Axion Cold Dark Matter in View of BICEP2 Results

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(-0.05)(+0.07) as reported by the BICEP2 Collaboration, then half of the CDM axion parameter space is ruled out. Namely, in the context of single-field slow-roll inflation, for axions to be 100% of the CDM, the Peccei-Quinn symmetry must be broken after the end of inflation, so that axion nonadiabatic primordial fluctuations are compatible with observational constraints. 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) = (71 +/- 2 mu eV) (alpha(dec) + 1)(6/7), where alpha(dec) is the fractional contribution to the cosmic axion density from decays of axionic strings and walls.