Gravitational waves helping to expose black holes, dark matter and theoretical particles

Gravitational waves – the invisible ripples predicted by Albert Einstein in the fabric of space – open up a new astronomical era that allows scientists to see parts of the universe that we once thought invisible, such as black holes, dark matter and theoretical subatomic theory. particles called axions.

Nearly 100 years after Einstein predicted their existence as part of his theory of general relativity, gravitational waves were first detected in 2015 by scientists working on the Gravitational Wave Observatory Laser Interferometer (LIGO), which earned them the Nobel Prize in Physics.

The small disturbances detected by the giant instrument were created by two black holes that collided with each other at 1.3 billion light years from Earth. When these two very heavy objects collided, they distorted space and time.

"The deformation spreads like ripples on a lake," said Professor Paolo Pani, theoretical physicist at Sapienza University in Rome, Italy. "These are gravitational waves."

All objects with a mass will create their own light dip in the structure of space-time, creating what we call gravity. But only cataclysmic events involving the heaviest objects, such as black holes and neutron stars, can create gravitational waves large enough to be detected on Earth. They radiate through the universe at the speed of light, crossing almost everything in their path.

But the ability to detect these waves also provides astronomers with new ways to see the universe. Professor Pani directs the DarkGRA project with the goal of using gravitational waves to probe some of the greatest mysteries of the universe, including heavy and exotic stars, dark matter and black holes.

Previously, astrophysicists were forced to infer the presence of black holes by examining the behavior of the material surrounding them. Thought to be the super-heavy remains of collapsed stars, the gravity they produce is so great that even light does not escape. Anything beyond the limit of a black hole, called the event horizon, stays there.

"That's why we can not see black holes," said Professor Pani. "Instead, we see an absence of light from them.The black holes are still a big mystery."

Gravitational waves, however, allow scientists such as Professor Pani to view them directly. "They are sort of a messenger of space-time around these objects, without using an intermediary," he said.

By studying the characteristics of these waves, it is possible to obtain information on the mass, the rotation, the radius and the speed of these previously invisible objects. "The goal of our project is to understand the gravitational wave observations of very compact objects, in order to exclude or confirm other types of objects," said the Professor Pani.

According to general relativity, the fusion of two very compact objects – such as white dwarfs, neutron stars or black holes – will result in the collapse of the final object into a black hole. But there are other theories that predict that they could also form objects of mass and radius similar to those of black holes, but without event horizon. These mysterious compact objects would therefore have a surface that would reflect gravitational waves.

"If there is a surface, after a fusion of objects, there should be gravitational wave echoes, so a signal reflected by the surface," explained Professor Pani. It should be possible to detect these echoes in the signals picked up here on Earth.

Black matter

However, there is another explanation that would lead to black holes producing echoes unexpectedly or other unexplained gravitational waves – they could be sitting in a bath of dark matter, a hypothetical form of matter that has not yet been view, but who could account for it. for 85% of all matter in the universe. This too could produce a distinctive gravitational wave.

"The dark matter interacts very little with anything else, so it is very difficult to test it in the laboratory," said Professor Pani. But by looking for distinct signals in gravitational waves, scientists could "see" it for the first time.

Some gravitational observations can only be explained by the presence of dark matter, which we can not see, or by a modification of our laws of gravity. Professor Ulrich Sperhake, theoretical physicist at the University of Cambridge, UK, and lead scientist of the StronGrHEP project, described gravitational waves as a "new window to the universe" that could help us solve these mysteries.

If there is all this dark matter hanging around two black holes during their fusion, it will absorb energy.

This would mean that in a black hole collision like that detected by LIGO, the gravitational waves would be a little different from what they would be without dark matter.

An observation puzzle that they could explain is why galaxies rotate faster than their size suggests. "The speed of rotation is related to the mass that is inside," said Professor Sperhake. So, if a galaxy rotates faster than the mass we can see, there are two possible explanations: we have to either modify our fundamental theories of how gravity works, or there is dark matter in galaxies that we do not have. can not see.

One idea that Professor Sperhake is studying is to extend Einstein's general relativity with a new theory, called gravitational gravitational scalar tensor. This suggests that the universe is filled with an additional field – similar to a magnetic or electric field – that has not yet been detected.

This would mean that the supernova explosion of a dying star would not only be visible as a burst of gravitational waves, but that there would be a subsequent glow of gravitational waves that we could detect. We could direct LIGO to areas of the sky where stars exploded – known as supernovae – to try to detect such reverberation from the scalar field that could persist for centuries after the actual explosion.

Sperhake also studies whether dark matter could be explained by theoretical subatomic particles called axions. He tries to model what could be the resonance of black hole gravitational waves if these particles were present.

"I would say axions are one of the best candidates for dark matter," he said. The next step is to apply its models to the data collected by LIGO to determine if the theory and the observation match.

Beautiful theory

Dr. Richard Brito joined Professor Pani's group in Italy earlier this year as part of his own project, FunGraW, to use gravitational waves to test for the existence of axionic particles. But he will also use them to test Einstein's theory and determine if it is incorrect on a large scale.

"If we see objects almost as compact as black holes but without a horizon of events, it means that general relativity is wrong at these scales," he said.

This could have important day-to-day implications. The theory of general relativity is crucial for the daily operation of GPS, for example. But finding that Einstein's theory collapses on a large scale does not mean that it should be thrown in the trash. On the contrary, an addendum might be necessary.

"You would have trouble matching the mathematical clarity of Einstein's theory," Professor Sperhake said. "This is not just amazing because of all the fantastic predictions that it does.This has the advantage of being a beautiful theory." And physicists interestingly consider beauty as an important ingredient of a theory. "

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