Revisiting Black Holes as Dark Matter

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Could dark matter be made of intermediate-mass black holes formed in the beginning of the universe? A recent study takes a renewed look at this question.

Galactic Lurkers

The nature of dark matter has long been questioned, but the recent discovery of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) has renewed interest in the possibility that dark matter could consist of primordial black holes in the mass range of 10–1000 solar masses.

dark matter

The relative amounts of the different constituents of the universe. Dark matter makes up roughly 27%. [ESA/Planck]

According to this model, the extreme density of matter present during the universe’s early expansion led to the formation of a large number of intermediate-mass black holes. These black holes now hide in the halos of galaxies, constituting the mass that we’ve measured dynamically but remains unseen.

LIGO’s first gravitational-wave detection revealed the merger of two black holes that were both tens of solar masses in size. If primordial black holes are indeed a major constituent of dark matter, then LIGO’s detection is consistent with what we would expect to find: occasional mergers of the intermediate-mass black holes that formed in the early universe and now lurk in galactic halos.

Quasar Microlensing

There’s a catch, however. If there truly were a large number of intermediate-mass primordial black holes hiding in galactic halos, they wouldn’t go completely unnoticed: we would see signs of their presence in the gravitational microlensing of background quasars. Unseen primordial black holes in a foreground galaxy could cause an image of a background quasar to briefly brighten — which would provide us with clear evidence of such black holes despite our not being able to detect them directly.

quasar microlensing

A depiction of quasar microlensing (click for a closer look!). The microlensing object in the foreground galaxy could be a star (as depicted), a primordial black hole, or any other compact object. [NASA/Jason Cowan (Astronomy Technology Center)]

A team of scientists led by Evencio Mediavilla (Institute of Astrophysics of the Canaries, University of La Laguna) has now used our observations of quasar microlensing to place constraints on the amount of dark matter that could be made up of intermediate-mass primordial black holes.

Poor Outlook for Primordial Black Holes

Mediavilla and collaborators used simulations to estimate the effects of a distribution of masses on light from distant quasars, and they then compared their results to microlensing magnification measurements from 24 gravitationally lensed quasars. In this way, they were able to determine both the abundance and masses of possible objects causing the quasar microlensing effects we see.

The authors find that the observations constrain the mass of the possible microlensing objects to be between 0.05 and 0.45 solar masses — not at all the intermediate-mass black holes postulated. What’s more, they find that the lensing objects make up ~20% of the total matter, which is barely more than expected for normal stellar matter. This suggests that normal stars are doing the majority of the quasar microlensing, not a large population of intermediate-mass black holes.

What does this mean for primordial black holes as dark matter? Black holes in the range of 10–200 stellar masses are unlikely to account for much (if any) dark matter, Mediavilla and collaborators conclude — which means that LIGO’s detection of gravitational waves likely came from two black holes collapsed from stars, not primordial black holes.

Citation

E. Mediavilla et al 2017 ApJL 836 L18. doi:10.3847/2041-8213/aa5dab

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