### Nobel Prize for Physics brings to limelight efforts of scientists in deep space exploration

The Nobel Prize in Physics for 2020 was shared. While one half of it was awarded to English mathematical physicist Sir Roger Penrose “for the discovery that black hole formation is a robust prediction of the general theory of relativity”, the other half was jointly divided between German astrophysicist Dr. Reinhard Genzel and American astronomer Dr. Andrea Ghez “for the discovery of a supermassive compact object at the centre of our galaxy.” Interestingly the exhaustive and ongoing work on the black hole has strong Indian connections. The foundational work on this exotic phenomena was done by Indian scientists A.K. Raychaudhuri and C.V. Vishveshwara. In fact, it is argued that without the legacy left behind by Raychaudhuri and Vishveshwara it would be difficult that the scientific and research community could have come thus far on understanding black holes. One of the best and biggest legacy of science is that researchers build on the work of their predecessors as an ongoing process.

**The concept of a black hole**

A black hole is a place in space where gravity pulls so much that even light cannot get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying. Because no light can get out, people can’t see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars. Even light that passes inside the black hole’s spherical surface of no return, called the event horizon, gets sucked in, which makes black holes invisible. Interestingly, Nobel Prize-winning Indian-American astrophysicist S. Chandrasekhar, the nephew of the legendary Nobel Laureate C.V. Raman, who predicted at what mass a star could or could not collapse into a black hole. This is called the ‘Chandrasekhar Limit’ in astrophysics. The limit explains that when a star’s mass is lighter than 1.4 times that of the sun, it eventually collapses into a denser stage called a “white dwarf”. When heavier than 1.4, a white dwarf can continue to collapse and condense, evolving into a black hole or a supernova explosion.

**The formation of a black hole**

Some black holes form when a massive star collapses into itself. But many mysteries still remain around these strange objects. A black hole can be formed by the death of a massive star. When such a star has exhausted the internal thermonuclear fuels in its core at the end of its life, the core becomes unstable and gravitationally collapses inward upon itself and the star’s outer layers are blown away. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space. A black hole takes up zero space, but does have mass – originally, most of the mass that used to be a star. And black hole gets “bigger” (technically, more massive) as they consume matter near them. The bigger they are, the larger a zone of “no return” they have, where anything entering their territory is irrevocably lost to the black hole. This point of no return is called the event horizon. Eventually, by growing and consuming material – planets, stars, errant spaceships, other black holes – astronomers think they evolve into the supermassive black holes that they detect at the centres of most major galaxies.

**Detecting a black hole**

**Detecting a black hole**

A black hole cannot be seen because strong gravity pulls all of the light into the middle of the black hole. But scientists can see how the strong gravity affects the stars and the gas around the black hole. Scientists can study stars to find out if they are flying around, or orbiting, a black hole. When a black hole and a star are close together, high-energy light is made. This kind of light cannot be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light. Prof. Tapas Kumar Das, a senior theoretical physicist based at Harish Chandra Research Institute, Allahabad, works on black hole shadow imaging. A telescope network consisting of eight radio telescopes forming the Event Horizon Telescope (EHT) helped astronomers to image the black hole at such a large distance. It acted as an earth-sized virtual observatory to study the black hole. The EHT network telescopes in Hawaii, Mexico, Spain, Chile and Antarctica use the earth’s rotation to serve as a single giant telescope. It can resolve objects into precision of 15 to 20 micro arcseconds, almost like trying to spot a typical golf ball on the moon. These telescopes operated at range of frequencies first generated by Indian physicist Jagdish Chandra Bose in Bengal about a 100 years ago.

**Theories of Noble prize winners**

**Dying stars form black holes**

Sir Roger Penrose won half the prize for his seminal work which proved, using a series of mathematical arguments, that under very general conditions, collapsing matter would trigger the formation of a black hole. This rigorous result opened up the possibility that the astrophysical process of gravitational collapse, which occurs when a star runs out of its nuclear fuel, would lead to the formation of black holes in nature. He was also able to show that at the heart of a black hole must lie a physical singularity – an object with infinite density, where the laws of physics simply break down. At the singularity, our very conceptions of space, time and matter fall apart and resolving this issue is perhaps the biggest open problem in theoretical physics today.

Penrose invented new mathematical concepts and techniques while developing this proof. Those equations that Penrose derived in 1965 have been used by physicists studying black holes ever since. In fact, just a few years later, Stephen Hawking, alongside Penrose, used the same mathematical tools to prove that the Big Bang cosmological model – our current best model for how the entire universe came into existence – had a singularity at the very initial moment. These are results from the celebrated Penrose-Hawking Singularity Theorem. The fact that mathematics demonstrated that astrophysical black holes may exactly exist in nature is exactly what has energized the quest to search for them using astronomical techniques. Indeed, since Penrose’s work in the 1960s, numerous black holes have been identified.

