Scientists working at the frontier of particle physics are proposing the existence of a theoretical exotic, ultra-light “boson star” with a mass billions of times smaller than that of the electron and thinking about seeking a ‘darker’ origin of the ripples in spacetime, at the same time proving the existence of a dark-matter particle. Theories about the origin of dark matter in the universe –one of the biggest questions in science–vary from suggesting that it may be older than the Big Bang to the existence of particles the size of galaxies.
Beyond the Standard Model
The question of what particles make up dark matter –“dark” in the sense that it doesn’t emit radiation or hardly physically interact with anything except through its gravitational attraction –is a crucial one for modern particle physics. Observations indicate that dark matter exists, but apparently something other than the particles in the Standard Model constitutes it.
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 from the merger of two of the most mysterious entities in the Universe — black holes and neutron stars.–a 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 center 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 Boson Star Alternative
The alternative explanation says Nicolás Sanchis-Gual, a postdoctoral researcher at the University of Aveiro and at the Instituto Superior Técnico (University of Lisbon), opens a new direction for the study”: 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.”
A new finding involves the first observation of boson stars, as well as of their building block, a new particle known as the ultra-light boson. that have been proposed as the constituents of what we know as dark matter. 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.
Eliminates “Forbidden Black Hole”
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, explains team co-leader Juan Calderón Bustillo, a Marie Curie Fellow at the Galician Institute of High Energy Physics: “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.”
Although the analysis tends to favor “by design” the merging black holes hypothesis, says astrophysicist Toni Font, at he University of Valencia and one of the co-authors, “the 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.”
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, says co-author, Carlos Herdeiro from the University of Aveiro. “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 a ‘darker’ origin of the ripples in spacetime, and a prove the existence of a dark matter particle.