Axion cold dark matter in view of BICEP2 results
L. Visinelli, P. Gondolo
TL;DR
The paper investigates whether axions can comprise all cold dark matter in light of the BICEP2 measurement of tensor modes. By distinguishing PQ symmetry-breaking scenarios relative to inflation, it shows that the BICEP2 result excludes PQ breaking before inflation (Scenario B), leaving post-inflation breaking (Scenario A) where the axion density is set by vacuum realignment and possible string-wall decays. In ΛCDM, this yields a narrow axion mass window around $m_a \approx 71 \mu{\rm eV}$ modulated by the defect contribution parameter $\alpha^{\rm dec}$, with the corresponding decay constant $f_a \approx 8.7\times10^{10}\, {\rm GeV}\,(\alpha^{\rm dec}+1)^{-6/7}$. The work links early-Universe inflationary physics to present-day axion searches, guiding experiments toward the tens-of-microelectronvolt mass range and highlighting the role of topological defects in setting the relic density.
Abstract
The properties of axions that constitute 100% of cold dark matter (CDM) depend on the tensor-to-scalar ratio $r$ at the end of inflation. If $r=0.20^{+0.07}_{-0.05}$ as reported by the BICEP2 collaboration, then "half" of the CDM axion parameter space is ruled out. Namely, the Peccei-Quinn symmetry must be broken after the end of inflation, and axions do not generate non-adiabatic primordial fluctuations. The cosmic axion density is then independent of the tensor-to-scalar ratio $r$, and the axion mass is expected to be in a narrow range that however depends on the cosmological model before primordial nucleosynthesis. In the standard $Λ$CDM cosmology, the CDM axion mass range is $m_a = \left(71 \pm 2\right) μ{\rm eV} \, (α^{\rm dec}+1)^{6/7}$, where $α^{\rm dec}$ is the fractional contribution to the cosmic axion density from decays of axionic strings and walls.