Axion cold dark matter in view of BICEP2 results

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 $\Lambda$CDM cosmology, the CDM axion mass range is $m_a = \left(71 \pm 2\right) \mu{\rm eV} , (\alpha^{\rm dec}+1)^{6/7}$, where $\alpha^{\rm dec}$ is the fractional contribution to the cosmic axion density from decays of axionic strings and walls.

Luca Visinelli

Assistant Professor of Theoretical Physics (RTT)

Theoretical physicist working across particle physics, astrophysics, cosmology, and experimental searches.