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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 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. The hypothetical stars are among the simplest exotic compact objects proposed and constitute well founded dark matter candidates. Within this interpretation, the team is able to 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 24 February 2021.

The team is co-led by Dr. Juan Calderón Bustillo, a former professor from 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 came from the University of Valencia, the University of Aveiro and Monash University. Samson Hin Wai Leong, a second-year undergraduate 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 in the most violent events of the Universe, carrying information about their sources. Since 2015, the advanced detectors of the Laser Interferometer Gravitational Wave Observatory (LIGO) and Virgo have observed 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 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 of a new black hole family — intermediate-mass black holes. According to Professor Tjonnie Li, 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 centre 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 merged has a “forbidden” size. The alternative explanation proposed by the team brings a new direction for 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 ‘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.”

The team compared the GW190521 signal to computer simulations of boson star mergers and found that these actually explain the data slightly better than the analysis conducted by LVC. The result implies that the source would have 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 favour “by design” the merging black holes hypothesis, a boson star merger is actually slightly preferred by the data, although in a non-conclusive way. Despite the computational framework of the current boson star simulations being still fairly limited and subject to major improvements, the team will further develop a more evolved model and study similar gravitational wave observations under the boson star merger assumption.

According to another co-author, Professor Carlos Herdeiro from the University of Aveiro, 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. Moreover, the team can actually 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 would 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 programme 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.”

Reference: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.081101