A long time ago, seven billion years ago to be precise, two blackholes—deemed physically impossible to exist—collided to produce a cataclysmic explosion. On May 21, 2019, astronomers detected four short wiggles of gravitational waves passing through the Earth. The source of these signals has been traced back to this seven-billion-year-old collision, which happened more than ten billion light-years away.
The discovery of this most massive source of gravitational waves to be detected so far were published in a peer-reviewed journal this week. The ripples coming from this insanely far away collision were picked by Laser Interferometer Gravitational-Wave Observatory (LIGO) based in the United States and its counterpart in Italy named Virgo. The discovery was published exactly five years after the first-ever gravitational wave was detected in September 2015.
The gravitational waves are the disturbances or ripples in the space-time continuum caused by some of the most violent and energetic processes in the Universe. The existence of gravitational waves was first predicted by Albert Einstein in 1916 through his general theory of relativity and was empirically proved a century later.
The short gravitational signal received by the LIGO in May 2019 was named GW190521 from a collision of two massive black holes. LIGO detected the ripples in the space-time continuum as four short wiggles, which lasted one-tenth of a second. It was detected from a source which is five gigaparsecs away, thus making it one of the most distant gravitational sources to be detected by LIGO.
The signals were generated from two binary black hole mergers, both of whom lie in the ‘impossible’ range of black holes in terms of mass. The research paper explains that the two highly spinning black holes—about 66 and 85 times more than the mass of the Sun—collided and merged. The collision released an enormous amount of energy that spread across the universe in the form of gravitational waves.
But this in itself has come as a surprise for the scientists. As per the statement from the Australian National University (ANU), whose scientists were involved in the discovery, the merger of the two black holes has led to the formation of a class black hole known as intermediate-mass black holes (IMBH) that has remained elusive for scientists so far.
All the black holes detected till date largely fall into two categories: stellar-mass black holes (less than 100 solar masses) and a supermassive black hole (more than 1,000 solar masses). The black holes which have masses more than 100 to between 100,000 solar masses are IMBH.
As per the research, these two black holes collided to form a black hole weighing 142 times the mass of the Sun. It is till date the biggest black hole detected through gravitational waves observation.
This detection has offered scientists a rare glimpse at one of the classes of black holes never detected before. That’s’ why astronomers call it an ‘impossible’ black hole. “We are very excited to have achieved the first direct observation of an IMBH in this mass range. We also saw how it formed, confirming that IMBHs can be produced through the merger of two smaller black holes,” said Professor Susan Scott from the ANU Research School of Physics, a co-author on the publication.
Supermassive black holes (SMBH) are those that are usually at the centre of the galaxies and were formed during the early years of the Universe. Stellar black holes, on the other hand, have a mass less than 100 times that of the Sun and are formed due to the collapse of stars.
Another ‘impossible’ aspect of the current discovery is the nature of the merging black holes. Astronomers hypothesize that the stars between 65-130 times the mass of the Sun go through the process known as pair-instability, which results in the star to be blown apart completely and thus leaving nothing behind, definitely not a black hole.
Now, the heavier (85) amongst the two merging black holes is said to be the first-ever detection which is within the pair of instability mass gap. So, the question bothering physicists is how does a black hole with a mass in right in-between this ‘instability’ range exist?
One popular theory is that the black hole is not a product of a collapsing star, but was a result of a similar merger that might have happened before. But, we are yet to find any such evidence. “The fact that we’re seeing a black hole in this mass gap will make a lot of astrophysicists scratch their heads and try to figure out how these black holes were made,” says Nelson Christensen, member of Virgo.
The study was published in the journal Astrophysical Journal Letter this week and can be accessed here.
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Black hole, Star, Virgo interferometer, Gravity, Gravitational wave, Intermediate-mass black hole, Collision
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