High Energy Physics

We study the implications of precision measurements of light-element abundances, in combination with the Cosmic Microwave Background, for scenarios of physics beyond the Standard Model that generate large inhomogeneities in the baryon-to-photon ratio. We show that precision Big Bang Nucleosynthesis (BBN) places strong constraints on any mechanism that produces large-scale inhomogeneities at temperatures around or below the TeV scale. In particular, we find that fluctuations of order 25% on comoving length scales larger than the horizon at T ~ 3 TeV are incompatible with the observed light-element abundances. This sensitivity to early-universe physics arises because baryon-number inhomogeneities homogenize primarily through diffusion, a slow process. As a result, BBN serves as a novel probe of baryogenesis below the TeV scale, readily ruling out some proposed scenarios in the literature. We discuss the implications for electroweak baryogenesis, and further show that precision BBN provides a new probe of first-order phase transitions that generate gravitational waves in the pHz–mHz frequency range. This yields constraints on the electroweak phase transition, as well as first-order phase transitions that have been suggested as an explanation of the pulsar timing array signal. Finally, we comment on the future prospects for improving this probe

The QCD axion is not only a leading contender as a solution to the Strong CP problem, it is also a natural dark matter candidate. In particular, the post-inflationary axion is particularly attractive because of the possibility of a unique prediction of the axion mass if axion makes up all of dark matter. On the other hand, QCD axion models often suffers from the so-called axion quality problem, where the axion shift symmetry is unprotected from quantum gravity effects. In this talk I will discuss the tension between solutions to the quality problem and viable cosmology for post-inflationary QCD axions. I will start with a simple Z_N solution as an illustrative example of the close connection between the quality problem and the domain wall problem. I will then analyze proposals in the literature that involve more complex symmetry structure and show that they share a set of cosmological issues. Our study suggests that a viable post-inflationary QCD axion model is likely to have a non-standard cosmological history which could significantly impact the prediction of the "correct" axion mass.