Gravitational waves, the invisible waves in the fabric of space predicted by Albert Einstein, are opening a new era in astronomy that allows scientists to see parts of the universe that were previously thought invisible, such as black holes, dark matter and theoretical subatomic Particles called axions.
Almost 100 years after Einstein predicted its existence as part of his theory of general relativity, gravitational waves were first detected in 2015 by scientists working at the Laser Interferometer Gravitational Wave Observatory (LIGO), earning them the Prize Nobel of Physics.
The slight disturbances that the giant instrument picked up were created by two black holes that collide with each other at 1.3 billion light years from Earth. When these two superheavy objects collided, they deformed space and time.
"The deformation propagates like ripples in a lake," explained Professor Paolo Pani, a theoretical physicist at the Sapienza University in Rome, Italy. "These are gravitational waves."
All objects with mbad will create their own slight fall in the fabric of spacetime, creating what we call gravity. But only cataclysmic events involving heavier 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, pbading through almost everything in their path.
But the ability to detect these waves also now gives astronomers new ways of seeing the universe. Prof. Pani is leading the DarkGRA project in an attempt to use gravitational waves to explore some of the biggest mysteries in the universe, including heavy and exotic stars, dark matter and black holes.
Previously, astrophysicists have been forced to infer the presence of black holes by observing the behavior of the surrounding material. Thought to be the super-heavy remains of collapsed stars, the gravity they produce is so great that not even light escapes. Everything that pbades the boundary of a black hole, known as the event horizon, remains there.
"That's why we can not see the black holes," said Prof. Pani. "Instead, we see an absence of light in them, black holes are still a great mystery."
However, gravitational waves are allowing scientists like Prof. Pani to see them directly. "They are a kind of space-time messenger around these objects, without using any intermediary," he said.
By studying the characteristics of these waves it is possible to obtain information about the mbad, rotation, radius and speed of these previously invisible objects. "The objective of our project is to understand the observations of gravitational waves from very compact objects, so that we can discard or confirm other types of objects," said Prof. Pani.
According to general relativity, the fusion of two very compact objects, such as white dwarfs, neutron stars or black holes, will cause the final object to collapse to form a black hole. But there are alternative theories that predict that they could also form objects of a mbad and a radius similar to black holes, but without an 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 the objects, there should be echoes of gravitational waves, therefore a signal that is reflected from the surface", explained Prof. Pani. It should be possible to detect these echoes in the signals captured here on Earth.
However, there is another explanation that would lead to black holes unexpectedly producing echoes or other characteristics of inexplicable gravitational waves: they could be sitting in a bath of dark matter, a hypothetical form of matter that has not yet been seen, but it is believed that It's the cause. for 85% of all matter in the universe. This could also produce a distinctive gravitational wave.
"Dark matter interacts very little with anything else, so it is very difficult to test it in the laboratory," said Prof. Pani. But by looking for different 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 changing our laws of gravity. Professor Ulrich Sperhake, a theoretical physicist at the University of Cambridge, UK, and lead scientist on the StronGrHEP project, described gravitational waves as a "new window to the universe" that could help us unravel these mysteries.
If there is all this dark matter hanging around two black holes as they merge, then this would absorb energy.
It would mean that in a collision of a black hole like the one detected by LIGO, gravitational waves would look a little different than they would be without dark matter.
An observational puzzle in which they could shed light is why galaxies rotate faster than their size suggests they should. "The speed of rotation is related to the mbad inside," said Prof. Sperhake. So, if a galaxy is spinning faster than the mbad we can see, there are two possible explanations: either we need to alter our fundamental theories of how gravity works or there is dark matter in galaxies that we can not see.
One idea that Prof. Sperhake is investigating is to extend Einstein's general relativity with a new theory, called scalar tensorial gravity. This suggests that the universe is filled with an additional field, similar to a magnetic or electric field, that has not yet been detected.
It 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 glow of gravitational waves that we could detect. We could direct LIGO to regions of the sky where the stars have exploded, known as supernovas, to try to detect a later brightness of the scalar field that may persist for centuries after the actual explosion.
On the other hand, Professor Sperhake is investigating whether dark matter could be explained by theoretical subatomic particles called axions. He is trying to model what the echoes of the gravitational waves of black holes would look like if these particles were present.
"I would say that axions are one of the best candidates for dark matter," he said. The next step is to apply their models to the data that LIGO collects to see if theory and observation are coincidental.
Dr. Richard Brito joined Prof. Pani's group in Italy earlier this year as part of his own project, FunGraW, to use gravitational waves to prove the existence of axion particles. But he will also use them to prove Einstein's theory and if it can be wrong on large scales.
"If we see objects almost as compact as black holes but without an event horizon, that means that general relativity is incorrect on those scales," he said.
It could have important daily implications. The theory of general relativity is crucial for the daily functioning of GPS, for example. But finding that Einstein's theory breaks down on a large scale does not mean that it should be discarded. Rather, an addendum might be necessary.
"It would be difficult for him to match the mathematical clarity of Einstein's theory," said Professor Sperhake. "Not only is it amazing because of all the fantastic predictions it has, it has the appeal of being a beautiful theory, and physicists find it interesting that beauty is an important ingredient in a theory."
Gravitational waves provide doses of reality over extra dimensions
Gravitational waves that help expose black holes, dark matter and theoretical particles (2019, April 5)
retrieved on April 5, 2019
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