Ph.D. Candidate: Adam Scherlis
Research Advisor: Peter Graham
Date: Friday, November 22, 2019
Time: 2:00 pm
Location: PAB 102/103
Title: Scalar Fields in Cosmology
Scalar fields play a unique role in physics due to the rich dynamics of their expected values, which can drive cosmic inflation or spontaneously break symmetries. I will discuss recent work studying two of the most well-known scalars, the QCD axion and the Higgs boson, and some of the less-known things they can do during the inflationary era.
The QCD axion, a proposed solution to the strong CP problem, is also an ultralight dark matter candidate. For the minimal QCD axion model it is generally believed that overproduction of dark matter constrains the axion mass to be above a certain threshold, or at least that the initial misalignment angle must be tuned if the mass is below that threshold. We demonstrate that this is incorrect. During inflation, if the Hubble scale is low, the axion tends toward an equilibrium. This means the minimal QCD axion can naturally give the observed dark matter abundance in the entire lower part of the mass range, down to masses of nanoelectronvolts (or decay constants up to almost the Planck scale). The axion abundance is generated by quantum fluctuations of the field during inflation. This mechanism generates cold dark matter with negligible isocurvature perturbations. In addition to the QCD axion, this mechanism can also generate a cosmological abundance of axion-like particles and other light fields.
The Higgs boson, responsible for electroweak symmetry breaking, is also a candidate for the inflaton. Recently, the problem of unitarity violation during the preheating stage of Higgs inflation with a large non-minimal coupling has been much discussed in the literature. This problem can be also interpreted as a strong-coupling problem for the Higgs field when its inflaton component crosses the origin. We point out that the existence of these problems is highly dependent on the choice of higher-dimensional operators. Correspondingly, the typical energy of produced particles during the first stage of preheating can remain comparable to or below the cut-off scale of the theory. As an example, we have numerically calculated particle production in the presence of one such operator. We also exhibit a possible UV-completion that gives rise to such an operator.