The “most intense black hole collision” detected by gravitational waves could actually be a fusion of boson stars

Artist’s rendering of the collision of two boson stars, together with the emitted gravitational waves. Credit: Nicolás Sanchis-Gual and Rocío García-Souto

An international team of scientists led by the Galician Institute of High Energy Physics and the University of Aveiro, including a graduate from the Physics Department of the Chinese University of Hong Kong (CUHK), has proposed the collision of two exotic compact objects known as boson stars as an alternative explanation for the origin of the gravitational wave signal GW190521. Hypothetical stars are among the simplest proposed exotic compact objects and are well-founded dark matter candidates. Within this interpretation, the team is able to estimate the mass of a new constituent particle of these stars, an ultralight boson with a mass billions of times less than that of the electron. His analysis has been published in the journal Physical Review Letters on February 24, 2021.

The team is co-directed by Dr. Juan Calderón Bustillo, former professor in the CUHK Physics Department and now “La Caixa Junior Leader – Marie Curie Fellow”, at the Galician Institute of High Energy Physics, and Dr. Nicolás Sanchis – Gual, postdoctoral researcher at the University of Aveiro and at the Instituto Superior Técnico (University of Lisbon). Other collaborators came from the University of Valencia, the University of Aveiro and the Monash University. Samson Hin Wai Leong, a sophomore at CUHK, also participated.

Gravitational waves are waves in the fabric of space-time that travel at the speed of light. Predicted in Einstein’s Theory of General Relativity, they originate in the most violent events in the Universe, carrying information about their sources. Since 2015, the advanced detectors of the Gravitational Wave Observatory with laser interferometer (LIGO) and Virgo have observed around 50 gravitational wave signals originating from the coalescence and fusion of two of the most mysterious entities in the Universe: black holes and neutron stars.

In September 2020, LVC, the joint body of LIGO Scientific Collaboration and Virgo Collaboration, announced the detection of the gravitational wave signal GW190521. According to the LVC analysis, in which the CUHK group led by Professor Tjonnie Li, associate professor in the Department of Physics at CUHK was deeply involved, the signal was consistent with the collision of two black holes 85 and 66 times the mass of the Sun, which produced a final solar mass of 142 dungeon. The latter was the first member of a new family of black holes found: intermediate-mass black holes. According to Professor Tjonnie Li, this discovery was of utmost importance because these black holes had long been considered the missing link between stellar-mass black holes that form from the collapse of stars and the supermassive black holes that lurk. at the center of almost every galaxy.

Despite its importance, the observation of GW190521 poses an enormous challenge to today’s understanding of stellar evolution, because one of the merged black holes is a “forbidden” size. The alternative explanation proposed by the team brings a new direction to the study. Dr. Nicolás Sanchis-Gual explained: “Boson stars are objects almost as compact as black holes but, unlike them, they do not have a surface or a ‘no return’ event horizon. When they collide, they form a boson star that can become unstable, eventually collapsing into a black hole and producing a signal consistent with what LVC observed last year. Unlike regular stars, which are made up of what we commonly know as matter, boson stars are made up of ultralight bosons. These bosons are one of the most attractive candidates for dark matter that makes up about 27% of the Universe. “

The team compared the GW190521 signal with computer simulations of boson star mergers and found that they actually explained the data slightly better than the analysis performed by LVC. The result implies that the font would have different properties than those indicated above. Dr. Juan Calderón Bustillo said: “First, we would no longer be talking about colliding black holes, which eliminates the problem of dealing with a forbidden black hole. Second, because the boson star mergers are much fainter, we infer a much closer distance than that estimated by LVC. This leads to a much higher mass for the final black hole, 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 although the analysis tends to favor the hypothesis of black hole fusion “by design”, the data slightly prefer a fusion of boson stars, although in an inconclusive way. Although the computational framework for current boson star simulations is still quite limited and subject to major improvements, the team will develop a more evolved model and study similar gravitational wave observations under the assumption of boson star fusion.

According to another co-author, Professor Carlos Herdeiro from the University of Aveiro, the find not only involves the first observation of bosonic stars, but also that of their building block, a new particle known as an ultralight boson. These ultralight bosons have been proposed as components of what we know as dark matter. Furthermore, the team can measure the mass of this putative new dark matter particle and a value of zero is ruled out with high confidence. If confirmed by subsequent analysis of GW190521 and other gravitational wave observations, the result would provide the first observational evidence for a long-sought dark matter candidate.

Samson Hin Wai Leong, a student who joined CUHK’s summer research internship program, added: “I worked with Professor Calderón Bustillo on the design of the software for this project, which successfully sped up the study’s calculations, and we were finally able to publish our results immediately after LVC published its analysis. It is exciting to work on the frontier of physics with the multicultural team and think about looking for a ‘darker’ origin of the waves in space-time, while demonstrating the existence of a particle of dark matter ”.

Reference: “GW190521 as a fusion of Proca stars: a potential new 8.7 × 10 vector boson−13eV ”by Juan Calderón Bustillo, Nicolas Sanchis-Gual, Alejandro Torres-Forné, José A. Font, Avi Vajpeyi, Rory Smith, Carlos Herdeiro, Eugen Radu and Samson HW Leong, February 24, 2021, Physical Review Letters.
DOI: 10.1103 / PhysRevLett.126.081101

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