Mint Explainer: Why black holes are on India’s radar

Isro on 1 January launched India's first X-ray polarimeter satellite to study black holes.
Isro on 1 January launched India's first X-ray polarimeter satellite to study black holes.


India’s newest space mission, Isro’s XPoSat, will help researchers understand our planet and the universe better. It will also anchor India more firmly on the world space map, with Isro planning at least 12 missions this year

If you watched Christopher Nolan’s 2014 movie Interstellar, you would be familiar with the word black hole, even if you do not fully understand what it means. Released a decade ago, Interstellar achieved a high level of scientific accuracy in its portrayal of a black hole called Gargantua, with the help of Nobel laureate Kip Thorne, despite taking some artistic liberties. 

While Interstellar popularised the idea of black holes, physicists and research labs continue to expand their research of such interstellar phenomena to further understand the origins of our universe.

In December 2021, the US-based National Aeronautics Space Agency launched the imaging X-ray polarimetry explorer mission to explore the remnants of supernova explosions, the particle streams emitted by black holes and other cosmic events. 

On 1 January, the Indian Space Research Organisation welcomed the New Year with the successful launch of its first X-ray polarimeter satellite (XPoSat) that would offer insights into celestial objects such as black holes and neutron stars. This, even as Isro races to launch 12 more missions this year–including Gaganyaan-1 and Mangalyaan-2. Mint explains the significance of XPoSat and why the mission could take us closer to understanding the origin of our planet.

What is XPoSat’s purpose?

XPoSat, expected to last for about 5 years, comprises two scientific payloads in a low earth orbit (close to the earth’s surface)–the primary payload, POLIX (polarimeter instrument in X-rays), to measure the polarimetry parameters (degree and angle of polarisation); and the XSPECT (X-ray spectroscopy and timing) payload to provide spectroscopic information. X-ray polarimetry is used to understand how intense gravitational forces draw matter toward black holes.

Why are black holes important?

When a giant star dies in a supernova, it can either be completely destroyed, become a black hole, or a neutron star. A neutron star is so dense that just a teaspoon of neutron star material would weigh 10 million tonnes, and research suggests that neutron star collisions are one of the universe’s main sources of heavy elements like gold and uranium.

Black holes, too, are for real. A black hole is anything but empty space, and is called so because even light cannot escape from it. Black holes were predicted by Einstein’s theory of general relativity, which showed that when a massive star dies it leaves behind a small, dense remnant core. 

His general relativity equations showed that if the core’s mass is more than about three times the mass of the sun, the force of gravity overwhelms all other forces and produces a black hole.

Simply put, observing and thinking about the properties of black holes have yielded enormous insights about the nature of the universe and the role they may play in the formation of galaxies.

Why is more research needed on this?

As Nasa puts it, you may think of a star 10 times more massive than the sun but squeezed into a sphere approximately the diameter of New York City. As the star collapses, the surface of the star nears an imaginary surface called the ‘event horizon’, where time appears to stand still, and the star can collapse no more–it is a frozen collapsing object. 

Black holes comprise the event horizon, and ‘singularity’–a point at its centre that is infinitely small and infinitely dense. The laws of general relativity have given physicists a good understanding of the ‘event horizon’ but they are yet to fully understand ‘singularity’.

How significant is India’s role in space research?

India continues to play a significant role researching black holes. Consider these examples. 

The country expects to set up a Laser Interferometer Gravitational-Wave Observatory (LIGO)-India, an advanced gravitational-wave observatory, by 2030. 

Gravitational waves, described as ‘ripples’ in space-time, are caused by some of the most violent and energetic processes in the universe, such as massive accelerating objects. The strongest gravitational waves are produced by cataclysmic events such as colliding black holes, supernovae (massive stars exploding at the end of their lifetimes), and colliding neutron stars. 

As an example, four years ago, researchers confirmed the detection of a collision between a black hole and a neutron star, which sent gravitational waves rippling across at least 900 million light-years to reach earth (one light year equals to the 9 trillion kilometres that light travels in a year at 300,000 km per second).

Further, Nasa’s premier X-ray observatory was named the Chandra X-ray Observatory in honour of the late Indian-American Nobel laureate Subrahmanyan Chandrasekhar, who calculated that all stars larger than 1.4 times the mass of our sun would eventually run out of fuel and collapse.

Professor C.V. Vishveshwara, who passed away in Bengaluru in early 2017, was one of the true pioneers of black hole physics. Among his other achievements, he provided mathematical proof of the stability of Schwarzschild black holes (the simplest of black holes with mass but no electric charge and no spin), suggesting that if black holes are indeed produced, they would also continue to exist.

And India’s first dedicated satellite, AstroSat, launched by Isro in September 2015, observed for the very first time rapid variability of high energy X-ray emissions from a black hole system. In August 2021, Indian researchers discovered three supermassive black holes merging together to form a triple active galactic nucleus. 

India’s new XPoSat mission, along with the multiple space missions planned for this year, will only strengthen these efforts, and help the country embellish its name in space too.

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