On May 21, 2019, the LIGO and Virgo interferometers recorded a gravitational wave signal that was emitted about 16 billion light years away. Referred to as GW190521, this would be the result of the merging of two black holes that weigh 66 and 85 times the mass of the Sun, respectively.
The collision would have occurred about 7 billion years ago and would have created a black hole with a mass 142 times that of the Sun. The 9 remaining solar masses would have been converted into energy in the form of gravitational waves. But what if it was something else?
A new study by Juan Calderón Bustillo of the IGFAE (Galician Institute of High Energy Physics) in Spain suggests that GW190521 may not be the result of a binary black hole merging.
For Bustillo and his coworkers, the gravitational wave could be due to mysterious and theoretical objects known as boson stars. The problem arises from the fact that stars capable of producing black holes as massive as those believed to be the source of GW190521 must usually be significantly more massive than their supernovae. Since orders of magnitude do not follow logic, the researchers believe that the assumption previously made by their colleagues is inconsistent.
GW190521: the collision of two black holes? Photo credit: Raúl Rubio (Virgo Valencia Group, The Virgo Collaboration)
Bosons instead of fermions
As mentioned above, the boson stars are still something hypothetical. Unlike conventional stars, which are made up of fermions, they would be made up of bosons. Black holes are either of the “star” type, if they result from the implosion of a star, or “supermassive”.
Supermassive black holes are at the center of galaxies. In the case of stars believed to be the source of gravitational wave GW190521, their implosion would normally have eroded the star’s core, leaving nothing behind that could collapse into a black hole by gravity.
This led Juan Calderón Bustillo’s team to initially suspect a collision between two smaller black holes. In the end, however, the boson star thesis was favored. Note that boson stars like black holes can reach millions of solar masses in a very compact size.
In contrast to fermions, which can be elementary particles (electrons) or composite particles (protons), bosons in particular are basic particles (photons, gluons, Higgs bosons).