The excellent Symmetry magazine, jointly produced by Fermilab and SLAC, has a fun article following up on a recent debate held at the Smithsonian Museum of Natural History. The topic was "The Dark Side of the Universe", and the pretext was the question of whether progress in tackling the deep issues of dark energy and dark matter will come from the particle physicists or from the cosmologists. Representing these constituencies were Joe Lykken and Rocky Kolb, both physicists from Fermilab, and both capable of representing the science while putting on an all-singing, all-dancing show.
The particle physics vs. cosmology setup (or Mr. Accelerator vs. Mr. Telescope, as Symmetry puts it) is a nice idea, and must have made for a lively exchange, enabling Joe, Rocky, and moderator Michael Turner to cover all the relevant bases. Hopefully though, given the track records of the participants, the audience will have come away with a message precisely the opposite of that implied by the debate format - namely that particle physics and cosmology couldn't be more closely related. There was a time, of course, when astronomers gazed at the skies, and dutifully reported on their contents, while particle physicists built ever more sophisticated microscopes with which to peer at the subatomic world. These endeavors seem to occupy two extremes of scale, and it is not obvious at first glance that they should have too much to do with one another. But as we've come to realize over the years, the key to understanding the behavior of the universe lies in understanding the microphysical, or fundamental, properties of its constituents, while precision cosmological observations can provide crucial information and constraints on particle physics models in regimes that we can't dream of accessing with terrestrial accelerators. An extremely recent sign of this can be seen in the flurry of activity in particle phenomenology driven almost exclusively by results reported by cosmological observations, in this case from the PAMELA experiment (I first mentioned this in my post on the ICHEP conference earlier this year). But the connections go back much further, as I was reminded of this week when I attended a series of short talks at Cornell, where I'm currently on leave, in memory of Edwin Salpeter, a renowned astrophysicist who very recently passed away. Salpeter began his career in particle physics, writing papers on field theory, the best known of which contains his discovery, with Bethe, of the Salpeter-Bethe equation describing two-body bound states in quantum field theory. But Salpeter was interested in astrophysics and is equally well-known for the Salpeter process in which applying the ideas of nuclear physics to the behavior of the interior of stars he showed how helium nuclei can fuse to form Carbon. Thus began our understanding of how astrophysics gave rise to most of the periodic table. These particle physics-cosmology connections are now commonplace, and there is a reasonable argument that there no longer exists a clear distinction between the fields, but rather that they represent a continuum of approaches to understanding the fundamental building blocks of the universe. As I've mentioned before, whether one comes at the issues from the cosmology or the particle physics side, there are a host of common questions that arise. We observe dark matter through cosmology, but a definitive understanding of its nature may well rely on our particle accelerators; our universe is observed to contain negligible primordial antimatter, but we may not know why without colliders or data from a new generation of neutrino experiments; clusters of galaxies crystalized from the primordial plasma, but their origin may lie in quantum mechanical fluctuations at the earliest times; and I could go on and on. These days, Mr. Accelerator and Mr. Telescope can't live without one another.