There may be as many as 100 million black holes in the Milky Way galaxy, according to scientists, including one of Indian origin, who conducted a cosmic survey to calculate and categorise the enigmatic, dark objects.
The celestial census began more than a year and a half ago, shortly after the news that the Laser Interferometer Gravitational-Wave Observatory (LIGO) had detected ripples in the space-time continuum created by the distant collision of two black holes, each the size of 30 suns.
"Fundamentally, the detection of gravitational waves was a huge deal, as it was a confirmation of a key prediction of Einstein's general theory of relativity," said James Bullock, University of California, Irvine (UCI) in the US.
"But then we looked closer at the astrophysics of the actual result, a merger of two 30-solar-mass black holes. That was simply astounding and had us asking, 'How common are black holes of this size, and how often do they merge?'" Bullock said.
Scientists assume most stellar-remnant black holes - which result from the collapse of massive stars at the end of their lives - will be about the same mass as our Sun.
To see the evidence of two black holes of such epic proportions coming together in a cataclysmic collision left some astronomers puzzled.
The new research was an attempt to interpret the gravitational wave detections through the lens of what is known about galaxy formation and to form a framework for understanding future occurrences.
According to Manoj Kaplinghat, professor at UCI, the number of black holes of a given mass per galaxy will depend on the size of the galaxy.
The reason is that larger galaxies have many metal-rich stars, and smaller dwarf galaxies are dominated by big stars of low metallicity.
Stars that contain a lot of heavier elements, like our sun, shed a lot of that mass over their lives.
When it comes time for one to end it all in a supernova, there is not as much matter left to collapse in on itself, resulting in a lower-mass black hole.
Big stars with low metal content do not shed as much of their mass over time, so when one of them dies, almost all of its mass will wind up in the black hole.
"We have a pretty good understanding of the overall population of stars in the universe and their mass distribution as they're born, so we can tell how many black holes should have formed with 100 solar masses versus 10 solar masses," Bullock said.
"We were able to work out how many big black holes should exist, and it ended up being in the millions - way more than I anticipated," he said.
In addition, to shed light on subsequent phenomena, researchers sought to determine how often black holes occur in pairs, how often they merge, and how long it takes.
They wondered whether the 30-solar-mass black holes detected by LIGO were born billions of years ago and took a long time to merge or came into being more recently (within the past 100 million years) and merged soon after.
"We show that only 0.1 to one per cent of the black holes formed have to merge to explain what LIGO saw," Kaplinghat said.
"If the current ideas about stellar evolution are right, then our calculations indicate that mergers of even 50-solar- mass black holes will be detected in a few years," he said.
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