Artistic representation of a collision of two forest stars, together with the released 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), has suggested the collision of two exotic compactors objects known as boson stars as an alternative explanation for the origin of the gravitational wave signal GW190521. The hypothetical star is one of the simplest proposed compact objects and forms well-founded candidates for dark matter. Within this interpretation, the team is able to estimate the mass of a new particle component of these stars, an ultralight boson with a mass billions of times smaller than the electron’s mass. Their analysis was published in the journal Physical overview letters on February 24, 2021.
The team is led by Dr Juan Calderón Bustillo, a former professor of the Department of Physics at CUHK and now ‘La Caixa Junior Leader – Marie Curie Fellow’, at the Galician Institute of High Energy Physics, and Dr Nicolás Sanchis -Gual , a postdoctoral researcher at the University of Aveiro and at the Instituto Superior Técnico (University of Lisbon). Other collaborators come from the University of Valencia, the University of Aveiro and Monash University. Samson Hin Wai Leong, a second-year undergraduate student at CUHK, also participated.
Gravitational waves are wrinkles in the tissue of space-time that move at the speed of light. Predicted in Einstein’s General Theory of Relativity, they originate in the most violent events in the universe, with 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 that originated in the fusion 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 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 of the Department of Physics 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 142 solar mass black hole. The latter was the first member ever found in a new black hole family – black holes with intermediate mass. According to Professor Tjonnie Li, this discovery was of paramount importance because such black holes have long been considered the missing link between the black mass holes that result from the collapse of stars, and the supermassive black holes that form in the middle of almost every galaxy.
Despite its significance, the observation of GW190521 poses an enormous challenge to the current understanding of stellar evolution, because one of the black holes joined together has a ‘forbidden’ size. The alternative explanation that the team proposes offers a new direction for the study. Dr. Nicolás Sanchis-Gual explained: ‘Boson stars are objects that are almost as compact as black holes, but unlike them they have no’ no return ‘surface or event horizon. When they collide, they form a forest star that can become unstable, which can eventually collapse into a black hole and produce a signal similar to what LVC observed last year. Unlike ordinary stars, which consist of what we commonly know as matter, boson stars consist of ultra-light bosons. This forest zone is one of the most attractive candidates for the formation of dark matter that makes up about 27% of the universe. ‘
The team compared the GW190521 signal with computer simulations of fusion of forest stars and found that it actually explains the data better than the analysis done by LVC. The result implies that the source will have different properties than those mentioned earlier. Dr Juan Calderón Bustillo said: ‘Firstly, we will no longer talk about black holes colliding, which eliminates the problem of dealing with a forbidden black hole. Second, because mergers of boson stars are much weaker, we deduce a much smaller distance than those estimated by LVC. This results in a much larger mass for the final black hole of about 250 solar masses, so the fact that we saw the formation of an intermediate mass of black hole remains true. ‘
Professor Toni Font, of the University of Valencia and one of the co-authors, explained that although the analysis tends to favor a fusion of the black holes’ hypothesis, a fusion of boson stars is actually preferred by the data, although a non-conclusive way. Despite the fact that the computational framework of the current bosonster simulations is still quite limited and subject to major improvements, the team will further develop a more developed model and study similar gravity wave observations under the assumption of the bosonster fusion.
According to another co-author, Professor Carlos Herdeiro of the University of Aveiro, the finding involves not only the first observation of boson stars, but also that of their building block, a new particle known as the ultralight boson. Such ultralight forest zones are represented as the constituents of what we know as dark matter. In addition, the team can measure the mass of this putative new dark matter particle and the value of zero is thrown away with great confidence. If confirmed by the subsequent analysis of GW190521 and other gravitational wave observations, the result would provide the first evidence for a long-sought candidate for dark matter.
Samson Hin Wai Leong, a student who joined the summer undergraduate research internship program at CUHK, added: ‘I worked with Professor Calderón Bustillo on the design of the software for this project, which covers the calculations of the study successfully accelerated, and finally we were able to publish our results immediately after LVC published their analysis. It is exciting to work with the multicultural team on the frontier of physics and to think about finding a ‘darker’ origin of the ripples in space-time, and at the same time the existence of a dark matter particle to prove. ‘
Reference: “GW190521 as a merger of Proca stars: a potential new vector bean of 8.7 × 10−13eV ”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, 24 February 2021, Physical overview letters.
DOI: 10.1103 / PhysRevLett.126.081101