A gravitational wave signal generated more than 7,000 million years ago – the largest that has been captured to date – has become one of the most controversial and exciting phenomena in physics today, because with the laws of the universe in hand
In May 2019 two sophisticated detectors capable of capturing these tiny ripples in space-time predicted by Einstein saw this signal.
An international team with hundreds of scientists from both detectors – LIGO, in the US, and Virgo, in Europe – analyzed this wave for months, which lasted barely a tenth of a second. In September they came to a conservative conclusion — boring — or as some physicists called it “vanilla flavor” : it was the result of the merger of two black holes. But there was a problem. At least one of those black holes could not exist with the laws of stellar physics in hand. In the minds of many physicists, a multitude of more risky and exciting explanations emerged, since they involved a glimpse of completely new phenomena, unknown particles, something as exotic and unexpected as a bean ice cream.
Today, part of the LIGO and Virgo scientists publish a new study in which they jump into the pool with a very risky but plausible explanation: that signal was not produced by two black holes but by two transparent stars made of never-observed particles that number in the trillions times lighter than an electron. They are called ultralight bosons and in theory they can be the explanation to one of the greatest enigmas in the universe: what is dark matter , the mysterious component that makes up 27% of the universe while known matter makes up only 5%?
These stars They were theorized in the late 1950s and described in greater detail in the following decade. It would be stars made of particles that do not emit light, such as black holes. But instead of being a large dark point in the sky, they would be completely transparent to our eyes. Until now it has not been possible to verify its existence because the necessary technology and models that explain its behavior are lacking.
"Once created, a black hole is not made of anything, it is just an area of the universe in which if you enter you will never be able to leave" Juan Calderón, IGFAE
When that gravitational wave that should not exist was captured in May 2019, Font's team had been working for years on a mathematical model capable of predicting the behavior of boson stars. Physicists call them “black hole mimicking objects” because of the properties they share with these bodies.
“The biggest difference between the two is that an ultra-light boson star does not have an event horizon [the point of no return], so that it would not swallow us without the possibility of ever leaving. There would be an escape route ”, explains Juan Calderón Bustillo, theoretical physicist at the University of Valencia and Virgo collaborator.
Calderón has been responsible for the statistical analysis of the new study on the gravitational wave signal, which is published today in the prestigious journal Physical Review Letters . His colleague Nicolás Sanchís-Gual, from the University of Lisbon, has made the mathematical simulations of these objects. The team has compared which model best explains the captured signal, black holes or their own. The results show that the second is about eight times more likely, explains Calderón.
It is an exciting first result, but very preliminary. In statistical terms, the probability that what these scientists say is true is about two sigmas. But to claim a discovery in physics, five sigmas are needed: a single possibility in almost two million that what is claimed is false.
The most interesting evidence these physicists provide is that they have calculated the mass of the ultralight boson that would form these stars . His result fits with the theoretical predictions of these particles.
Bosons are one of the two basic types of elementary particles that nature is made of. There are four bosons that transmit force, one of them well known transmits electromagnetic force: the photon, the particle of light. There is another famous boson with another function: to provide mass to the rest of the elementary particles, the Higgs boson . The rest of the matter is made of another elementary particle – fermions -, like the electron. Here ends the description of the basic building blocks of matter that make up everything we humans see and touch. It is a model that only describes 5% of the universe. The rest are completely unknown things: dark matter, which would make up 27% of the cosmos, and dark energy , which would explain the remaining 68%.
Ultralight bosons would inhabit that unknown territory. These particles would interact with conventional matter only by their gravitational force. "If they exist, they could accumulate and form dark matter stars," explains Calderón. This would explain the effects of dark matter in the universe, which are very evident, because without its gravitational pull, galaxies like the Milky Way would collapse and possibly could not host planets with intelligent life. Proving that ultralight bosons are responsible for these effects would be a historic finding, as it would open up a new dimension of physics and our understanding of the cosmos.
“If we are really looking at a star made up of axions [dark matter particles] we will see more signals of this type. This story will evolve over time ”Rainer Weiss, Nobel Prize in Physics 2017
The authors of the work for now are very cautious. "This is just a proof of concept, an indication that gravitational waves can be used in a heterodox way to discover new physics," explains Font. "Our study says that there may be an ultralight boson that could be dark matter. We are based for now on a single case, but the probabilities that we are correct are slightly higher than that they are black holes, ”he points out.
When two boson stars collide, they join to form a larger one, but almost instantly collapse to spawn a black hole. Proving this is important because it would be a new way to create black holes that does not need conventional stars. But trying it is devilishly difficult. "Once created, a black hole is not made of anything, it is just an area of the universe in which if you enter you will never be able to leave," explains Calderón. The only way to test the boson star theory is in the first part of the gravitational wave, which lasts less than one-hundredth of a second. "Soon we will have more signals of this type, we will be able to apply our model to it and know if what we say makes more sense," says Font.
The American physicist Rainer Weiss, winner of the Nobel Prize in Physics in 2017 for being one of the parents of LIGO , provide your opinion on the work. "This gravitational wave signal has two things out of the ordinary," he explains to this newspaper. “It implies the existence of very large mass bodies and a head-on collision. Both are very rare among all the gravitational waves that we have detected so far. If we are really dealing with a star made up of axions [dark matter particles] we will see more signals of this type. This story will evolve over time, ”he warns.
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