Dark matter is the most common matter in the Universe, accounting for nearly 85% of the total mass of the cosmos. But while we know dark matter is out there, we have no real idea what it is. Lots of ideas have been proposed, such as WIMPs or exotic particles known as axions, but another idea is that it might be caused by black holes. Not ones that formed from dying stars or the giants in the hearts of galaxies, but primordial black holes that formed soon after the big bang. 

Most models of primordial black holes propose that they formed during the first few milliseconds of the Universe. Depending on the model their hypothetical sizes range from tiny ones the mass of a small mountain, to large ones dozens of times more massive than the Sun. There has been a great deal of effort to observe primordial black holes, but so far there has been no signs of them. For this reason, primordial black holes have not been a popular solution to the dark matter problem. There would have to be lots of primordial black holes out there to account for dark matter, and their presence would be seen by things like the gravitational lensing of starlight.

The recent detection of gravitational waves, however, has rekindled interest in dark matter black holes. The LIGO observation saw the merger of two black holes roughly 30 solar masses in size. This is curious, because we’d expect stellar collapse black holes to have less mass than that, and intermediate mass black holes found in star clusters typically have more mass than that. It’s hard to imagine how the LIGO black holes could have formed from either stars or mergers, but they do fall within the upper range of primordial black hole models. They also fall within the mass range that’s hardest to rule out as a dark matter candidate.

A recent paper in the Astrophysical Journal Letters argues that black holes like the ones seen by LIGO could account for dark matter. To support this idea, Alexander Kashlinsky looked at fluctuations in the cosmic infrared background. Unlike the cosmic microwave background, which is the thermal remnant of the big bang, the cosmic infrared background is caused by a wide range of processes that produce infrared light, such as heated gas within galaxies. Fluctuations in this background can tell us about the structure of its sources. Kashlinsky compared infrared fluctuations with the distribution of known sources such as galaxies, and found some of the fluctuations couldn’t be accounted for by known sources. The scale of these fluctuations agrees with a dark matter distribution of LIGO-mass black holes. So it’s possible that black holes could explain dark matter after all.

There are reasons to be cautious however. In addition to black holes, there are other models that could explain the fluctuations of the infrared background. There’s also the fact that while LIGO has found black holes on the order of 30 solar masses, we don’t know if such black holes are common or rare. However further observations of the infrared background and the gravitational waves from black hole mergers can answer these questions. If LIGO-mass black holes turn out to be quite common, then the great mystery of dark matter could be solved by primordial black holes after all.

Paper: A. Kashlinsky. LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies. The Astrophysical Journal Letters, Volume 823, Number 2 (2016) arXiv:1605.04023 [astro-ph.CO]

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The Nature website has recently written about a preprint paper postulating that primordial black holes could collide with and destroy neutron stars.  The paper goes on to argue that because we have not observed neutron star explosions these primordial black holes are not likely to exist.  Of course that isn’t surprising.

The basic idea of a primordial black hole is that it would form during the early moments of the universe when the mass-energy density was really high.  We know that there were fluctuations in the early universe, as evidenced from the cosmic microwave background.  If some fluctuations got large enough fast enough, then they could collapse into a black hole before the inflationary period started to even things out.

Primordial black holes hold a strange position in astrophysics.  On the one hand, they would be an interesting solution to the dark matter problem.  Given what we know of dark matter, we know it can’t interact with light strongly, and it can’t be baryonic (meaning it can’t be made of stuff like protons and neutrons).  If primordial black holes had formed before the period where protons and neutrons formed, then they would be non-baryonic.  If they were of small enough mass, then they wouldn’t interact strongly with light.  So they seem like a really good candidate for dark matter.

On the one hand, there isn’t any evidence for primordial black holes.  None.  If primordial black holes were dark matter, then they would make up the majority of mass in our galaxy. If they were around we’d see some of them gravitationally lens stars they pass in front of them.  Some of them would collide with each other, and we’d see a burst of light.  If they were small enough, some of them would evaporate in a burst of Hawking radiation.  Now apparently some would collide with neutron stars and destroy them.  We’ve looked for all of these things and found nothing.  So far, every observational test we’ve done has turned up nothing.

There is still a very small mass range (about the mass of an asteroid) where observational evidence hasn’t completely ruled them out.  If primordial black holes had this very specific range, and only this range, then they could still make up at least part of dark matter and remain undetected so far.  But that is extraordinarily unlikely.  This is why most astronomers have moved on from black holes as a solution to the dark matter problem, and are instead looking for things like WIMPS (weakly interacting massive particles).

Sometimes what seems like a really good idea just doesn’t work out.

Paper:  P Pani and A Loeb. Exclusion of the remaining mass window for primordial black holes as the dominant constituent of dark matter. arXiv:1401.3025

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