of the light elements - then proceeds with a summary of open questions. This is then followed by a discussion of the challenges these pose for fundamental physics, and some of the possible solutions and how they may be tested in upcoming experiments. In particular, I focus on candidates for dark matter, and their possible origin in beyond the standard model physics at the TeV scale, on models of baryogenesis, and on the logical possibilities for the origin of cosmic acceleration - the cosmological constant, dynamical dark energy and modified gravity. Obviously this is a cursory treatment, since a detailed discussion isn't really what people expect from a public lecture. And there are huge numbers of topics I'd love to touch on but just don't have time for, like inflation, and topological defects, and extra dimensions, and ... You get the picture. But when you're speaking to an audience like this one, people tend to already have questions about all kinds of things that you didn't explicitly mention in the talk. And not just any questions, but subtle and technical ones. So afterwards I spent a while discussing the status of tests of the inflationary paradigm, the anthropic principle and its implementation through eternal inflation in the string theory landscape, the problem of measure in the multiverse, and other related topics. Not your usual collection of post-public lecture questions, and a lot of fun!