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Explained: Two black holes merged billions of years ago, but it is puzzling scientists now

LIGO and Virgo detect signals from gravitational waves released in the merger. One of the two parent black holes was of an unusual “intermediate mass”, which challenges traditional scientific knowledge. What is generally understood, and what explains this unusual size?

Written by Kabir Firaque | New Delhi |
Updated: September 5, 2020 5:04:41 pm
black hole merger, gravitational wave observatory LIGO, discovery of black holes merger, indian express explainedArtist's concept illustrating a hierarchical scheme for merging black holes. (Credit: LIGO/Caltech/MIT/R Hurt [IPAC])

Billions of years ago, a collision between two black holes sent gravitational waves rippling through the universe. In 2019, signals from these waves were detected at the gravitational wave observatory LIGO (United States) and the detector Virgo (Italy).

What has excited scientists, however, is the mass of one of the parent black holes, which defies traditional knowledge of how black holes are formed.

The discovery and the analysis are described in two research papers. One, in Physical Review Letters, details the discovery and proposes possible ways in which the unusual merger may have taken place. The other paper, in The Astrophysical Journal Letters, discusses the signal’s physical properties.

What exactly was detected?

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It was a signal from a gravitational wave, a relatively new field of discovery. Gravitational waves are invisible ripples that form when a star explodes in a supernova; when two big stars orbit each other; and when two black holes merge. Travelling at the speed of light, gravitational waves squeeze and stretch anything in their path.

Gravitational waves were proposed by Albert Einstein in his General Theory of Relativity over a century ago. It was only in 2015, however, that the first gravitational wave was actually detected — by LIGO. Since then, there have been a number of subsequent detections of gravitational waves.

The signal detected at LIGO and Virgo, as described by the LIGO Collaboration, resembled “about four short wiggles” and lasted less than one-tenth of a second.

Where did it come from?

Subsequent analysis suggested that GW190521 had most likely been generated by a merger of two black holes. The signal likely represented the instant that the two merged. It was calculated to have come from roughly 17 billion light years away, and from a time when the universe was about half its age.

But these findings led to further questions. One of the two merging black holes falls in an “intermediate mass” range — a misfit that cannot be explained by traditional knowledge of how black holes form.

Why is it unusual?

All the black holes observed so far belong to either of two categories. One category ranges between a few solar masses (one solar mass is the mass of our Sun) and tens of solar masses. These are thought to form when massive stars die.

The other category is of supermassive black holes. These range from hundreds of thousands, to billions of times that of our sun.

According to traditional knowledge, stars that could give birth to black holes between 65 and 120 solar masses do not do so — stars in this range blow themselves apart when they die, without collapsing into a black hole.

But in the merger leading to the GW190521 signal, the larger black hole was of 85 solar masses —well within this unexpected range, known as the pair instability mass gap. It is the first “intermediate mass” black hole ever observed. (In fact, the smaller black hole too is borderline, at 66 solar masses.)

The two merged to create a new black hole of about 142 solar masses. Energy equivalent to eight solar masses was released in the form of gravitational waves, leading to the strongest ever wave detected by scientists so far.

How could the black hole of unusual mass have formed?

The researchers suggest that the 85-solar-mass black hole was not the product of a collapsing star, but was itself the result of a previous merger. Formed by a collision between two black holes, it is likely that the new black hole then merged with the 66-solar-mass black hole — leading to gravitational waves and the signal received by LIGO and Virgo.

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