Scientists working at the frontier of particle physics are proposing the existence of a theoretical exotic, ultra-light boson with a mass billions of times smaller than that of the electron. They are 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 stellar-mass black holes weighing in at 85 and 66 solar masses. The final merger product was an intermediate-mass black hole with 142 solar masses, and the remaining 9 solar masses were radiated away as energy in the form of gravitational waves. The discovery was of paramount importance because such intermediate-mass black holes had long been 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. Specifically, standard models of stellar evolution cannot form black holes with 85 times the mass of the sun.
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 known 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.”
Ultra-Light Dark Matter?
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. Ultra-light dark matter candidates are only a small fraction the mass of an electron, in contrast with the more popular cold dark matter, which includes several candidates that are tens to hundreds times the mass of a proton.
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 the 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.”
The Last Word -J.Antonio Font
“The inference studies on GW190521 carried out by the LIGO VIRGO KAGRA Collaboration (LVK) reported a primary black hole mass of about 85 million suns (Msun),” wrote Antonio Font in an email reply to The Daily Galaxy asking how the observation of GW190521 poses a challenge to current understanding of stellar evolution; and has subsequent analysis confirmed the existence of the ultra-light boson?
“This mass is within the pair-instability supernova mass gap,’ Font explained, “a range of masses roughly between 50 Msun and 130 Msun, where black holes are not expected to form from the gravitational collapse of a massive star at the end of its evolution. While the existence of this gap seems to be a solid theoretical result, its particular boundaries are known to be affected by factors not very well understood, e.g. the rotation of the star, uncertainties on nuclear reactions rates or episodes of rapid accretion at black hole birth.
“It seems, however unlikely,’ Font continued, “that the lower limit of the gap can rise to a value close to 85 Msun. As a result, there have been a number of alternative explanations for GW190521 including hierarchical captures, highly non-quasi-circular mergers, high-mass torus-black hole systems, or even exotic proposals like mergers of primordial black holes or collisions of hypothetical bosonic stars, the latter being our own proposal.
“We are currently reevaluating our analysis with a few of the most massive observations reported in GWTC-3, finding good agreement with the value of the ultra-light boson mass we inferred from the GW190521 signal. While this further supports our claim of a close degeneracy between two theoretical models (black hole collisions vs boson star collisions), it by no means implies (let alone confirms) the existence of ultra-light bosons. Strong support for their existence could come from the detection of continuous gravitational waves from boson clouds around spinning black holes.”
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 ‘darker’ origin of the ripples in spacetime, and prove the existence of a dark matter particle. The G2190521 event was detected near the edge of our observable universe at a distance of 5.3 gigaparsecs (17 billion light years). More nearby mergers of black holes spanning the stellar-mass / intermediate-mass limit may help confirm the nature of these elusive objects.