Unexpected black hole
The 85 solar masses black hole is such a great mystery because of how massive stars are coming to an end, according to scientists.
Despite the fact that stars are seething nuclear furnaces, they are in perfect balance: the star is compressed with immense force by its own gravity, while the radiation from its nucleus pushes matter out again. But massive stars sometimes reach such high fusion temperatures at their core that this balancing act is disrupted. Individual light particles (photons) absorb enough energy to form pairs of electrons and positrons (the electron’s anti-particles). This change temporarily reduces the pressure in the star’s core, causing the star to compress itself and become much hotter.
Current theories predict that if a star has a mass between 60 and 130 solar masses, this compression and heating will lead to an explosive chain reaction, a “pair instability” supernova. In such a supernova, a star is so thoroughly destroyed that the erupted remnants cannot implode into a black hole.
Oddly, the larger of the two black holes that caused GW190521 “fall right in the range where you would expect pair instability,” says Berry. The bottom line is that it is theoretically impossible for such a star to create a black hole.
“If you find a black hole between 52 and 133 masses of the Sun, it could not have come from the imploded remnants of a single star,” said Priyamvada natarajan, a theoretical astrophysicist from Yale University and expert on black holes who was not involved in the new study. “Nature now tells us that there are countless ways for black holes to reach these kinds of masses.”
Hostile take-over?
In an accompanying article in The Astrophysical Journal LettersThe LIGO-Virgo team discusses several scenarios in which the merger – and the formation of the curious black holes – could have taken place. Most promisingly, at least one of the two black holes, if not both, is the result of a fusion of two smaller, “more ordinary” black holes.
“That’s my favorite screenplay,” says Steinn Sigurdsson, an astrophysicist from Pennsylvania State University who was also not involved in the discovery.
In certain cosmic conditions such a ‘two-stage fusion’ would be plausible. One possibility that would increase the likelihood of such an event would be if the fusion took place within an accretion disk of gas orbiting the central and supermassive black hole of a galaxy.
There is promising evidence that GW190521 did indeed occur in such an environment. Last June, Graham was one of the authors of an investigation in the Physical Review Letters describing a flash of light observed in the same patch of night sky where GW190521 occurred about 34 days after the gravitational waves echoed through Earth. Graham’s team argues that this flash may have originated when the fused black hole was launched right through an accretion disk of gas around a supermassive black hole. The gas was heated in such a way that it started to glow.
Yet there remains a difference between the research into the fusion and the findings about the flash of light: the distance. In Graham’s study, the flash of light is located in a galaxy some eight billion light years from Earth, while the results of the LIGO-Virgo observation indicate a merger of black holes more than seven billion away. light years. Graham admits that the coinciding locations of both events in the night sky may be coincidental. “If you observe enough events, these very rare things may be aligned from Earth,” he says.
Natarajan has a different explanation for the merger. In a study published in 2014 in the journal Science appeared and of which she was one of the authors, it was calculated that in the still young universe, smaller black holes could fatten up at lightning speed by bouncing around dense star clusters and feeding on matter. A forthcoming study detailing this idea discusses the possibility that certain types of star clusters could produce a pair of black holes, each between 50 and 75 solar masses, which could then merge.
“That’s why I find this very exciting of course!” Natarajan says.
Aside from all the theoretical work that GW190521 has undoubtedly yielded, scientists believe the riddles will not be solved until LIGO and Virgo observe more collisions of this kind. “With a single event you can always rely on special and accidental circumstances,” says Sigurdsson. “But once you have observed multiple events, you can extract a lot of information from all kinds of models.”
This article was originally published in English on NationalGeographic.com
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