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According to the General Relativity of Albert Einstein, mass is a property of matter that “tells” the space-time how to curve. When this mass is subjected to changes in acceleration causes variations in this curvature that propagate through the universe at the speed of light and that are known as gravitational waves . These waves penetrate the Earth and cause here tiny distortions (literally “lengthen and shorten” the space-time) that Einstein thought that it would be impossible to detect. But from 2015, the observatories of gravitational waves LIGO (in the Usa) and Virgo (in Italy) have been attracting.

These investigations allow to test general Relativity and to access parts of the universe that are beyond the scope of astronomy by electromagnetic waves (which is done through X-rays or visible radiation, for example), especially in the distant universe. So far they have observed gravitational waves from ten mergers of pairs of black holes and a pair of neutron stars.

Now, the observatories have managed to capture the merger of a black hole and a mysterious object whose mass makes it to be very exotic: it is in a middle point between the smaller black holes known and the biggest neutron stars, so that astronomers do not know what it is. During this fusion, which has received the name of GW190814, it caught the encounter between a black hole 22.2 24.3 solar masses and another object of 2,5 2,67 solar masses, at a distance of 800 million light years from Earth, and that created a new black hole of around 25 solar masses ( here you can see an animation of such a merger). These observations have been accepted to be published in “The Astrophysical Journal Letters”.

The object of the most extreme of the family

“The most important thing from this observation is that the object less massive of the collision is in a range of masses where the observational data, both in gravitational waves as electromagnetic radiation, does not abound, so very new ,” he explained to ABC’s José Antonio Font , a researcher from the University of Valencia, co-author of the study and a member of the Virgo cluster, the european observatory of gravitational waves.

scientists do not know if the smaller object of the pair that is merged is a black hole or a neutron star – LIGO/Caltech/MIT/R. Hurt (IPAC)

“For the moment, however, we do not know what that is: you can be a black hole or a neutron star,” he continued. “In either case, would be the object of the most extreme of their respective family : the neutron star more massive or the black hole less massive than we’ve observed”.

A shock phantom to 800 million light years

on August 14, the observatories LIGO and Virgo detected a signal that lasted just about 10 seconds , which announced a forthcoming merger between a black hole and an object more massive than the largest neutron star and less massive than the smaller black holes. Estimated the localization of this fusion in a wide region of the sky and immediately sent an alert to the scientific community what to seek with telescopes of electromagnetic waves. Unlike what happened in 2017, the date on which it was possible to observe the collision between two neutron stars detected by gravitational waves, saw nothing.

as has been explained to the scientists in a press release, there may be several reasons: this merger is very far away, six times more than the 2017, to reach a distance of 800 million light years away, or that the collision has occurred between two black holes, an event which does not release radiation. Another possibility is that the object less massive, of about 2.6 solar masses, was a neutron star, but that was gobbled up very quickly by the black hole, as if it had been the victim of a pacman .

is There a hollow mass?

what, Then, happened during this merger? By what has been observed until now, and according to the models of formation of neutron stars, the most massive “weigh” as 2.5 suns. on the other hand, it is believed that the smaller black holes are around five solar masses. Therefore, it is considered that there is a “mass gap”, that is, hollow masses, in the which there are none of these objects, either by a natural phenomenon, either because it has not been able to observe until now. However, this study would rethink that idea, to question what is known about the limits of the neutron star, and part of what they say, the theories about the evolution of stars.

“This event is interesting because it puts to the test the models of formation of neutron stars and gives us the idea that there are objects in that “mass gap “, ” explained Mario Pérez , a researcher at the Institute of High Energy Physics (IFAE), in Barcelona. “However, you are going to be needed more events to be able to understand this population and its nature. Will arrive in the next few years.”

In the limits of the subject

as has been explained to José Antonio Font, it is believed that most of the neutron star around the 1,4 solar masses and 10 or 12 kilometres in radius. This depends on the material of which they are made and are based on what is known as the equation of state of dense matter . “Not knowing with precision what is the equation of state of neutron stars, because of the huge densities of these objects are not accessible to experimentation in the laboratory, we cannot determine its maximum mass and maximum radius,” he explained. However, with the observations made until now, the scientists claim that the maximum mass of the neutron star is between two and three solar masses.

as for black holes, in theory, can have any mass, according to Font, although “if we look at the observational data we have, the minimum mass of the black holes observed in our galaxy (in X-ray binaries) is about five times the mass of the Sun.” However, as has been pointed out Mario Perez, some observations have suggested the existence of black holes of about 3.3 solar masses.

Representation of a neutron star – Casey Reed/Penn State University

therefore, if the object less mass detected in the fusion outside a black hole, one would have to consider theories about the formation of these objects after the collapse of stars or by what have been observed so few as small, according to Font.

Another option that has been pointed out to this researcher is to think in objects more speculative, such as black holes primordial stars bosons . This possibility is “more interesting intellectually”, according to Mario Perez, and “opens the door to explain the dark matter with black holes primordial”.

Given that the sensitivity of gravitational wave detectors is constantly improving, with time it will accumulate new observations. For the moment there are already dozens that are being tested: “ This will allow to have a statistic enough to be able to begin to provide answers to questions such as that GW190814 raises,” said José Antonio Font. So you will better understand what is the role of black holes and neutron stars in the universe.