Black holes are among the most mysterious objects in the universe and have occupied the popular imagination for decades. The oft-repeated enthusiastic description — "even light cannot escape it!" — has gotten old, but still speaks to the ability of black holes to tickle the human mind.

As active as the human imagination is about these cosmic giants, black hole physics itself has been dynamic, and a new development promises to stretch our understanding.

Historically, scientists have known black holes to be of two types — "stellar mass" and "supermassive". The names give away one or more of the characteristics of each type.

Stellar-mass black holes are formed from the death of stars and have a mass anywhere from a few times the mass of the sun to a few tens of solar masses. (A solar mass is defined as the mass of the sun, which is about “2” followed by 30 zeroes in kilograms.)

Supermassive black holes, on the other hand, are, well, "supermassive". Astrophysicists think of that adjective in the context of black holes to mean upwards of 10 lakh solar masses.

Little is known about how supermassive black holes are formed, but they are known to be at the centres of galaxies and believed to have originated in the early universe.

Our home galaxy, the Milky Way, has one of these giants residing at the centre. Its mass is about 40 lakh times the mass of the sun.

Whichever end of the mass spectrum a black hole may be placed, comprehending that kind of mass is next to impossible because there aren't any meaningful references for it here on Earth. Nothing compares.

In this mass-based classification, though, there is a gap — a range of mass where no black hole has been known to exist, although it should in theory.

Essentially, one class of black hole ends with a few tens of solar masses, and the other begins roughly at 10 lakh solar masses.

What happens in between? Do they not make black holes in the intervening mass category?

Here is where the discovery of the gravitational wave on 21 May 2019, and announced on 2 September 2020, assumes significance.

A black hole of 142 solar mass has been detected finally, confirming the presence of medium-sized black holes. This hole becomes the first bonafide entrant into the third class of black holes, beyond the stellar and supermassive categories.

Two black holes — at least one (if not both) of whose mass was such that it shouldn’t even have existed in the first place, as per the standard model for how stars die — kept closing in as they went around each other, only to collide and form one uncomfortably high-mass black hole. The result was the most massive binary black hole merger detected as yet.

The crash is thought to have occurred some 17 billion light years away, when the universe was half its age. It speaks to the unimaginable magnitudes of space that “news” of that event reached us on Earth only now.

The signal caught in the LIGO-Virgo web of three gravitational-wave detectors — called “GW190521” after its date of detection, i.e., year '19, month 05, day 21 — was a signature of that event which led to the formation of a 142 solar mass black hole.

“I’m very excited about GW190521 because this is really the first time there is good evidence of a black hole of mass greater than 100 solar masses,” said Dr Somak Raychaudhury, director of the Inter-University Centre for Astronomy and Astrophysics, an institute leading the effort to set up LIGO India. “There have been claims before from X-ray binary detections, but the evidence hasn’t been strong.”

The constituent black holes were 66 and 85 solar masses. The heavier of the two black holes shouldn’t exist. Stars of helium core mass over 64 times that of the sun and under 135 solar masses eat themselves up and leave nothing behind. Yet, this one exists, or did exist, before it was turned into a more massive black hole.

Black holes in the mass category of 100-100,000 solar masses were not known to exist before, but they have had a name — "intermediate-mass black hole". Some have made educated guesses about their existence and some others have remained sceptical. Certainly such black holes were never directly detected before. That changed in May 2019.

Speaking to Swarajya in April this year, Dr Karan Jani, a black hole astrophysicist and part of the international team behind the latest discovery, had said: "There is this mass gap. You can have a black hole from a star that is a hundred times bigger (massive) than the sun, and the galactic centres have (black holes) a million times bigger than the sun. From a 100 (solar mass) to a million, it doesn't seem like a natural way that the universe makes black holes."

Does the universe have a preference for making certain kinds of black holes, Dr Jani wondered, because the mathematics of Einstein's theory of gravity did not distinguish between black holes on the basis of mass.

The GW190521 detection suggests that there are indeed black holes in the intermediate-mass category. And now that they are verified to exist, they pose challenges to our understanding of black hole formation. Thankfully, it is the kind of problem that scientists welcome.

The challenges come from the fact that conventional scientific explanations for the death or collapse of stars don’t account for the larger of the black holes that merged. This development now compels a relook at some of the earlier ideas, especially concerning nuclear reactions going on in stellar cores and how they determine black hole formation.

Perhaps, contrary to present understanding, stars in the mass range of 64-135 solar masses can explode in a supernova to produce a black hole.

There may be, additionally, intermediate steps in a black hole’s journey that takes it higher up the ranks of mass — more precisely, a series of black hole mergers — on its way to becoming a supermassive black hole. That could explain how these giants come about, and intermediate-mass black holes would be integral to that process.

Dr Raychaudhury says: “Supermassive black holes have mass millions to billions of times the mass of the Sun. How are these formed? If they are formed by merging the smaller ones over time, then there must be the intermediate-mass ones.

“With the discovery of intermediate-mass black holes, this shows that we can possibly form supermassive black holes from merging smaller ones.”

The implication is of a family tree of black holes, comprising everyone from the ancestors to the youngest living holes. One curious feature would be that the newest black hole would be the heftiest in the lineage.

The question arises: if black holes have to merge one after another successively to become more massive each time, shouldn’t these black holes be living in close proximity?

For that, astrophysicists propose, black holes would have to reside in closely packed environments that could facilitate mergers on repeat. These places could be discs of active galactic nuclei or dense star clusters.

Active galactic nuclei are powerful and luminous energy sources in the universe situated at the centres of many galaxies. Star clusters are large groups of stars in relative close proximity.

However, it may even be that hierarchical mergers of black holes till they grow supermassive are rare events. This is because the velocity imparted on a black hole on account of a merger kicks it out of its surrounding, probably reducing the chances of a subsequent collision.

These questions and possibilities promise to propel astrophysics in new directions.

The latest gravitational-wave detection has also allowed a closer look at the different stages of black hole formation and the testing of our understanding of gravity.

There are three stages known presently. At first, two black holes orbit around each other and spiral inwards — a stage called “inspiral”. Next is the self-explanatory “merger” stage, where two black holes come together. Last is the ringdown when, as explained in an official summary, “the remnant black hole “rings“ like a struck bell before it settles down into a stable, final state.”

Earlier detection of gravitational waves allowed the study of inspiral and merger stages more clearly because of the low masses of the black holes observed. But GW190521 has taken scientists into the late merger and ringdown stages, thanks to the presence of high-mass black holes. It turns out that the general theory of relativity passed the tests presented by GW190521.

All of this may only be the beginning for gravitational-wave observation of intermediate-mass black holes. As the twin LIGO facilities in the United States and the Virgo centre in Europe get better, our abilities to spot more anomalous black holes stand to improve.

In about four years, the LIGO India facility is expected to kick off too and join the international effort to look for gravitational-wave sources. However, many Indians are already involved in this work worldwide, something that excites Dr Raychaudhury, who is hopeful for the future. “With LIGO India, we will discover many more of these objects.”

Two international ground-based detectors, Einstein Telescope and Cosmic Explorer, are expected to be established. A space-based detector, Laser Interferometer Space Antenna, is scheduled to take off in the early 2030s. Working in unison, these centres would be able to launch a more extensive search for this new class of black hole, among many other cosmic gems.

Virgo collaboration

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