Can LIGO Find the Missing Dark Matter?

Today promises to be an exciting day in the world of gravitational-wave detections. To keep with the theme, we thought we’d use this opportunity to take a renewed look at an interesting question about the Laser Interferometer Gravitational-Wave Observatory (LIGO) and dark matter: if dark matter is made up of primordial black holes, will LIGO be able to detect them?

Black Holes in the Early Universe

The black holes we generally think about in the context of gravitational-wave detections are black holes formed by the collapse of massive stars. Indeed, LIGO’s detections have thus far been of merging black holes weighing between 7 and 35 solar masses — the perfect sizes to have formed from massive-star collapses.

But another type of black hole has also been proposed: primordial black holes, which theoretically formed in the early universe as a result of the direct collapse of density fluctuations during the Big Bang. These proposed black holes could have initial masses anywhere from an impossibly small 10^-8 kg (that’s ~10^-38 solar masses) to thousands of solar masses.

Dark Matter as Black Holes

Could dark matter — the missing, unseen matter in our universe — be composed of as-yet-undetected primordial black holes? Over several decades, scientists have narrowed down possible mass ranges for these hypothetical black holes based on models and observations. In a recent publication, Pennsylvania State University scientists Ryan Magee and Chad Hanna now consider these past constraints together with LIGO’s observations thus far.

Magee and Hanna suggest a population of primordial black holes with a mass distribution peaking between 0.06 and 1 solar mass, which they argue could account for all of the missing dark matter. The authors’ model for this population is consistent with LIGO’s observations, as well as the strongest constraints placed by microlensing observations.

LIGO’s Performance

Would LIGO be able to detect the black holes the authors predict? In theory, LIGO is or will be sensitive to signals from the mergers of black holes between ~0.01 and 100 solar masses, even out to extragalactic distances.

Magee and Hanna offer a prediction to test their model: with the mass distribution of black holes predicted by their model, 1% of LIGO’s detections would be of black holes less massive than our Sun. Since such small black holes can’t be formed by stellar collapses, such a detection would be a smoking gun supporting the model of primordial black holes as dark matter.

Based on LIGO’s specs, this prediction suggests that if Magee and Hanna’s model is correct, then within one year of operating advanced LIGO at design sensitivity, we will have found signs of a primordial black hole mass distribution. Now we just have to wait and see how things pan out!

Citation

Ryan Magee and Chad Hanna 2017 ApJL 845 L13. doi:10.3847/2041-8213/aa831c