It's Not Just Black Holes: Unusual New Source Of Gravitational
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It’s not just black holes: unusual new source of gravitational waves

The signals came from the merger of two neutron stars, the first detected in history.

Recreation of the new source of gravitational waves.

As if it were a military operation, an army made up of more than 3,500 scientists from around the world, supported by land and air by dozens of telescopes, has fulfilled a mission: to find out where the strange gravitational waves recorded in the past 17 years came from. August by the two LIGO detectors in the US The response has been spectacular: they are the result of the collision between two neutron stars, the smallest and densest known. It is the first time in history that this phenomenon has been detected.

Gravitational waves – disturbances in space-time predicted by Einstein – had already been detected before in four black hole mergers. But as soon as they saw the new signal –called GW170817–, the scientists realized that it responded to a very different event: an emission of electromagnetic radiation accompanied it. It could not be another collision of black holes, which do not emit light. The origin of this new wave was a mystery.

After weeks of hard work and in the midst of secrecy worthy of an espionage agency, the scientific operation has borne fruit. The cataclysm of neutron stars was associated with a spectacular gamma-ray burst in a galaxy 130 million light-years away called NGC 4993.

“This observation represents the birth of a new and powerful field that we call multi-messenger astronomy,” Barry C. Barish, a pioneer in hunting these waves and one of the three awards, tells Sinc by email. Nobel Prize in Physics in 2017. As Rainer Weiss, another of the laureates, explains to this medium, “the discovery of the fusion of neutron stars through gravitational waves, together with the measurement of gamma radiation with the Fermi satellite, together with the observations with electromagnetic telescopes, they form a beautiful example of the science that we can do with this multi-messenger astronomy”.

One of the many Spanish scientists who participated in the discovery, José Antonio Font, qualifies it as “historic”. The intense working hours these two months have prevented him from taking a single day off. “I’ve had to put my life on hold for a bit, but it’s worth it,” the principal investigator of the virgo group from the University of Valencia.

From NASA Goddard Space Flight Center (USA), Eleonora Troy did not expect to detect something like this so soon – the scientific collaboration LIGO It has been running for just two years since its last renovation. “Not even in my wildest dreams did I think we were going to get these results on the first try,” he admits to Sinc.

Very different from black holes

The data recorded by LIGO revealed that the objects that have generated this new signal were not as large as the holes and their mass ranged between 1.1 and 1.6 times that of the Sun, measurements that were consistent with those of neutron stars. These objects, which are formed by the explosion of supernovae, are about 20 kilometers in diameter. In addition, the signal lasted longer than that recorded in previous events (about 100 seconds).

“It is the strongest gravitational wave signal that has been detected so far,” he highlights. Sascha Husa, member of the Relativity and Gravitation Group of the Universitat de les Illes Ballears that is part of LIGO.

From Italy, scientific collaboration Virgo he took the LIGO witness and pinpointed the position of the collision more precisely with his other detector: the signal came from a relatively small region of the sky in the southern hemisphere.

“Coincidentally, this signal was in one of Virgo’s almost blind spots and that’s why he couldn’t see it so clearly, although that indirectly helped the location, since it indicated that he was right in that blind spot,” says Font, who also directs the Department of Astronomy and Astrophysics at the University of Valencia.

caught from the air

In parallel, the gamma-ray space telescope Fermi, who has been orbiting the Earth for almost ten years in search of astrophysical phenomena like this, noticed the presence of gamma rays, arriving two seconds after the detection of gravitational waves.

Using data from LIGO and Virgo, the telescope further pinpointed the location of the collision, and then everyone went to work locating the signal. In total, seventy ground and space observatories – some of them Spanish – were able to observe the event in its different wavelengths, a new point of light similar to that of a star. NASA, the European Space Agency and the European Southern Observatory are three of the organizations that have participated in the discovery.

“With the telescope’s DECam instrument Victor White from Chile we discovered an optical counterpart and there was also a gamma ray burst. All this light indicates with great certainty that they must be neutron stars and not black holes”, he points out to Sinc. Daniel Holz, researcher at the Department of Astronomy and Astrophysics at the University of Chicago (USA).

