Gravitational waves: the origin of the enigmatic black holes of Ligo and Virgo becomes clearer

It is hard to believe, but it has already been six years since the passage of a gravitational wave was detected directly on Earth.. The study of binary pulsars had already provided indirect indications for decades of the reality of these waves. The calculations carried out from the equations of the general relativity ofEinstein had in fact shown that for these neutron stars the emission of waves was accompanied by a loss ofenergy fast enough for binary systems that it was becoming possible on a human scale to see a decrease in orbital period of the two stars which were therefore approaching each other. The speed of evolution could be calculated and decades of measurements with radio telescopes, like that ofArecibo, had led to the verification with astonishing precision of the predictions on this subject, which already left little doubt about the existence of gravitational waves.

The detectors Ligo and Virgo therefore made it possible to verify that these emissions of waves eventually also conducted black holes in a binary system to come together to the point of colliding and merging, emitting fantastic amounts of energy, again in the form of gravitational waves, to the point of sometimes converting up to several masses pure solar energy carried away by these waves in much less than a second.

We know that according to the same principle of neutron star collisions and between black holes and neutron stars were to occur and there too the two detectors highlighted the waves produced.

Abnormally massive black holes for a simple stellar origin

Remember that we have thus entered the era of gravitational astronomy thanks to Kip Thorne and Rainer Weiss on one side of the Atlantic, Alain Brillet and Thibault Damour on the other side. But, we must not forget also all the other members of the Ligo and Virgo collaborations which have enabled the detection and analysis of gravitational waves directly on Earth as well as, of course, so many other names associated with their quest for decades. and who unfortunately passed away (Ron Drever, Vladimir Braginsky, etc).

Just like the spectrum of a star’s radiation tells us a lot about a star, the gravitational wave spectrum can tell us the masses, angular moments and orbital parameters like theeccentricity black holes in a binary system. And so it was a surprise when the astrophysicists relativists calculated that the masses of the first gravitational wave source detected on Earth were respectively about 36 and 29 solar masses for the two black holes progenitors of GW150914 (GW being the acronym for gravitational wave, gravitational wave in English, and 150914 indicating the date of the observation, September 14, 2015).

Indeed, although we are in orders of magnitude masses of massive stars, the evolutionary theory stellar scale applied to the birth of black holes by collapse of these stars at the end of their lives has a strong tendency to produce stellar black holes much less massive. In fact, the rare stellar black holes previously detected, in particular by emissions in X-rays from accretion disks which sometimes surround them, did not exceed 15 solar masses and we therefore do not understand very well how these compact stars were able to form. Ligo and Virgo have also provided other examples.

The case of GW190521 is even more problematic. Analysis of the signal from the source observed by Ligo and Virgo on May 21, 2019, indeed revealed that it was the product of the merger of two black holes of respectively 85 and 65 times the mass of the Sun about. The end product of this merger must have been a 142 solar mass black hole, which means that’s the equivalent of almost eight solar masses that have been converted into pure gravitational radiation.

The researchers therefore quickly proposed various scenarios to try to explain the existence of such anomalies.

Fusion scenarios at the heart of galaxies and globular clusters

These scenarios are all based on the same process called hierarchical fusion.

In one of these scenarios, everything takes place in the accretion disk of a supermassive black hole at the heart of galaxies, more precisely when one is in the presence of a active nucleus of galaxies. In this type of nuclei supplied with gas very often by filaments of cold matter, stars and black holes will tend to concentrate.

In the accretion disk surrounding the central giant black hole, the forces of friction somehow caused by the gas present will tend to sediment, so to speak, the black holes of classic stellar masses.

Calculations carried out during simulations show that a ring located at about 300 times the radius of theevent horizon of the central black hole is the place of an accumulation of stellar black holes where they are trapped. They will tend to merge to give by accretion – much like the planets formed in the protoplanetary discs – black holes with masses larger than those usually obtained by gravitational collapse ofgiant stars in systems binaries.

In the other scenario, again based on simulations, the mergers between black holes of stellar masses are catalyzed by encounters between these black holes in the particular environment of globular clusters. Remember that these are assemblies of particularly dense old stars, which can bring together on average a few hundred thousand stars in a sphere whose diameter is only a few tens to a hundredlight years at most. We therefore have inter-star distances of the order of one light-year on average, but which can be of the order of the size of the Solar system in their heart.

But, how to test these scenarios or at least accredit the hypothesis of the hierarchical fusion process? We have the beginning of an answer with an article published in Nature Astronomy by researchers from Center for Computational Relativity and Gravitation from Rochester Institute of Technology and the University of Florida.

According to this hypothesis, the orbits black holes before fusion must be particularly eccentric since binary systems form by mutual captures of stellar black holes and not by evolution of two massive stars forming a double star.

The researchers therefore resumed the analyzes of the signals detected by Ligo and Virgo and focused in particular on the case of GW190521. It was also necessary to generate signals representing binary systems with strong eccentricity and various masses to compare them to the signal of GW190521. Astrophysicists have therefore carried out hundreds of new complete numerical simulations with supercomputers laboratories, local and national, which took almost a year.

In the end, the signal of GW190521 is indeed better explained with an orbit with a marked eccentricity, which confirms the hypothesis of the existence of hierarchical fusion processes.