Merger of Two Boson Stars Could Explain Existence of Dark Matter

An international research team has now demonstrated that the heaviest collision of black holes to be ever visualized and created by the gravitational-wave GW190521 could be more mysterious than previously believed—that is, the merger of a pair of boson stars.

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The team was headed by the Galician Institute of High Energy Physics (IGFAE) and the University of Aveiro.

The new study would be the first proof of the presence of these theoretical objects that represent one of the key candidates to create dark matter, which constitutes 27% of the Universe.

Gravitational waves are essentially ripples in the fabric of spacetime traveling at the pace of light. These gravitational waves appear in the most intense events of the Universe, carrying data about their sources.

Thanks to the two LIGO detectors (Hanford and Livingston in the United States) and Virgo detectors (Cascina in Italy), humans were able to identify and understand gravitational waves since 2015.

So far, such detectors have already identified about 50 gravitational-wave signals, and all these signals emerged during the merger and collision of two of the most enigmatic entities in the Universe—neutron stars and black holes—enabling humans to gain a deeper understanding of these objects.

However, the potential of gravitational waves goes much further than this, because these should ultimately provide proof for formerly unseen and even unanticipated objects and offer a better understanding of the existing mysteries, like the nature of dark matter. But the latter could have already occurred.

Then in September 2020, the LIGO and Virgo partnerships (LVC) declared the gravitational-wave signal GW190521 to the world.

According to the researchers’ study, the gravitational-wave signal was consistent with the impact of two heavy black holes, whose mass is 66 and 85 times that of the Sun, which created an ultimate black hole that has 142 solar masses.

The latter black hole was actually the first of the latest, formerly unseen, black-hole series—the intermediate-mass black holes.

This finding is very significant, because these black holes were, in fact, the missing link between two famous black-hole families—that is, the giant black holes that hide in the core of nearly all galaxies, including the Milky Way, and the stellar-mass black holes that emerge from the disintegration of stars.

This observation also came with an immense challenge. Based on the understanding of how stars live and die, the heaviest of the striking black holes (85 solar masses) may not develop from the disintegration of a star toward the end of its life, which opens up a host of possibilities and doubts about its origin.

In a new article recently published in the Physical Review Letters journal, a research team has suggested an alternative explanation for the origin of the GW190521 signal—the impact of two unusual objects, called boson stars, which are one among the most solid candidates to form the so-called dark matter and constitute 27% of the Universe.

The team was headed by Dr Juan Calderón Bustillo, “La Caixa Junior Leader – Marie Curie Fellow,” from the Galician Institute of High Energy Physics (IGFAE), a joint center of the University of Santiago de Compostela and Xunta de Galicia, and Dr Nicolás Sanchis-Gual, a postdoctoral researcher from the University of Aveiro and the Instituto Superior Técnico (University of Lisbon).

This also included colleagues from the University of Valencia, Monash University, and The Chinese University of Hong Kong.

Within the context of this understanding, the researchers were able to predict the mass of a novel particle constituent of these stars—an ultra-light boson whose mass is a billionth of times smaller than that of a single electron.

When the researchers compared the GW190521 signal with the computer simulations of the mergers of boson stars, they observed that these phenomena really explain the data somewhat better than the study performed by Virgo and LIGO detectors. The outcome indicates that the source would have varied properties than cited before.

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 LIGO and Virgo. 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.

Dr Juan Calderón Bustillo, Galician Institute of High Energy Physics

Dr Nicolás Sanchis-Gual, explained, “Boson stars are objects almost as compact as black holes but, unlike them, do not have a “no-return” surface. 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 LIGO and Virgo observed.”

Unlike regular stars, which are made of what we commonly known as matter, boson stars are made up of what we know as ultralight bosons. These bosons are one of the most appealing candidates for constituting what we know as dark matter.

Dr Nicolás Sanchis-Gual, Postdoctoral Researcher, University of Aveiro and Instituto Superior Técnico

The researchers observed that while the study tends to prefer “by design” the merging black-holes hypothesis, a boson star merger is truly favored by the data, albeit in a non-conclusive manner.

Our results show that the two scenarios are almost indistinguishable given the data, although the exotic boson-star one is slightly preferred. This is very exciting since our boson-star model is as of now very limited and subject to major improvements. A more evolved model may lead to even larger evidence for this scenario and would also allow us to study previous gravitational-wave observations under the boson-star merger assumption.

Jose A. Font, Professor, University of Valencia

This outcome involves the initial visualization of boson stars and also that of their building block, a new particle called an ultra-light boson. These ultra-light bosons have been suggested as the constituents of the so-called dark matter, which constitutes about 27% of the discernible Universe. 

According to Professor Carlos Herdeiro from the University of Aveiro, “one of the most fascinating results is that we can actually measure the mass of this putative new dark-matter particle, and that a value of zero is discarded with high confidence. If confirmed by subsequent analysis of this and other gravitational-wave observations, our result would provide the first observational evidence for a long-sought dark matter candidate.”

Journal Reference:

Bustillo, J. C., et al. (2021) GW190521 as a Merger of Proca Stars: A Potential New Vector Boson of 8.7×10⁻¹³  eV. Physical Review Letters. journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.081101.