As in previous detections, confidentiality and coordination between teams from so many countries has been essential, something that is not easy; LIGO is made up of some 1,200 people, to which are added another 1,300 from different institutions that have collaborated in the discovery.

“It’s the spirit of collaboration. Everyone adds up, feels part of something, something big that marks history, “he says. Alicia M. Sintes, principal investigator of the LIGO collaboration at the University of the Balearic Islands.

The brightest after the Big Bang

The observations revealed that the gravitational waves were produced by two neutron stars in spiral orbit. Medium-mass stars that die as supernovae give rise to these types of extremely dense objects. Just one teaspoon of its material is equivalent to a mass of about a billion tons.

“The stars that are smaller become white dwarfs and the heavier ones, black holes,” he explains. Christoph Adami, Professor of Physics and Astronomy at Michigan State University (USA).

About 130 million years ago, both stars were in their last spiral orbits, about 300 kilometers apart. As they rotated faster and came closer together, they warped and distorted the space-time around them, emitting energy in the form of gravitational waves before colliding with each other.

“That gives rise to the most spectacular fireworks you can imagine,” he describes. Gonzalo J. Olmo, researcher at the Institute of Corpuscular Physics and the Department of Theoretical Physics at the University of Valencia.

At the time of the collision, most of the two neutron stars merged into a very dense object and simultaneously emitted a kind of fireball of gamma rays. “After the Big Bang, there is nothing as luminous as these eruptions,” Font details.

For the first time, the researchers have detected a kilonova, the process in which the material left behind after the collision is ejected outwards. Optical observations show that heavy elements like lead and gold are created in these collisions and distributed throughout the universe.

“The detection of the kilonova emission opens a way to understand the cosmic chemical enrichment resulting from heavy elements and also to know the final phases of stellar evolution,” he stresses. Elena Pian, a researcher at the National Institute of Astrophysics in Bologna (Italy), who participated in the discovery.

The era of multimessengers

The simultaneous detection of the different signals that have come to us from the collision, both gravitational and electromagnetic waves – what physicists call messengers – ushers in a new era in astrophysics. This is how he expresses it Stefano Covino, who has also collaborated in the discovery. “A new range of possibilities opens up for today’s researchers to study the universe,” says this researcher from the National Institute of Astrophysics in Merate (Italy).

The only messengers that have not been detected have been the elusive neutrinos, which are thought to be formed in such mergers. However, the different devices that search for them from Earth have not found any signal associated with the collision recorded two months ago.

“Neutrino detectors are not directional, that is, they are not like telescopes that you can point in a certain direction,” he clarifies. Roberto Emparan, researcher at the Institute of Cosmos Sciences of the University of Barcelona and ICREA research professor.

What they have been able to record is the Hubble constant, a measure of the speed at which the universe is expanding that is very useful for cosmology. This and other findings will be published from today in Nature, Physical Review Letters and in a long list of publications. “It is probably going to become the astronomical event that has been studied the most in the history of astrophysics,” predicts Font. “It will give us work for many years,” he adds.

So much information from a single collision has come as a surprise to the thousands of researchers involved in the project. With a single event they have been able to contrast theories that other generations of astrophysicists formulated when they were children.

“Any scientist’s dream is to solve at least one big mystery in the course of their career, but imagine solving three or four in a couple of weeks! No one could have expected it,” exclaims Troja.

discovery of the year

Experts who have not participated in the finding agree on its importance. “Observing this type of event will allow us to explore physics beyond what we would dream of doing in the laboratory,” says Olmo. Although for Emparan it is not at the same level as the first detection of gravitational waves, he emphasizes that it will be the first in a series of discoveries.

“They are going to allow us to understand much better what neutron stars are, where heavy elements are formed and many other things”, lists the physicist, who puts a cinematographic simile. If with gravitational waves we only listened to the soundtrack of a movie, with the detection of electromagnetic waves we can see the scenes, “in technicolor and Dolbi sound”, he compares.

For Adami we are probably facing “the discovery of the year”. In his opinion, discovering new mergers of neutron stars will help us measure the rate at which the universe is expanding with greater precision, which will serve to better understand the nature of the Big Bang. “It is as if we had been given a microscope when before we could only see with the naked eye. The object we are seeing is the entire universe,” he maintains.