General Relativity in Everyday Life

My first seminar at ICCUB is live on youtube!

A note on the questions: I favored field binaries, i.e., binaries that form in the galaxy, over a dense environment/dynamic evolution scenario. Most stars form in binaries because of the way the disc fragments. Then when the stars explode, some remnants stay in binaries. Thus, the field binaries go through a channel that is somewhat better understood -- common envelope, supernova collapse that does not kick the black hole much. However, both scenarios are equally possible at this stage. Neither is ruled out by LIGO data.

The field binaries provenience would be more likely only IF the black holes are born without spin, which they could be due to inefficient angular momentum coupling, but we don't know that this is the case. Naively, one would expect BHs (and NSs) to acquire high spin during the collapse process from a larger to a much smaller object -- it's just that neither BHs nor the observed young neutron stars seem to have much spin. So, either they are born with high spin and there is some mechanism that spins them down like r-modes for neutron stars and super-radiance for black holes as Roberto pointed out during my seminar OR they are born without spin. A proof for super-radiance would be a scenario like Roberto mentioned, where there is high spin below a certain mass, when the super-radiance process is active, and no spin above that mass or the other way around. It would be more interesting to have them born spinning and see the spin-down mechanism at work -- but we have not seen such proof yet. For neutron stars, the most obvious proof would be what LIGO calls continuous waves from, e.g., unstable r-modes in newborn neutron stars, which have a specific gravitational wave frequency at 4/3 x frequency of the star vs. the usual 2 x frequency of the star for mountains, and would hopefully be close enough to identify the source and its environment.

IF they are born with spin, then the globular cluster/dynamic scenario is more likely. Then the BHs are likely to be both spinning and precessing, but the precession would not be visible for the equal mass scenario.

What would be proof that BHs can be born with spin?
Black holes with spin in an equal mass binary would be proof that they are born with spin and that there is some instability that only works in some cases. I don't see that they would have to be large. Perhaps if they are small and we know they are black holes, they are more likely to be born this way and not be the result of a merger.