Domain wall and isocurvature perturbation problems in axion models

Abstract

Axion models have two serious cosmological problems, domain wall and isocurvature perturbation problems. In order to solve these problems we investigate the Linde's model in which the field value of the Peccei-Quinn (PQ) scalar is large during inflation. In this model the fluctuations of the PQ field grow after inflation through the parametricresonance and stable axionic strings may be produced, which results in the domain wall problem. We study formation of axionic strings using lattice simulations. It is found that in chaotic inflation the axion model is free from both the domain wall and the isocurvature perturbation problems if the initial misalignment angle θa is smaller than O(10−2).Furthermore, axions can also account for the dark matter for the breaking scale v ≃ 1012−16 GeV and the Hubble parameter during inflation Hinf≲1011−12 GeV in general inflation models.