**Black holes play yo-yo with stars**

**Black holes play yo-yo with stars**

Reinhard Genzel and Andrea Ghez, who each lead a team that discovered the presence of a supermassive black hole, at the centre of our Milky Way galaxy. Genzhel and Ghez used the world’s largest telescopes (Keck Observatory and the Very Large Telescope) and studied the movement of stars in a region called Sagittarius A* at the centre of our galaxy. They both independently discovered that an extremely massive – four million times more massive than our sun – invisible object is pulling on these stars, making them move in very unusual ways. This is considered the most convincing evidence of a black hole at the centre of our galaxy.

**Indian Pioneers**

### Amal Kumar Raychaudhuri

Dr. Amal Kumar Raychaudhuri was born in a Baidya family coming from Barisal, (now in Bangladesh), on 14 September 1923. He was just a child when the family migrated to Kolkata. He finished his early education in Tirthapati Institution and later completed matriculation from Hindu School, Kolkata. Raychaudhuri established the ‘Raychaudhuri Equation’ in 1955 to describe gravitational focusing properties in cosmology. This equation is extensively used in general relativity, quantum field theory, string theory and the theory of relativistic membranes. This paper investigates the issue of the final fate of a gravitationally collapsing massive star and the associated cosmic censorship problems and space-time singularities therein with the help of Raychaudhuri Equation. It is a conjecture that the universe is emerged from a big bang singularity where all the known laws of physics break down. On the other hand, when the star is heavier than a few solar masses, it could undergo an endless gravitational collapse without achieving any equilibrium state. This happens when the star has exhausted its internal nuclear fuel which provides the outwards pressure against the inwards pulling gravitational forces. Then for a wide range of initial data, a space-time singularity must develop.

It is also a conjecture that such a singularity of gravitational collapse from a regular initial surface must always be hidden behind the event horizon of gravity; this is called the cosmic censorship hypothesis. Thus cosmic censorship implies that the final outcome of gravitational collapse of a massive star must necessarily be a black hole which covers the resulting space-time singularity. So, causal message from the singularity cannot reach the external observer at infinity. Raychaudhuri equation plays a pioneer role in cosmology to describe the gravitational focusing and space-time singularities. The Penrose–Hawking singularity theorems try to explain the conditions under which gravitation produces ‘singularities’. The Raychaudhuri Equation is a fundamental lemma for this series of theories on black holes. Penrose, in collaboration with cosmologist Stephen Hawking, used the Raychauduri equation published in the journal Physical Review in 1955, for a mathematical description of black holes in 1969.

**C. V. Vishveshwara**

Prof. Vishveshwara, the founding director of the Jawaharlal Nehru Planetarium in Bengaluru, where he established the now famous REAP (Research Education Advancement Programme in Physical Sciences) for undergraduate science students, made his mark in the study of black holes in the 1970s as a graduate student at the University of Maryland. He obtained BSc (Hons) in 1958 and MSc in 1959 from Mysore University. He received his A.M. degree from Columbia University, New York in 1964. He completed his Ph D from University of Maryland, USA in 1968. He was among the first to study ‘black holes’ even before they had been so named.

His calculations succeeded in giving a graphical form to the signal that would be emitted by two merging black holes – this was the waveform detected in 2015 by the LIGO collaboration, and contain the so-called ‘quasi normal modes’ – a ringdown stage that sounds like the ringing sound of a bell that is fading out. He was one of the first to analyse the structure of black holes employing space-time symmetries thereby demonstrating the existence of the ergosphere. He proved the stability of the non-rotating Schwarzschild black hole, a crucial factor that ensures its continued existence after formation. Further, he discovered the quasinormal modes of black holes. These modes of black hole vibrations would be one of the main targets of observation when the gravitational wave detectors, being set up all over the world, become functional. In recent years, he was investigating black holes in cosmological backgrounds, an important aspect of black hole physics that has hardly been explored. Vishveshwara has also made significant contributions to other areas of general relativity. In fact, years later after his work as a graduate student, on February 11, 2016, Laser Interferometer Gravitational-wave Observatory invited him to the synchronised announcement of the first discovery of gravitational waves at The Inter-University Centre for Astronomy and Astrophysics (IUCAA).

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**Compiled by: Preethi Jayaraman**

**Edited by: Trinity Mirror Online Team**