Two detectors separated by thousands of kilometers have captured the same signal that corresponds to the most powerful source of gravitational waves ever observed. As Albert Einstein predicted more than a century ago, the most violent phenomena in the cosmos produce these ripples of space-time – the stuff the universe is made of – that travel at the speed of light in all directions as if they were the ripples from a stone thrown into a pond. Upon reaching Earth after traveling vast cosmological distances, these waves are so weak that Einstein was skeptical that they could be captured.
The signal was captured on May 21, 2019 and lasted just a tenth of a second. After more than a year of study, the pattern that this characteristic vibration produced in the laser light beams of the LIGO detectors, in the US, and Virgo, in Italy, has allowed us to reconstruct how this phenomenon occurred.
The wave comes from the merger of two black holes and would be the largest captured to date, as explained in two studies this Wednesday by the almost 2,000 scientists from 19 countries who work with the data from both detectors. The collision occurred about 7 billion years ago – before the formation of the solar system and the Earth – when a black hole with a mass 85 times that of our Sun collided with another equivalent to about 66 solar stars. The interesting thing is that with the laws of general relativity in hand and what is known about the physics of stars, this phenomenon is impossible to explain: either the theory of stellar evolution must be changed or the black holes involved have a origin unknown and still mysterious.
This is the most important discovery in this field since the first gravitational wave was discovered in 2016.
Black holes are objects so dense that their force of gravity attracts anything that falls into their jaws, including light, so they are invisible. After the collision, these two monsters devoured each other and formed a black hole with a mass of 142 suns. The rest of the mass was transformed into energy that was thrown in all directions like the shock wave of a bomb that traveled at the speed of light, bending space and time in its path. LIGO and Virgo laser interferometers, which work like a kilometer-long fishing net made with beams of laser light capable of detecting vibrations in space-time up to 10,000 times smaller than the diameter of an atom, captured the already very weak signal, 7,000 millions of years later.
This is probably the most important discovery in this field since the first gravitational wave was discovered in 2016 and the LIGO boosters won the Nobel Prize in Physics just one year later. Toni Font, theoretical physicist at the University of Valencia and Virgo collaborator, explains why: “Until now, all the mergers of black holes and neutron stars that had been captured were vanilla flavor; almost all physicists liked them because they corresponded with what was to be expected ”. The fusion announced today is like a fabada ice cream: rare, possible and not liked by many because it takes them out of their comfort zone. “Today we are not prepared to understand this phenomenon and we cannot answer the many questions it opens up,” explains Font. “The most interesting thing is that the result of this fusion is a hole of 142 solar masses, something never observed and that for now we cannot understand either,” he highlights.
In the world of black holes there are two broad categories. The first is made up of stellar mass, which are formed when a star dies and its corpse collapses on itself to form the black hole. These usually do not exceed a few tens of solar masses. The second class are supermassive black holes, hulking monsters with masses of hundreds of thousands of stars like the Sun that lurk at the center of galaxies, including our own. Between these two types there is a huge empty space. Black holes are hardly known in the intermediate range between the two types mentioned.
According to stellar physics and Einstein’s relativity, a star between 65 and 120 solar masses that dies explodes like a bomb without leaving any trace. That is why the two black holes intuited by the detectors, of 85 and 66 solar masses, and their final product, cannot be the result of a stellar death: they must have an alternative origin that allows multiple explanations, from the most conservative to some that They could deserve another Nobel and revolutionize our understanding of the universe.
Collaborative scientists analyze the signal and reconstruct with powerful computers all the phenomena that could have produced it, always playing according to Einstein’s rules of general relativity. In two studies published today in Physical Review Letters and Astrophysical Journal Letters, ensure that the most likely explanation is that it is a merger. How could two theoretically impossible black holes have been formed? “Either the theory of stellar evolution that we handle is incomplete and we have to rewrite it, or else the two black holes involved do not come from stars that died, but were formed by earlier mergers of smaller black holes,” explains Font.
The second option would be possible in certain regions of the universe known as globular clusters, huge spheres made up of thousands of stars. Many of them die and form thousands of black holes that would be close enough to meet, attract, collide and merge.
The unknowns about this wave are greater than with the previous ones. Normally two black holes that gradually approach their orbits until colliding produce waves that last longer and whose frequency is increasing in what the physicists of LIGO and Virgo call a “chirp”, which ends with a high peak. Scientists have translated these waves into sound so that they can be heard at other times. The previous twitter provides much of the information about the masses, rotation or distance of the two black holes. On this occasion, the previous twittering has not been captured, only the final moment of the merger, a tenth of a second that hides many of the details about what produced it and how, explains Font.
This phenomenon allows from the most conservative explanations to some that could deserve another Nobel and revolutionize our understanding of the universe
Much more unlikely, but also more stimulating because they delve into the unknown, are two alternative explanations addressed in the second study. The first says that we are facing primordial black holes, clots of matter formed seconds after the birth of the universe after the Big Bang – 13.7 billion years ago – and that they would be made of dark matter. For the universe to look and behave as it does, 85% of all matter has to be dark, but what it is made of has never been found out. Getting it would be a historic discovery, a Nobel one.
In the 1970s, Stephen Hawking and Bernard Carr proposed the existence of these black holes so tiny that their mass would not exceed that of a mountain; but they concluded that all of them have already evaporated. Subsequently, the theoretical physicists Juan García-Bellido, from Autónoma de Madrid, and Sébastien Clesse, from the University of Louvain, proposed that there may be primordial black holes of dozens of solar masses and that together they could constitute all the dark matter in the universe, or at least part of it.
The finding of now resurrects this theory, but it stops its feet instantly, as those responsible warn that the possibility that they are primordial holes is very, very remote. “With these data we cannot affirm that they are primordial holes, only that we cannot rule out that possibility,” says García-Bellido. “We probably need to re-analyze the data with other previous assumptions and wait for more phenomena of this type to be detected,” he highlights.
Another less likely but amazing possibility is that the origin of this most powerful gravitational wave in history is a cosmic string, a kind of one-dimensional thread formed fractions of a second after the Big Bang and that, for now, only exists on paper fruit of the theories of some physicists. “It is extremely unlikely that this event was produced by a cosmic string,” the scientists write cautiously in their second study to conclude that the most likely option is the one that fits with what has been observed since 2016: a merger of two black holes. Returning to ice cream, Font recognizes that there are still so many unknowns that the most prudent thing for scientific collaboration has been to stick with vanilla, for now.