The “biggest black hole collision” detected by gravitational waves could actually be a merger between Bosonnian stars

Artistic representation of a collision of two Boson stars, 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 an undergraduate student from the Department of Physics at the Chinese University of Hong Kong (CUHK), proposed the collision of two exotic compact objects known as the boson stars as an alternative explanation for the origin of the gravitational wave signal GW190521. Hypothetical stars are among the simplest exotic compact objects on offer and are well-founded dark matter candidates. As part of 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 smaller than that of the electron. Their analysis was published in the journal Physical examination letters February 24, 2021.

The team is co-led by Dr Juan Calderón Bustillo, former professor of the Department of Physics of CUHK 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 student at CUHK, also participated.

Gravitational waves are ripples in the fabric of space-time that travel at the speed of light. Predicted in Einstein’s general theory of relativity, they originate from the most violent events in the Universe, carrying information about their sources. Since 2015, the advanced detectors of the Laser Interferometer Gravitational Wave Observatory (LIGO) and Virgo observed about 50 gravitational wave signals from the coalescence and fusion of two of the Universe’s most mysterious entities – black holes and neutron stars.

In September 2020, LVC, the joint body of the LIGO scientific collaboration and the 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 physics department at CUHK was deeply involved, the signal was consistent with the collision of two black holes of 85 and 66 times the mass of the Sun. , which produced a final solar mass of 142 black hole. The latter was the first ever discovered member of a new family of black holes – intermediate mass black holes. According to Professor Tjonnie Li, this discovery was of paramount importance because these black holes had long been considered the missing link between stellar-mass black holes that form from collapsing stars, and supermassive black holes. that lurk in the center of almost every galaxy.

Despite its importance, the sighting of GW190521 poses a huge challenge to the current understanding of stellar evolution, as one of the fused black holes is of 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 almost as compact objects as black holes but, unlike them, they do not have a surface horizon or ‘no return’ event. 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 call matter, Boson stars are made up of ultralight bosons. These bosons are one of the most attractive candidates to constitute dark matter forming about 27% of the Universe.

The team compared the GW190521 signal to computer simulations of boson star merges and found that these actually explained the data slightly better than the analysis conducted by LVC. The result implies that the source would have properties different from those indicated above. Dr Juan Calderón Bustillo said: “First, we would no longer talk about the collision of black holes, which eliminates the problem of dealing with a forbidden black hole. Second, since the boson star mergers are much weaker, we deduce a much closer distance than estimated by LVC. This leads to a much larger mass for the final black hole, of around 250 solar masses, so the fact that we saw the formation of an intermediate mass black hole remains true.

Professor Toni Font, University of Valencia and one of the co-authors, explained that although the analysis tends to favor the black hole fusion hypothesis “by design”, a star boson fusion is in the works. fact slightly preferred by the data, albeit 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 further develop a more evolved model and study similar gravitational wave observations under the fusion hypothesis. Boson stars.

According to another co-author, Professor Carlos Herdeiro of the University of Aveiro, the discovery involves not only the first sighting of boson stars, but also that of their building block, a new particle known as ultralight boson. Such ultralight bosons have been proposed as constituents of what we call dark matter. Additionally, the team can actually measure the mass of this putative new dark matter particle and a value of zero is discarded with great 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 undergraduate research internship program added, “I worked with Prof. Calderón Bustillo on the software design for this project, which accelerated with the study calculations were successful, and ultimately we were able to publish our results immediately after LVC released its analysis. It’s exciting to work on the frontier of physics with the multicultural team and to think about the search for a “darker” origin of ripples in space-time, while proving the existence of a particle. of dark matter.

Reference: “GW190521 as a Proca Stars merger: 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 examination letters.
DOI: 10.1103 / PhysRevLett.126.081101