Fusion of Boson stars could explain massive black hole collision and prove the existence of dark matter

An international team of scientists led by the Galician Institute of High Energy Physics (IGFAE) and the University of Aveiro shows that the heaviest black hole collision on record, produced by gravitational wave GW190521, could in fact be something even more mysterious: the fusion of two boson stars. This would be the first proof of the existence of these hypothetical objects, candidates for dark matter, supposed to represent 27% of the mass of the universe.

Gravitational waves are ripples in the fabric of space-time that travel at the speed of light. These come from the most violent events in the universe, carrying information about their sources. Since 2015, the two LIGO detectors in the United States and the Virgo detector in Cascina, Italy, have detected and interpreted gravitational waves. To date, these detectors have already observed around fifty gravitational wave signals. All of this was born from the collisions and mergers of black holes and neutron stars, allowing physicists to deepen their knowledge of these objects.

However, the promise of gravitational waves goes much further than that, as they should eventually provide us with evidence of previously unobserved and even unexpected objects, and shed light on current mysteries like the nature of dark matter. However, the latter may have already taken place.

In September 2020, the LIGO and Virgo (LVC) collaboration announced to the world the gravitational wave signal GW190521. According to their analysis, the signal was consistent with the collision of two heavy black holes, 85 and 66 times the mass of the sun, which produced a final black hole with 142 solar masses. The resulting black hole was the first of a new family of previously unobserved black holes: intermediate-mass black holes. This discovery is of utmost importance, as these black holes were the missing link between two well-known black hole families: stellar-mass black holes that form from collapsing stars and supermassive black holes that reside at the center of almost every galaxy, including the Milky Way.

In addition, this observation came with a huge challenge. If what we think we know about how stars live and die is correct, the heaviest colliding black hole (85 solar masses) might not be able to form from a star’s collapse at the end of his life, which opens up a range of doubts. and possibilities about its origins.

In an article published today in Physical examination letters, a team of scientists led by Dr Juan Calderón Bustillo at the Galician Institute of High Energy Physics (IGFAE), a joint center of the University of Santiago de Compostela and Xunta de Galicia, and Dr Nicolás Sanchis -Gual, postdoctoral researcher at the University of Aveiro and the Instituto Superior Técnico (Univ. Lisboa), in collaboration with collaborators from the University of Valencia, Monash University and Chinese University of Hong Kong , have proposed an alternative explanation for the origin of the GW190521 signal: the collision of two exotic objects known as Boson stars, which are one of the most likely candidates for explaining dark matter. In their analysis, the team was able to estimate the mass of a new particle that makes up these stars, an ultralight boson with a mass billions of times smaller than electrons.

The team compared the GW190521 signal to computer simulations of boson-star mergers and found that these actually explained the data slightly better than the analysis conducted by LIGO and Virgo. The result implies that the source would have properties different from those indicated above. Dr Calderón Bustillo says: “First, we would no longer be talking about the collision of black holes, which eliminates the problem of handling a ‘forbidden’ black hole. Second, because the Boson star mergers are much weaker, we deduce a much closer distance. than that estimated by LIGO and Virgo. This leads to a much larger mass for the final black hole, of around 250 solar masses, so the fact that we saw the formation of an intermediate mass black hole remains true. ”

Dr Nicolás Sanchis-Gual says: “Boson stars are objects almost as compact as black holes but, unlike them, have no ‘no-return’ surface. When they collide, they form a boson star that can become unstable, eventually collapsing into a black hole and producing a signal consistent with what LIGO and Virgo observed. Unlike regular stars, which are made of what we commonly call matter, Boson stars are made up of what we call ultralight bosons. These bosons only make one of the most attractive candidates to constitute what we call dark matter. ”

The team found that while the analysis tends to favor the black hole fusion hypothesis, a boson star fusion is actually preferred by the data, albeit inconclusive. Professor Jose A. Font of the University of Valencia says: “Our results show that the two scenarios are almost indistinguishable from the data, although the exotic star boson hypothesis is slightly preferred. This is very exciting, because our boson star model is, at the moment, very limited and subject to major improvements. A more evolved model could lead to even greater evidence for this scenario and would also allow us to study previous observations of gravitational waves under the boson-star fusion hypothesis. ”

This result would involve not only the first sighting of the boson stars, but also that of their building block, a new particle known as the ultralight boson. Professor Carlos Herdeiro of the University of Aveiro says: “One of the most fascinating results is that we can actually measure the mass of this putative new dark matter particle, and that a value of zero is discarded with a great confidence. If confirmed by analysis of this and other gravitational wave observations, our result would provide the first observational evidence for a much sought-after dark matter candidate. ”