Astrophysicists spot biggest black hole collision so far, and a mystery begins to unfurl

The black hole collision was observed by the LIGO and Virgo collaborations, which involve nearly 1,000 scientists working around the globe.

Artist's impression of binary black holes about to collide | Mark Myers, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)

Bengaluru: Astrophysicists have observed the collision of the most massive black holes detected so far through the Advanced LIGO and Advanced Virgo observatories in the US and Italy, respectively. 

The merger, named GW190521, was observed on 21 May 2019, just after 8.30 am IST, and resulted in another black hole. GW stands for gravitational waves, the ripples in space-time caused by violent and energetic processes in the universe (such as black hole collisions), a phenomenon that is helping researchers study space like never before.  

Among the colliding black holes, one weighed 85 times the mass of the sun, while the other was 66 times the solar mass. The remnant left over from the merger is a black hole with a mass that’s approximately 142 times that of the sun. The merger occurred 17.2 billion light years away, and has offered scientists a glimpse at a class of black holes that has proved elusive so far. 

Never before have astrophysicists made a direct observation of an intermediate mass black hole, or one that weighs between 100 and 100,000 solar masses. 

The LIGO and Virgo collaborations involve nearly 1,000 scientists working around the globe. India has also begun working on the Indian observatory, LIGO-India or IndIGO (Indian Initiative in Gravitational-wave Observations), which will be located in Hingoli, Maharashtra. 

Owing to its location, LIGO-India is expected to play a crucial role in triangulating the source of gravitational waves in space, as well as obtain additional data. 

The gravitational wave research groups at IIT-Bombay and IIT-Gandhinagar played key roles in the findings, which took over a year to be analysed and confirmed. The results were published Wednesday in two papers in the journals The Physical Review Letters and the Astrophysical Journal Letters.

Signal decoding

Observed gravitational wave signals are converted and stored in the form of a sound wave called the chirp. 

The amount of sound produced by a black hole merger, or the time interval of a chirp or signal, is inversely proportional to the total mass of the two merging bodies, just as is the frequency of the signal recorded. 

GW190521 chirped only for 0.1 second in the LIGO-Virgo data, far shorter than that thrown up by other observed binary black hole mergers. It also peaked at the low frequency of 60 Hz. 

For comparison, the very first observed binary black hole merger in 2015 (GW150914) had a signal that was 0.2 seconds long and peaked at 150 Hz. The second definitive one, GW151226 binary black hole merger, was over a second long, peaking at 450 Hz. 

In the collision observed in May 2019, the two black holes were 85 times and 66 times the mass of the sun, making the larger black hole bigger than any black hole observed by gravitational waves, including black holes left over as remnants in merger events. 

The current remnant weighs in at 142 solar masses. The difference in the sum of the masses of the two colliding black holes and that of the remnant is about eight solar masses, which was expelled as energy during the merger and detected as gravitational waves. 

The chirps progress in three phases. 

The first phase carries information about the inspiral, where the two black holes dance around each other, drawing in closer. Then comes the merger, the signal emitted when the two bodies join together. And last is the ringdown, a final vibration emitted by the remnant before it settles into being a stable body in space. 

The ringdown is the loudest for heavier black holes, and the GW190521 signal provided an analysis of the ringdown phase that was unprecedented in its loudness and clarity.

Analysis of this signal further confirms theories put forth by Albert Einstein in his general theory of relativity.

Black hole formation clues

The merging of such massive black holes challenge human understanding of black hole formation. 

Black holes fall roughly into three categories. 

The most massive ones, called supermassive black holes (SMBH), can hold up to a billion times the mass of our sun. They are thought to have formed during the early days of the universe, enabling them to reach their monstrous masses.

The smallest black holes, meanwhile, formed from supernova explosions of massive stars. They have masses of less than 100 times that of the sun, and are called stellar mass black holes. 

In between these two are intermediate mass black holes, which measure from 100 to about 100,000 times solar masses. 

There had been no observations of black holes in this mass range so far, making their detection a quest in the world of astrophysics. Astronomers are also not sure how they form.

It is thought that black holes with 65 to 120 solar masses cannot be formed by a collapsing star. This makes the formation of the two black holes involved in the merger observed in May 2019 a mystery. 

The gravitational wave event expands our understanding of black hole formation, and offers the theory that binary black hole mergers can lead to the creation of more massive black holes, which can in turn merge with more black holes, creating even more massive ones. 

Researchers speculate that the 85 solar mass black hole could have formed from the merger of two smaller black holes.