An international group of scientists led by specialists from the Institute of High Energy Physics of Galicia (Spain) and the University of Aveiro (Portugal) came to the conclusion that the gravitational wave GW190521 could be caused not by the collision of black holes, but by much more exotic bosonic stars. If this is true, then the researchers may have received the first-ever evidence of the existence of hypothetical objects that are candidates for dark matter. The article was published in the journal Physical Review Letters.
In September 2020, physicists announced the registration of the signal GW190521, which corresponds to the collision of two black holes, the mass of which is 85 and 66 times the mass of the Sun, as a result of which a black hole with a mass of 142 Suns was formed. This object is the missing link between supermassive black holes and stellar mass holes and belongs to a previously unseen family of black holes. However, it is not known how a black hole with a mass of 85 Suns could have formed.
To solve this problem, scientists suggested that both objects were actually bosonic stars, consisting of ultra-light bosons with a mass of billions of times less than the mass of electrons. An unlimited number of bosons (these include, for example, photons) are capable of occupying the same quantum state, which distinguishes them from fermions (for example, electrons), which are subject to the Pauli exclusion principle.
Researchers have modeled the merging of bosonic stars and compared the theoretical gravitational wave signal emitted by this event with GW190521. In this case, both objects will be at a closer distance from the Earth, and the resulting black hole will have a mass of 250 Suns.
Bosonic stars are as compact as black holes, but have no event horizon. However, upon collision, they become unstable and collapse into a black hole. The model suggests that this scenario is slightly preferable to a black hole merger, but further research will need to confirm or deny it.