An artist’s vision of a boson star collision. Credit: Nicolas Sanchis Gual and Rocio Garcia Souto.
With the advent of functional gravity observatories, a new window into space has opened for us. Thanks to it, we detect events that we could only dream about before. The problem is that we don’t always understand what gravity observatories actually observed.
We have already detected a number of gravitational waves. Most come from monumental black hole collisions, some of which have soaked neutron stars. But some of the detected gravitational waves are strange. Queen among them is the 2019 event, designated GW190521. The mainstream view is that it was the collision of two black holes with masses of about 85 and 66 Suns.
Nicholas Santis Gual. Credit: Valencia Virgo Group.
However, scientists suspect that behind the event GW190521, which took place at a distance of about 17 billion light years (Luminosity distance), could have been some exotic objects. A number of proposals have already appeared, but so far none have been fully implemented. Nicolas Sanchis-Gual of Spain’s Universitat de València and his colleagues believe it is a collision of boson stars.
So far, boson stars are completely hypothetical objects that should form bosons, i.e. particles with a symmetric wave function and integer spin, as a particle physicist would say. Bosons can be particles of interaction, like the good old photon or the shouted Higgs boson, but also particles of matter, for example mesons, tetraquarks or alpha particles (the helium-4 nucleus). In the specific case of the gravitational event GW190521, according to the authors of the study, it should be Proc’s stars (Proca star), i.e. actually Bose-Einstein condensates of massive bosons with spin 1.
As for boson stars, they would not be able to cope with ordinary bosons. If they exist, they are probably made up of some so far unknown bosons, which are very stable and, unlike photons, have some mass. One of the possible candidates are axions, popular today but also entirely hypothetical particles. Boson stars would reliably expand the collection of breathtaking objects, because they resemble more the hallucination of an overworked astronomer than a cosmic body. They should be invisible, transparent and, thanks to overwhelming gravity, would dramatically bend space.
Were they boson stars? Credit: Galician Institute of High Energy Physics.
As Sanchis-Gual admits, boson stars should actually be like black holes. Compared to the famous gravity monsters, however, they have the advantage of lacking an event horizon and a singularity. These are precisely the properties of black holes that we don’t understand very well and that, to put it politely, make us a mess in physics. Another added value of boson stars is that they could account for at least some of the dark matter we are still searching for.
Boson stars can’t just be found. They are invisible, and while they certainly create gravitational lenses, their effect would probably be indistinguishable from that of a traditional black hole. However, Sanchis-Gual et al. they believe that gravitational astronomers are the only ones who could identify these cosmic oddities if two boson stars collide and ripple through space-time with gravitational waves.
As part of the mentioned research, the scientists analyzed a total of three suspected gravitational events. In addition to the mentioned GW190521, also GW190426 and GW200220. While GW190521 fits nicely into their boson star collision models, the other two events probably weren’t such a collision. In time, hopefully, we’ll learn more.
Video: Light in the dark: GW190521 as a boson star merger (Nicolás Sanchis-Gual)
Literature
Physical Review D 108: 123020.
2024-01-11 13:25:21
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