Did LIGO and Virgo observe dark matter?

Gr@v members co-author a paper in Physical Review Letters suggesting GW190521 may be a hint of a new dark matter particle.



An international team of scientists led by the University of Aveiro and the Galician Institute for High Energy Physics shows, in a paper published in Physical Review Letters, that the “heaviest black hole collision” ever observed might actually be something even more mysterious.





Gravitational waves are ripples in the fabric of spacetime that travel at the speed of light. These originate in the most violent events of our Universe, carrying information about their sources. Since 2015, the humankind can detect and interpret gravitational waves thanks to the two LIGO detectors (Livingston and Hanford, USA) and Virgo (Cascina, Italy) detectors. To date, these detectors have already observed around 50 gravitational-wave signals. All of these were originated in the collision and merger of two of the most mysterious entities in the Universe, black holes and neutron stars, allowing us to deepen in our knowledge about these objects.

The promise of gravitational waves goes, however, much further than this as these should eventually provide us with evidence for previously un-observed and even un-expected objects and shed light on current mysteries like the nature of dark matter. The latter may, however, have already happened. 

On September 2020, the LIGO and Virgo collaborations 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, of 85 and 66 times the mass of the sun, which produced a final black hole with 142 solar masses. The latter black hole was the first of a new, previously unobserved, black-hole family: intermediate-mass black holes. This discovery is of paramount importance, as such black holes were the missing link between two well known black-hole families: the stellar-mass black holes that form from the collapse of stars, and the supermassive black holes that hide in the center of almost every galaxy.

In addition, this observation came with an enormous challenge. If what we think we know about how stars live and die is correct, the heaviest of the colliding black holes could not form from the collapse of a star at the end of its life. It is a sort of a forbidden mass for black holes.

In an article published today in Physical Review Letters, a team of scientists lead by Dr. Juan Calderón Bustillo, “La Caixa Junior Leader - Marie Curie Fellow”, at the Galician Institute of High Energy Physics, and Dr. Nicolás Sanchis-Gual, a postdoctoral researcher at the University of Aveiro and at the Instituto Superior Técnico (University of Lisbon), together with Dr. Carlos Herdeiro and Dr Eugen Radu (from Aveiro), as well as collaborators from Valencia, Monash University and The Chinese University of Hong Kong, has proposed an alternative explanation for the origin of the signal GW190521: the collision of two exotic objects known as bosonic stars, which are one of the most solid candidates to form what we know as dark matter. Within such interpretation, the team was able to estimate the mass of a new particle constituent of these stars, an ultra-light boson with a mass billionths of times smaller than that of the electron.

Dr. Nicolás Sanchis-Gual, explains: “Bosonic stars are objects almost as compact as black holes but, unlike them, do not have a “no-return” surface. When they collide, they form a heavier bosonic star that can become unstable, eventually collapsing to a black hole, and producing a signal consistent with what LIGO and Virgo observed. Unlike regular stars, which are made of what we commonly know as matter, bosonic stars are made up of what we know as ultralight bosons. These bosons are one of the most appealing candidates for constituting what we know as dark matter, which forms ~27% of the Universe.”.  

The team compared the GW190521 signal to computer simulations of bosonic-star mergers, and found that these actually explain the data slightly better than the analysis conducted by LIGO and Virgo. The result implies that the source would have different properties than stated earlier. In words of Dr. Calderón Bustillo: “First, we would not be talking about colliding black holes anymore, which eliminates the issue of dealing with a forbidden black hole. Second, because bosonic star mergers are much weaker, we infer a much closer distance than the one estimated by LIGO and Virgo. This leads to a much larger mass for the final black hole, of about 250 solar masses, so the fact that we have witnessed the formation of an intermediate-mass black hole remains true”.

The team found that even though the analysis tends to favour “by design” the merging black-holes hypothesis, a bosonic star merger is actually preferred by the data, although in a non-conclusive way. Prof. Jose A. Font from the University of Valencia says “Our results show that the two scenarios are almost indistinguishable given the data, although the exotic bosonic-star one is slightly preferred. This is very exciting since our bosonic-star model is as of now very limited and subject to major improvements. A more evolved model may lead to even larger evidence for this scenario and would also allow us to study previous gravitational-wave observations under the bosonic-star merger assumption”. 

This result would not only involve the first observation of bosonic stars, but also that of their building block, a new particle known as an ultra-light (vector) boson. Such ultra-light bosons have been proposed as the constituents of what we know as dark matter, which makes up around 27% of the observable Universe. Dr. Carlos Herdeiro, from University of Aveiro says that “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 high confidence. If confirmed by subsequent analysis of this and other gravitational-wave observations, our result would provide the first observational evidence for a long-sought dark matter candidate”. 


Image credit: Nicolás Sanchis-Gual and Rocío García-Souto