What if what LIGO detected was dark matter?

At the end of the summer of 2016, the LIGO observatory detectors detected the signal in the form of a gravitational wave from two black holes of about 30 solar masses orbiting each other closer and closer until they merged. This type of black holes could be abundant in the Universe, so much so that they could actually be the famous dark matter. This, at least, is the idea considered by Simeon Bird, of the Johns Hopkins University (USA), and his colleagues in an article published in Physical Review Letters.

Bird's team explored this possibility assuming that the rate of fusions of black holes of 30 solar masses occupied the place of dark matter. The results are consistent with the rate that is inferred from the observations of LIGO, but it is clear that much more data is needed to affirm that the connection between black holes and dark matter is more than just an idea.

Most of the matter of the universe can not be seen by any type of telescope since it does not emit or absorb any type of radiation. We know of its existence by the effects that its gravity causes, for example, in the rotation and structure of galaxies but we do not know its composition, among other things, precisely because it does not interact with radiation. This is what is called dark matter.

It is usually thought that dark matter is made up of microscopic particles, but there are certain theories of the primitive universe that predict the formation of black holes that could act as the constituents of dark matter. The size of these primordial holes could be between 20 and 100 solar masses.

Bird and his collaborators have considered as hypothesis that the universe was full of primordial black holes of 30 solar masses with a density consistent with the distribution models of dark matter. Making assumptions about the concentration and velocity of dark matter within galaxies, they have calculated how often these primordial black holes get close enough to each other to end up merging. It is this frequency of mergers that fits the estimate made by the LIGO researchers, who found that between 2 and 53 mergers per year should be produced for each cubic gigaparsec (the equivalent of one billion Milky Ways).

The idea is really attractive. However, there are very high uncertainties in dark matter models, so many more merge observations are needed to find specific dark matter signals, as a significant percentage in small galaxies.

Reference:

Simeon Bierd et al (2016) Did LIGO Detect Dark Matter? Physical Review Letters doi: 10.1103 / PhysRevLett.116.201301

About the author: César Tomé López is a scientific disseminator and editor of Mapping Ignorance