No massive black holes in the Milky Way halo.

Abstract

The gravitational wave detectors have shown a population of massive black holes that do not resemble those observed in the Milky Way^1-3 and whose origin is debated^4-6. According to a possible explanation, these black holes may have formed from density fluctuations in the early Universe (primordial black holes)^7-9, and they should comprise several to 100% of dark matter to explain the observed black hole merger rates^10-12. If these black holes existed in the Milky Way dark matter halo, they would cause long-timescale gravitational microlensing events lasting years¹³. The previous experiments were not sufficiently sensitive to such events^14-17. Here we present the results of the search for long-timescale microlensing events among the light curves of nearly 80 million stars located in the Large Magellanic Cloud that were monitored for 20 years by the Optical Gravitational Lensing Experiment survey¹⁸. We did not find any events with timescales longer than 1 year, whereas all shorter events detected may be explained by known stellar populations. We find that compact objects in the mass range from 1.8 × 10^-4M_⊙ to 6.3M_⊙ cannot make up more than 1% of dark matter, and those in the mass range from 1.3 × 10^-5M_⊙ to 860 M_⊙ cannot make up more than 10% of dark matter. Thus, primordial black holes in this mass range cannot simultaneously explain a substantial fraction of dark matter and gravitational wave events.