In May 2019, the gravitational wave observatories LIGO and Virgo discovered the fusion of two black holes. One had a mass of 85 suns while the other had 66 solar masses. The event was named GW190521 and was the largest merger seen to date. It created a black hole with 142 solar masses, making it the first gravitational wave observation of a medium-mass black hole. However, the event also raised several questions.
One of the greatest puzzles concerns the masses of the two original black holes. According to star models, black holes created by the collapse of a large star cannot be larger than about 65 solar masses. While the smaller of the two is just below that limit, the larger is definitely not. How did the black hole with 85 solar masses form?
One idea is that it is the result of smaller mergers. If there is a tight system of 4 to 6 black holes, they may have merged into a single large black hole over time. However, this cluster would also have to orbit the black hole with 66 solar masses to create the GW190521 fusion. There is evidence to suggest that blackhole clusters may exist, but whether they can merge quickly enough to explain GW190521 is not clear. Recently a team suggested a different solution. They suggest that the GW190521 was not the merger of two black holes, but a merger of two proca stars.
The observed fusion event GW190521. Photo credit: R. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration)
A proca star, or boson star as it is sometimes called, is a hypothetical object that corresponds to a white dwarf or neutron star. The building blocks of matter come in two general forms: fermions like electrons and quarks and bosons like gluons and Higgs. Fermions resist the same quantum state. When gravity tries to compress fermions, they push back through what is known as degeneracy pressure. This pressure prevents white dwarfs and neutron stars from collapsing under their weight.
Bosons have no problem with taking the same quantum state. When you super cool bosons, they can form a single quantum object known as a Bose Einsten condensate. Because of this, one might think that gravity would have no problem collapsing a mass of bosons into a black hole. However, it turns out that extremely dense bosons behave in unexpected ways in a strong gravitational field. The physics is pretty complicated, but the end result is that a boson star would reach a gravitational distance similar to that of white dwarfs and neutron stars. Hypothetically, a boson star like white dwarfs and neutron stars could be gravitationally stable. Even more. The degeneracy pressure is limited if white dwarfs cannot be larger than 1.4 solar masses and neutron stars can be larger than 2 – 3 solar masses. Under the right conditions, a boson star could easily have a mass of 85 suns or more.
A boson star as it would appear to the EHT. Photo credit: Olivares et al
Known boson matter failed to become a proca star, but certain dark matter theories suggest new types of bosons. When dark matter is made up of low-mass bosons, they can form into objects that are similar in size and mass to black holes. Since they would have strong magnetic fields (as opposed to black holes) we could possibly distinguish them from black holes by the way light lenses are around them. Or, as this latest work suggests, we may be able to identify them by their gravitational waves.
When the team looked at the data from the GW190521 event, it found that this was compatible with both a black hole merger and a proca star merger. The data agreed equally well with both models. Since the properties of a Proca star depend on the mass of the bosons that make up it, they used the GW190521 data to measure boson mass. The answer was extremely small. About one trillionth of the estimated neutrino masses.
None of this proves that Proca stars exist. All of the gravitational wave events that we have observed are perfectly explained by black hole fusions. However, the study gives us interesting ideas. A new bright boson particle could help us explain dark matter, and these dark matter particles could form into boson stars. It’s a bizarre idea, but we can’t rule it out just yet.
Reference: Juan Calderon Bustillo et al. “GW190521 as an amalgamation of proca stars: A potential new vector boson of 8.7 × 10? 13 eV.” Physical Review Letters 126.8 (2021): 081101.
Reference: Brito, Richard et al. “Proca Stars: Gravitation of Bose-Einstein Condensates from Solid Spin-1 Particles.” Physics Letters B 752 (2016): 291- 295.
Reference: Olivares, Hector, et al. “How to distinguish an accreting boson star from a black hole.” Royal Astronomical Society monthly bulletin 497.1 (2020): 521-535.