Primordial gravitational waves in a minimal model of particle physics
  and cosmology
  In this paper we analyze the spectrum of the primordial gravitational waves
(GWs) predicted in the Standard Model*Axion*Seesaw*Higgs portal inflation
(SMASH) model, which was proposed as a minimal extension of the Standard Model
that addresses five fundamental problems of particle physics and cosmology
(inflation, baryon asymmetry, neutrino masses, strong CP problem, and dark
matter) in one stroke. The SMASH model has a unique prediction for the critical
temperature of the second order Peccei-Quinn (PQ) phase transition $T_c \sim
10^8\,\mathrm{GeV}$ up to the uncertainty in the calculation of the axion dark
matter abundance, implying that there is a drastic change in the equation of
state of the universe at that temperature. Such an event is imprinted on the
spectrum of GWs originating from the primordial tensor fluctuations during
inflation and entering the horizon at $T \sim T_c$, which corresponds to $f
\sim 1\,\mathrm{Hz}$, pointing to a best frequency range covered by future
space-borne GW interferometers. We give a precise estimation of the effective
relativistic degrees of freedom across the PQ phase transition and use it to
evaluate the spectrum of GWs observed today. It is shown that the future high
sensitivity GW experiment -- ultimate DECIGO -- can probe the nontrivial
feature resulting from the PQ phase transition in this model.
