Abstract
We find that the adiabatic fluctuations produced in the primordial plasma by cosmological inflation resonantly excite the axion field during the QCD phase transition by pumping axions from low momentum modes to modes with momentum up to of order $\sqrt{3}m$ where $m$ is the axion mass. We derive the momentum distribution of the excited axions. The fraction of cold axions that get excited is of order one if the axion mass is larger than a few $\ensuremath{\mu}\mathrm{eV}$. The effect occurs whether inflation happens before or after the Peccei-Quinn phase transition.
Highlights
The axion is a hypothetical particle postulated to solve the strong CP problem, i.e., to explain why the strong interactions conserve the discrete symmetries P and CP in spite of the fact that these symmetries are broken in the Standard Model of particle physics as a whole [1–3]
Axions are independently motivated by cosmology because a population of cold axions is produced during the QCD phase transition, when the temperature is of order 1 GeV [4]
UPQð1Þ becomes spontaneously broken when the temperature falls below some critical value of order fa, during the so-called PQ phase transition. If this phase transition occurs before inflation, the axion field gets homogenized by the inflationary expansion, so that at the start of the QCD phase transition the axion field oscillations begin with almost the same amplitude everywhere
Summary
The axion is a hypothetical particle postulated to solve the strong CP problem, i.e., to explain why the strong interactions conserve the discrete symmetries P and CP in spite of the fact that these symmetries are broken in the Standard Model of particle physics as a whole [1–3]. UPQð1Þ becomes spontaneously broken when the temperature falls below some critical value of order fa, during the so-called PQ phase transition If this phase transition occurs before inflation, the axion field gets homogenized by the inflationary expansion, so that at the start of the QCD phase transition the axion field oscillations begin with almost the same amplitude everywhere. The quantum mechanical fluctuations in the axion field during inflation are not the only source of perturbations in the axion fluid in case inflation occurs after the PQ phase transition. It is possible to derive the momentum spectrum of cold axions with the greatest precision in case inflation occurs after the PQ phase transition, and this is one of our goals here.
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