LIGO-Virgo collaboration has found black holes as heavy as $M\sim 30M_{\odot }$ through the detections of the gravitational waves emitted during their mergers. Primordial black holes (PBHs) produced by inflation could be an origin of such BHs. While it is tempting to presume that these PBHs constitute all dark matter (DM), there exist a number of constraints for PBHs with $\mathcal{O}(10)M_{\odot }$ which contradict with the idea of PBHs as all DM. Also, it is known that weakly interacting massive particle (WIMP) that is a common DM candidate is almost impossible to coexist with PBHs. These observations motivate us to pursue another candidate of DM. In this paper, we assume that the string axion solving the strong $CP$ problem makes up all DM, and discuss the coexistence of string axion DM and inflationary PBHs for LIGO events.

• Received 10 October 2017

DOI:https://doi.org/10.1103/PhysRevD.96.123527