The Bearable Inhomogeneity of the Baryon Asymmetry

We study the implications of precision measurements of light element abundances in concordance 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) imposes strong constraints on any mechanism that produces large scale inhomogeneities at temperatures of the order or below a TeV. In particular, we find that inhomogeneities of the order of $20\%$ at comoving length scales larger than the comoving horizon at the temperature of $3~\mathrm{TeV}$ are in conflict with the measured light element abundances. This sensitivity to physics at such early times is because inhomogeneities in baryon number homogenize predominantly through diffusion, which is a slow process. BBN therefore acts as a novel probe of baryogenesis below the $\mathrm{TeV}$ scale, and readily rules out some of the proposed scenarios of baryogenesis in the literature. We discuss the implications for electroweak baryogenesis. In addition we show that precision BBN is a new probe of first-order phase transitions which produce a gravitational wave signal in the frequency range from pHz to mHz. This leads to constraints on the electroweak phase transition, as well as the first-order phase transitions that have been postulated to explain the pulsar timing array signal. We also discuss the future prospects of improvements in this probe.