The heaviest black hole collision might be a boson star merger, study

The result would provide the first observational evidence for a long-sought dark matter candidate.

According to Einstein’s General Theory of Relativity, violent events of the Universe originate from the gravitational waves that carry information about their sources. Since 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo have detected around 50 gravitational wave signals originated in the coalescence and merger of two of the most mysterious entities in the Universe — black holes and neutron stars.

In September 2020, LVC, the collaboration LIGO, and Virgo reported the gravitational wave signal GW190521. However, the origin of the gravitational wave signal remains unknown.

Now, an international team of scientists led by the Galician Institute of High Energy Physics and the University of Aveiro, including an undergraduate from the Department of Physics at The Chinese University of Hong Kong (CUHK), has offered an alternative explanation for the origin of the gravitational wave signal GW190521. They proposed that the collision of two exotic compact objects known as boson stars generated the gravitational wave signal.

The boson stars are among the simplest exotic compact objects proposed and constitute well-founded dark matter candidates. With this understanding, the group can assess the mass of new particle constituent of these stars, an ultra-light boson with a mass billions of times smaller than that of the electron.

The incoming signal was consistent with the collision of two black holes of 85 and 66 times the Sun’s mass, which produced a final 142 solar mass black hole. This discovery was of paramount importance because such black holes had been long considered the missing link between 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.

Regardless of its importance, the observation of GW190521 represents a tremendous test to the current understanding of stellar evolution, as one of the black holes merged has a “forbidden” size. The elective clarification proposed by the scientists brings another heading for the investigation.

Dr. Nicolás Sanchis-Gual explained, “Boson stars are objects almost as compact as black holes but, unlike them, they do not have a ‘no return’ surface or event horizon. When they collide, they form a boson star that can become unstable, eventually collapsing to a black hole and producing a signal consistent with what LVC observed last year. Unlike regular stars, which are made of what we commonly know as matter, boson stars are made up of ultra-light bosons. These bosons are one of the most appealing candidates for constituting dark matter forming around 27% of the Universe.”

Scientists compared the GW190521 signal to computer simulations of boson star mergers. They found that these explain the data slightly better than the analysis conducted by LVC. This means the source probably has different properties than stated earlier.

Dr. Juan Calderón Bustillo said, “First, we would not be talking about colliding black holes anymore, which eliminates the issue of dealing with a forbidden black hole. Second, because boson star mergers are much weaker, we infer a much closer distance than the one estimated by LVC. 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.”

Professor Toni Font, from the University of Valencia and one of the co-authors, explained that even though the analysis tends to favor “by design” the merging black holes hypothesis, a boson star merger is slightly preferred by the data, although in a non-conclusive way.”

Another co-author, Professor Carlos Herdeiro from the University of Aveiro, said, “the finding not only involves the first observation of boson stars but also that of their building block, a new particle known as the ultra-light boson. Such ultra-light bosons have been proposed as the constituents of what we know as dark matter.”

“We can measure the mass of this putative new dark matter particle, and a value of zero is discarded with high confidence. If it is confirmed by the subsequent analysis of GW190521 and other gravitational wave observations, the result will provide the first observational evidence for a long-sought dark matter candidate.”

Samson Hin Wai Leong, a student who joined the summer undergraduate research internship program of CUHK, added“I worked with Professor Calderón Bustillo on the design of the software of this project, which successfully speeded up the calculations of the study, and eventually we were able to release our results immediately after LVC published their analysis. It is thrilling to work at the frontier of physics with the multicultural team and think about seeking a ‘darker’ origin of the ripples in spacetime, at the same time proving the existence of a dark matter particle.”

Regardless of the computational system of the current boson star simulations being still relatively limited and subject to significant enhancements, the group will additionally build up a more evolved model and study comparative gravitational wave perceptions under the boson star assumption.

Journal Reference:
  1. Juan Calderón Bustillo et al. GW190521 as a Merger of Proca Stars: A Potential New Vector Boson of 8.7×10-13eV. DOI: 10.1103/PhysRevLett.126.081101

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