Abstract
The BICEP2 experiment determined the Hubble parameter during inflation to be about 1014 GeV. Such high inflation scale is in tension with the QCD axion dark matter if the Peccei–Quinn (PQ) symmetry remains broken during and after inflation, because too large axion isocurvature perturbations would be generated. The axion isocurvature perturbations can be suppressed if the axion acquires a sufficiently heavy mass during inflation. We show that this is realized if the PQ symmetry is explicitly broken down to a discrete symmetry and if the breaking is enhanced during inflation. We also show that, even when the PQ symmetry becomes spontaneously broken after inflation, such a temporarily enhanced PQ symmetry breaking relaxes the constraint on the axion decay constant.
Highlights
The BICEP2 experiment determined the Hubble parameter during inflation to be about 1014 GeV
Even when the PQ symmetry becomes spontaneously broken after inflation, such a temporarily enhanced PQ symmetry breaking relaxes the constraint on the axion decay constant
If the PQ symmetry is restored during inflation, there is no axion cold dark matter (CDM) isocurvature perturbations, as the axion appears only when the PQ symmetry is spontaneously broken some time after inflation [26, 27]
Summary
The identity of dark matter is one of the central issues in cosmology and particle physics. [31] is consistent with the BICEP2 result only in a corner of the parameter space We need another solution to suppress the axion isocurvature perturbations, as long as we assume that the PQ symmetry remains broken during and tension with the Planck result can be relaxed in the presence of small modulations in the inflaton potential [25] or hot dark matter/dark radiation. If, during inflation, the radial component of the PQ scalar, i.e. the saxion, takes a large field value, the PQ symmetry breaking operator is enhanced and the axion becomes heavy. Later in this letter we briefly consider the first solution to the tension between the BICEP2 results and the axion CDM isocurvature perturbations, i.e., the PQ symmetry restoration during or after inflation.
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