Did LIGO Detect Dark Matter?

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 20 M M bh 100 M where primordial black holes (PBHs) may constitute the dark matter. If two BHs in a galactic halo pass sufficiently close, they radiate enough energy in gravitational waves to become gravitationally bound. The bound BHs will 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.The nature of the dark matter (DM) is one of the most longstanding and puzzling questions in physics. Cosmological measurements have now determined with exquisite precision the abundance of DM [1, 2], and from both observations and numerical simulations we know quite a bit about its distribution in Galactic halos. Still, the nature of the DM remains a mystery. Given the efficacy with which weakly-interacting massive particlesfor many years the favored particle-theory explanationhave eluded detection, it may be warranted to consider other possibilities for DM. Primordial black holes (PBHs) are one such possibility [3-6].Here we consider whether the two ∼ 30 M black holes detected by LIGO [7] could plausibly be PBHs. There is a window for PBHs to be DM if the BH mass is in the range 20 M M 100 M [8,9]. Lower masses are excluded by microlensing surveys [10][11][12]. Higher masses would disrupt wide binaries [9,13,14]. It has been argued that PBHs in this mass range are excluded by CMB constraints [15,16]. However, these constraints require modeling of several complex physical processes, including the accretion of gas onto a moving BH, the conversion of the accreted mass to a luminosity, the self-consistent feedback of the BH radiation on the accretion process, and the deposition of the radiated energy as heat in the photon-baryon plasma. A significant (and difficult to quantify) uncertainty should therefore be associated with this upper limit [17], and it seems worthwhile to examine whether PBHs in this mass range could have other observational consequences.In this Letter, we show that if DM consists of ∼ 30 M BHs, then the rate for mergers of such PBHs falls within the merger rate inferred from GW150914. In any galactic halo, there is a chance two BHs will undergo a hard scatter, lose energy to a s...

Production of high stellar-mass primordial black holes in trapped inflation

“…Interestingly, massive PBHs being non-relativistic and weakly interacting gravitationally may constitute the cold dark matter, if they are formed prior to the big bang nucleosynthesis. The estimated merger rate for these PBHs, if they are clustered in compact sub-halos, can span a range that overlaps the BH merger rate inferred from GW150914 [7][8][9]. As such, GW150914 can be considered as an indirect signal of dark matter coming from the PBH sub-halo that is a conversion of a few M ⊙ dark matter into gravitational waves in a massive PHB coalescence.…”

“…[7] have considered a possibility that the BH binary associated with GW150914 may be of primordial origin, different from more traditional astrophysical sources -the two BHs are primordial black holes (PBHs) formed in the early Universe. Interestingly, massive PBHs being non-relativistic and weakly interacting gravitationally may constitute the cold dark matter, if they are formed prior to the big bang nucleosynthesis.…”

Cosmic microwave background limits on accreting primordial black holes

“…Chapline was the first to suggest that PBHs could make the dark matter (DM) [3]. Though this class of DM candidate has taken a back seat to the notion that DM is a new elementary particle [4][5][6][7][8], the idea of PBH dark matter was recently rekindled [9,10], following the first detection of two merging ∼ 30 M black holes by LIGO [11]. Given the increasingly constraining null searches for particle DM, PBHs and their observational consequences are worth reconsidering [12,13].…”

Constraining primordial black holes with the EDGES 21-cm absorption signal

“…The detection of gravitational waves from black holes mergers with mass Oð10ÞM ⊙ by the LIGO and VIRGO interferometers [27,28] has recently revived the interest of the community in this topic [29][30][31][32][33] (for a review see, e.g., [34]). At the present stage, the accumulated experimental data collectively constrain the fraction of PBH DM, f PBH , to be below unity [34,35], barring scenarios in which the radiation induced by PBH is strongly modified [36] or misinterpretations of the results of lensing experiments [37][38][39].…”