Artistic representation of a collision of two forest monsters, along 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 student from the Department of Physics at the Chinese University of Hong Kong (CUHK), has proposed the collision of two exotic compact objects known As forest monsters as an alternative explanation for the origin of the gravitational wave signal GW190521. The hypothetical stars are among the simplest exotic compact objects proposed and are valid candidates for dark matter. Within this interpretation, the team can estimate the mass of a new particle constituent of these stars, an ultra-light boson with a mass billions of times smaller than that of the electron. Their analysis has been published in the journal Physical Review Letters on February 24, 2021.
The team is co-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, postdoctoral fellow at the University of Aveiro and the Instituto Superior Técnico (University of Lisbon). Other employees came from the University of Valencia, the University of Aveiro and Monash University. Samson Hin Wai Leong, a second-year student at CUHK, also participated.
Gravitational waves are ripples in the fabric of spacetime 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 and contain information about their sources. Since 2015, the advanced detectors of the Laser Interferometer Gravitational Wave Observatory (LIGO) and Virgo have observed about 50 gravitational wave signals that resulted from the merging 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, which closely involved the CUHK group led by Professor Tjonnie Li, associate professor of the physics department of CUHK, the signal was consistent with the collision of two black holes 85 and 66 times the mass of the Sun, which eventually produced 142 times the mass of the Sun black holeThe latter was the first member ever found of a new black hole family – medium black holes. According to Professor Tjonnie Li, this discovery was of paramount importance because such black holes have long been considered the missing link between the stellar-mass black holes created by the collapse of stars and the supermassive black holes that hide in the sky. center of almost every galaxy.
Despite its significance, the observation of GW190521 presents a tremendous challenge to the current understanding of the evolution of stars, because one of the merged black holes has a “forbidden” size. The alternative explanation proposed by the team provides a new direction for the investigation. Dr. Nicolás Sanchis-Gual explained, “Forest monsters are objects nearly 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 forest monster that can become unstable, eventually collapsing into a black hole, producing a signal consistent with what LVC observed last year. Unlike ordinary stars, which are made of what we commonly know as matter, forest monsters are made up of ultra-light bosons. These bosons are one of the most attractive candidates to form dark matter that makes up about 27% of the Universe. “
The team compared the GW190521 signal to computer simulations of forest monster fusions and found that these actually explain the data slightly better than the analysis of LVC. The result implies that the resource would have different properties than previously stated. Dr. Juan Calderón Bustillo said, “First, we would stop talking about colliding black holes, eliminating the problem of dealing with a banned black hole. Second, because the forest monster fusions are much fainter, we derive a much smaller distance than that estimated by LVC. This leads to a much greater mass for the last black hole, of about 250 solar masses, so the fact that we witnessed the formation of a black hole of average mass remains true. “
Professor Toni Font, from the University of Valencia and one of the co-authors, explained that while the analysis favors the black hole merger hypothesis, the fusion of a forest monster is in fact somewhat preferred by the data, although in a non-conclusive way. Although the computational framework of current boson-star simulations is still quite limited and subject to major improvements, the team will further develop a more developed model and study similar observations of gravitational waves 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 forest monsters, 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. In addition, the team can measure the mass of this supposed new dark matter particle and a value of zero is discarded with great certainty. If confirmed by the 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 the CUHK summer internship program, added, “I worked with Professor Calderón Bustillo on the design of this project’s software, successfully speeding up the calculations, and in the end we were able to release our results immediately after LVC published their analysis. It is exciting to work with the multicultural team on the frontier of physics and think about the search for a ‘darker’ origin of the ripples in spacetime, while proving the existence of a dark matter particle. “
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