Black Holes

General relativity is a profoundly complex mathematical theory, but its description of black holes is amazingly simple. A stable black hole can be described by just three properties: its mass, its electric charge, and its rotation or spin. Since black holes aren’t likely to have much charge, it really takes just two properties. If you know a black hole’s mass and spin, you know all there is to know about the black hole.

This property is often summarized by the no-hair theorem. Specifically, the theorem asserts that once matter falls into a black hole, the only characteristic that remains is mass. You could make a black hole out of a Sun’s worth of hydrogen, chairs, or those old copies of National Geographic from Grandma’s attic, and there would be no difference. Mass is mass as far as general relativity is concerned. In every case the event horizon of a black hole is perfectly smooth, with no extra features. As Jacob Bekenstein said, black holes have no hair.

But with all its predictive power, general relativity has a problem with quantum theory. This is particularly true with black holes. If the no-hair theorem is correct, the information held within an object is destroyed when it crosses the event horizon. Quantum theory says that information can never be destroyed. So the valid theory of gravity is contradicted by the valid theory of the quanta. This leads to problems such as the firewall paradox, which can’t decide whether an event horizon should be hot or cold.

Several theories have been proposed to solve this contradiction, often involving extensions to relativity. But the difference between standard relativity and these modified theories can only be seen in extreme situations, making them difficult to study observationally. But a paper in Physical Review Letters shows how they might be studied through the spin of a black hole.

Many modified relativity theories have an extra parameter not seen in the standard theory. Known as a massless scalar field, it allows Einstein’s model to connect with quantum theory in a way that isn’t contradictory. In this new work, the team looked at how such a scalar field connects to the rotation of a black hole. They found that at low spins, a modified black hole is indistinguishable from the standard model, but at high rotations the scalar field allows a black hole to have extra features. In other words, in these alternative models, rapidly rotating black holes can have hair.

The hairy aspects of rotating black holes would only be seen near the event horizon itself, but they would also affect merging black holes. As the authors point out, future gravitational wave observatories should be able to use rapidly rotating black holes to determine whether an alternative to general relativity is valid.

Einstein’s theory of general relativity has passed every observational challenge so far, but it will likely break down in the most extreme environments of the universe. Studies such as this show how we might be able to discover the theory that comes next.

Reference: “Spin-Induced Black Hole Spontaneous Scalarization” by Alexandru Dima, Enrico Barausse, Nicola Franchini and Thomas P. Sotiriou, 1 December 2020, Physical Review Letters.

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New Delhi:

In a rare occurrence, a merger of three supermassive black holes from as many galaxies in our nearby universe was spotted by a team of Indian researchers, said the Ministry of Science and Technology. This indicates that small merging groups are ideal laboratories to detect multiple accreting supermassive black holes and increases the possibility of detecting such rare occurrences.

Supermassive black holes are difficult to detect because they do not emit any light. But they can reveal their presence by interacting with their surroundings.

The astrophysicists were studying a known interacting galaxy pair - NGC7733 and NGC7734 - when they detected unusual emissions from the centre of the latter and a large, bright clump along the northern arm of NGC7733, moving with a different velocity compared to the galaxy itself. Further investigation led to the discovery of a small, separate galaxy behind the arm. They named this galaxy NGC7733N.

This study, published as a letter in the journal Astronomy and Astrophysics, used data from the Ultra-Violet Imaging Telescope (UVIT) onboard the first Indian space observatory ASTROSAT, the European integral field optical telescope called MUSE mounted on the Very Large Telescope (VLT) in Chile and infrared images from the optical telescope (IRSF) in South Africa.

The UV and H-alpha images also supported the presence of the third galaxy by revealing star formation along with the tidal tails, which could have formed from the merger of NGC7733N with the larger galaxy. Each of the galaxies hosts an active supermassive black hole in their nucleus and hence form a very rare triple AGN system.

When the dust and gas from the surroundings fall onto a supermassive black hole, some of the mass is swallowed by the black hole, but some of it is converted into energy and emitted as electromagnetic radiation that makes the black hole appear very luminous. They are called active galactic nuclei (AGN) and release huge amounts of ionized particles and energy into the galaxy and its environment. Both of these ultimately contribute to the growth of the medium around the galaxy and ultimately the evolution of the galaxy itself.
According to the researchers from the Indian Institute of Astrophysics, a major factor impacting galaxy evolution is galaxy interactions, which happen when galaxies move close by each other and exert tremendous gravitational forces on each other. During such galaxy interactions, the respective supermassive black holes can get near each other. The dual black holes start consuming gas from their surroundings and become dual AGN.

Many AGN pairs have been detected in the past, but triple AGN are extremely rare, and only a handful has been detected before using X-ray observations. However, the team of researchers expects such triple AGN systems to be more common in small merging groups of galaxies. Although this study focuses only on one system, results suggest that small merging groups are ideal laboratories to detect multiple supermassive black holes.

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