Did LIGO detect dark matter?
(Mar 1, 2016)Abstract
We consider the possibility that the black-hole (BH) binary detected by LIGO
may be a signature of dark matter. Interestingly enough, there remains a window
for masses \$10\,M\_M\_bh 100\, M\_ødot\$ where
primordial black holes (PBHs) may constitute the dark matter. If two BHs in a
galactic halo pass sufficiently close, they can radiate enough energy in
gravitational waves to become gravitationally bound. The bound BHs will then
rapidly spiral inward due to emission of gravitational radiation and ultimately
merge. Uncertainties in the rate for such events arise from our imprecise
knowledge of the phase-space structure of galactic halos on the smallest
scales. Still, reasonable estimates span a range that overlaps the \$2-53\$
Gpc\$^-3\$ yr\$^-1\$ rate estimated from GW150914, thus raising the possibility
that LIGO has detected PBH dark matter. PBH mergers are likely to be
distributed spatially more like dark matter than luminous matter and have no
optical nor neutrino counterparts. They may be distinguished from mergers of
BHs from more traditional astrophysical sources through the observed mass
spectrum, their high ellipticities, or their stochastic gravitational wave
background. Next generation experiments will be invaluable in performing these
tests.