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Description
The present universe is made of relics from the early universe, such as baryons, dark matter, and radiations. In order to precisely estimate their present abundance, we need to know how the universe has evolved from the early epoch until today. In this talk, we discuss thermodynamic properties of high temperature plasma in the early universe and identify their impact on the abundance of cosmological relics. Here we consider two classes of relics, dark matter and primordial gravitational waves (GWs). For the study of dark matter, we revisit the estimation of the WIMP dark matter abundance from the standard freeze-out mechanism and evaluate its uncertainty due to the equation of state in the Standard Model. For the study of primordial GWs, we particularly focus on those predicted in a minimal extension of the Standard Model, called the Standard Model-Axion-Seesaw-Higgs portal inflation (SMASH) model. We point out that the SMASH model gives a non-trivial and unique prediction for the nature of the second order Peccei-Quinn phase transition, which is imprinted on the spectrum of primordial GWs in a frequency range relevant to future high-sensitivity GW experiments.
