Big Bang Nucleosynthesis
Big bang nucleosynthesis (BBN) is the production of light elements in the early universe. By comparing the predicted and observed element abundances (primarily deuterium and helium), we can determine the baryon density of the universe and constrain the evolution of the early universe. My first published paper (with Rocky Kolb) was an examination of the effect of massive neutrinos on BBN. With Jim Applegate and Craig Hogan, I published the first examination of the effects of a QCD phase transition on BBN. Recently, with Justin Menestrina (a Vanderbilt undergraduate), I studied the “dark radiation” produced by a particle decaying during the BBN epoch.
Large-scale structure is the distribution of matter, including both galaxies and dark matter, on the largest scales in the universe. Ed Bertschinger and I examined the density field produced by the convolution of a fixed density field with a distribution of points. Although this work was originally inspired by the (now defunct) cosmic string model for large-scale structure, it later became the mathematical basis of the halo model, which models the distribution of galaxies embedded in halos of dark matter. David Weinberg and I examined galaxy bias, showing that, under fairly general assumptions, it is necessarily linear on large scales. With Qingqing Mao, Cameron McBride, and Andreas Berlind, I applied the copula formalism, commonly used in mathematical finance, to the description of primordial density fields.
Cosmic Microwave Background
The cosmic microwave background (CMB) is a nearly-uniform radiation field filling the universe — a relic of the big bang. It was long known that the fluctuations that gave rise to galaxies would also leave a tell-tale imprint on the CMB. This imprint was first observed by the COBE satellite, and later analyzed in detail by the WMAP satellite. Between these two experiments, Manoj Kaplinghat, Mike Turner, and I published the second examination of the effects of a time-varying fine structure constant on the CMB fluctuation spectrum — Steen Hannestad scooped us by one day! But this led to a collaboration with Steen, whom I have never met, on a variety of other projects.
Dark Matter and Particle Physics in the Early Universe
The late 1970s and early 1980s saw the application of particle physics to cosmology, with perhaps the most fundamental result being the idea that the dominant form of matter in the universe is an exotic relic particle. Mike Turner and I published a standard approximation for the abundance of a thermal dark-matter relic (see., e.g, the Kolb and Turner cosmology textbook). In a study of the thermodynamics of decaying particles in the early universe, we showed that such particles never “heat up” the universe, as had been previously believed. Instead, the universe simply cools more slowly.
Supernova observations in the late 1990s showed, rather surprisingly, that the expansion of the universe is speeding up instead of slowing down. The simplest explanation for this is a cosmological constant, but more exotic explanations, which go under the generic name of “dark energy”, have also been explored. Andrew Liddle and I did some of the early work on scalar field models for dark energy. More recently, in separate projects with Anjan Sen and Sourish Dutta, I examined simplifications to the scalar field equation of motion in the case where the background cosmological evolution is close to a cosmological constant. The equations can be solved analytically in this case, giving a rather general prediction for the evolution of the equation of state with redshift. Lawrence Krauss and I explored some of the implications of the accelerating universe for the distant future (see also our Scientific American article under “Popular Science”). Paul Frampton, Kevin Ludwick, and I examined models in which the future expansion is fast enough to destroy bound structures but not to produce a future singulary — a class of models we have dubbed the “little rip”.