Theory HEP Seminar

Cosmic Axiverse Background

Chris Dessert

CCA

Any light species in thermal equilibrium in the early universe, such as an axion, will contribute to the effective number of relativistic species Neff. In the context of the Axiverse, potentially hundreds of axions exist in the spectrum of nature and can thermalize with the Standard Model bath, but Planck data constrains the number of additional thermalized scalar degrees of freedom to nine. I discuss a computation of the Axiverse contribution to Neff under various assumptions for the structure of the axion couplings to the SM.