The burst of gravitational waves captured by LIGO in September 2015 could not have resulted from the formation of a pair of black holes in the interior of one large star, astronomers say in an article published in the journal Physical Review Letters.
“The results of our calculations show that if the pair of black holes that gave rise to the outbreak of GW150914 originated in the interior of one star, then their merger would be shorter than it actually is. The patterns of real and virtual gravitational waves begin to coincide only when the density of the star in our model drops to a vacuum level,”says Joseph Fedrow of Kyoto University, Japan.
In September 2015, virtually immediately after the updated LIGO was turned on, scientists discovered a burst of gravitational waves generated by merging black holes with a total mass of 65 Suns. LIGO subsequently recorded five more events, also caused, with one exception, by mergers of large black holes.
This discovery launched a new round of controversy among scientists: how exactly such pairs of black holes could have arisen and whether it is possible to trace the history of their formation by how the merger takes place.
Some astronomers believe that black holes in such pairs are born alone and only after a very long time come closer to each other and merge. This theory imposes very strict restrictions on the frequency of mergers and the place where they can occur - in fact, pairs of black holes arise only in superdense globular clusters on the outskirts of galaxies.
There is also an alternative theory, free from such shortcomings. Its supporters believe that pairs of black holes form immediately after the death of especially large stars, whose mass exceeds the solar mass by 50-60 times. In the last stages of life in such luminaries, anomalously dense regions may arise, which make them literally tear apart, turning into two neutron stars or black holes.
As proof, the authors of this idea pointed to one of the main mysteries - a burst of gravitational waves, for the discovery of which American scientists received the Nobel Prize in physics this year. The fact is that the Fermi telescope then recorded a weak gamma-ray flare, which appeared at about the same time and in the same part of the sky where the GW150914 burst was.
Fedrow and his colleagues decided to check whether this could really be so, for which they created a computer model of a star with a mass of 60 Suns, in theory capable of generating a pair of black holes of the required mass. By observing the formation of these objects and their subsequent collision, the scientists compared the obtained profiles of gravitational waves with data from LIGO and tried to achieve identity.
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As it turned out, the high density of the bowels of even very "aged" and bloated stars noticeably accelerated the process of merging of black holes: they slow down their motion, "clinging" to the superdense gas surrounding them. As the calculations of scientists show, as a result of this deceleration, the length of the flash will be reduced by 1.5-3 times.
Therefore, the gravitational waves generated by such black holes, "twins", will be noticeably different from those arising from the merger of single black holes. However, none of the scenarios calculated by Fedrow and his colleagues are similar to GW150914 and other bursts of gravitational waves recorded by LIGO. All of this suggests that the coincidence of a gamma-ray burst and a burst of gravitational waves was probably a mere coincidence.
As the researchers emphasize, the absence of traces of such "twins" does not necessarily mean that they, in principle, do not exist in the Universe, although the chances of this are rather small. Further observations of LIGO, ViRGO and the new gravitational wave detectors, Fedrow said, will help test whether this is the case or not.
