Disentangling the potential dark matter origin of LIGO's black holes

Abstract

The nature of dark matter remains one of the biggest open questions in physics. Intriguingly, it has been suggested that dark matter may be explained by another recently observed phenomenon: the detection of gravitational waves by LIGO. LIGO's detection of gravitational waves from merging stellar mass black holes renewed attention toward the possibility that dark matter consists solely of black holes created in the very early universe and that these primordial black holes are what LIGO is presently observing. Subsequent work on this topic has ruled out the possibility that dark matter could consist solely of black holes similar to those that LIGO has detected with masses above 10 solar masses. However, LIGO's connection to dark matter remains an open question and in this work we consider a distribution of primordial black holes that accounts for all of the dark matter, is consistent with LIGO's observations arising from primordial black hole binaries, and resolves tension in previous surveys of microlensing events in the Milky Way halo. The primordial black hole mass distribution that we consider offers an important prediction--LIGO may detect black holes smaller than have ever been observed with ~1% of the black holes it detects having a mass less than the mass of our Sun. Approximately one year of operating advanced LIGO at design sensitivity should be adequate to begin to see a hint of a primordial black hole mass distribution. Detecting primordial black hole binaries below a solar mass will be readily distinguishable from other known compact binary systems, thereby providing an unambiguous observational window for advanced LIGO to pin down the nature of dark matter.