Did Ligo Detect Dark Matter?
Alvise Raccanelli (left) gave an overview of the possibility that primordial black holes be dark matter candidates. He started with a description of the black hole merger observed by Ligo. That could be a chance observation, or maybe solar-mass black holes are more common than expected.
The standard cosmological model involves dark matter as cold. But what if it were due to primordial black holes? So one can look at constraints of these objects. The constraints come from microlensing, from wide binaries, and from disk kinematics. Running simulations they found that a model with black holes as dark matter could be viable, in a window between 20 and 100 solar masses per object.
The CMB anisotropy measurements seemed to constrain this further, but a reanalysis shows that under the worst possible assumptions and more detailed modeling the constraint was shown to not hold.
Another constraint on primordial black holes could come from observing globular clusters with the observed age and population, as black holes moving through a globular cluster would destroy it. This was also shown to not hold, as the disruption was shown to not hold in general cases.
In a toy model, all black holes are in the 30 solar mass range, which seem to survive the constraints. But of course a distribution of masses would change the picture; yet there seems to still be a lot of room for primordial black holes contributing significantly to the DM budget of the universe.
It is also possible – and this is a disturbing possibility – that there exist primordial black holes of all possible masses, and they conspire to contribute to the DM budget while being very hard to rule out or detect.
In big halos, the velocity dispersion is very high, so even if black holes meet each other, they do not create binary systems. In small halos it is more likely that they form a binary and emit gravitational waves. So how small should these halos be ? 99% should be in halos of less than a million solar masses. A cross-correlation of galaxy catalogs and gravitational wave maps, then one could understand whether this picture holds. If gravitational waves come from areas where a lot of stars are, the black holes could be inferred to be due to star evolution. If GWs come from areas where there are few stars, this could indicate primordial black holes.
Alvise he then discussed several potential ways to ascertain whether primordial black holes can be the explanation of DM in the universe. They are all requiring lots of more precise data, but a solution can eventually be found. So are black holes the source of dark matter in the universe ? The answer is that it is a plausibility argument that at the moment cannot be disproven.