Dark matter and LIGO

Question

In addition to the functioning LIGO detectors (two in USA and one in Italy), I am aware there are some gravitation wave detectors under construction. More detectors should provide more statistical data and I suppose some of these detectors may even be more sensitive.

Can these detectors give us some new insight about the nature of dark matter? With respect to dark matter, what can we detect using gravitational waves? Is the limiting factor the precision of the detectors or there is some other issue?

Answer 1

We will start getting more data before long from gravitational detectors. Virgo comes online soon, but LIGO is to be taken down for a while with updates. When fully functioning, and a couple others added in, we should start detecting maybe 10 a year, and after 10 years we'd have about 100 black hole (BH) mergers detected. And measured. The statistics as to whether there are enough mid sized (10-20 to 100 solar mass) BHs to make up dark matter will then be better known.

The other two confirmed detections of BHs at LIGO were smaller ones than the first, 8/14 solar masses for the second, and maybe twice that much for the third. Nothing bugger than the first BH merger detection.

There is a lot of mass to make up to have it make up the dark matter. About 5 times more than visible matter. With more statistics we'll see how likely the mid sized ones are.

Also, with more interferometers well start to be able to locate the sources, much more than the quarter or so hemisphere estimates we have now. Whether that'll be enough to determine if they are in the halo of a galaxy, which is where they should be more likely, remains to be seen - I doubt it'd be that good. But we should be able to start correlating the locations with otherwise observed galaxies, and perhaps with $\gamma$ ray bursts, or X ray sources, and start tying some astrophysics to them.

What is not clear to me is if enough calculations/simulations have been done to estimate how many of these primordial BH there should be, and see some of them from microlensing.

Finally, at some point (doesn't look like anytime in the early 2020's, expensive) we'll have some space based gravitational interferometers to see gravitational waves for the first second of the universe, and see some remnants of their formation. Also we might detect their effect on the CMB.

Some of these astrophysical and gravitational wave observations should start pointing things one way or the next.

What is true is we might be able to determine the contributions of BHs to dark matter. But it is still too early to say if it looks positive or not. Before the discovery of the midsized BHs, MACHO parameter space had been cut back to not much on the possible sources of dark matter. But midsized BHs were not thought to be prevalent. So MACHOs have a new chance.

See http://www.sciencemag.org/news/2017/02/dark-matter-made-black-holes

Answer 2

LIGO has the potential to give insights into the possibility of one component of dark matter: primordial black holes. Key targets of gravitational wave interferometers are merging binary systems (either black holes or neutron stars), and so if enough black hole binaries could be detected over time, scientists might be able to constrain the mass ranges of primordial black holes, thus giving us a better understanding of whether they could be good dark matter candidates.

This was explored in some detail in a recent paper, Bird et al. (2016). The authors consider mass ranges from $20$ to $100$ solar masses. They state that if primordial black holes have masses of roughly the same as those from the first detection in 2015 (GW150914), then the rates of mergers of primordial black holes should be consistent with the merger rates derived from that event.

That said, the authors assumed that the hypothetical black holes had masses like those of the black holes detected in that first event. LIGO has since had other detections (GW151226 and GW170104), involving binaries of different masses. It's possible that Bird et al. would change their calculations based on these later discoveries.