New Delhi:
Did you know that everything that we see around us, stars, planets, galaxies and gas make up a mere 4 per cent of the universe? This 4 per cent of the universe is considered “known matter” while 26 per cent is dark matter and the remaining 70 per cent is dark energy. Indian-origin astrophysicist Professor Priyamvada Natarajan believes it is an “embarrassment” that we know so little of the universe. But what do we know of the “unknown”?
In an exclusive conversation with NDTV’s Pallava Bagla, Professor Natarajan details the unknowns of dark energy, dark matter, and black holes.
Dark Energy And Dark Matter
“We know that the universe is expanding. In 1998, it was discovered that the universe is not just expanding, but that the expansion is accelerating. And we believe that dark energy is causing the universe’s accelerating expansion,” said Professor Natarajan.
Describing dark matter as “elusive, invisible entities”, Professor Natarajan said every galaxy in the universe has some dark matter in it. “Dark matter is smeared lightly everywhere in the universe. It’s a particle that was probably created in the very early universe. But we don’t know any other properties of the particle.”
Dark matter is likely made of particles that do not interact with light which is why scientists don’t get any signals.
“Light is a cosmic messenger. Everything we see and know about the universe comes from visible phenomena. The entities, dark matter, dark energy, and even black holes, don’t participate directly in interacting with light. And that’s why they’ve remained so elusive,” explained Professor Natarajan.
The astrophysicist hopes we can figure out dark matter and dark energy in her lifetime. India has an underground detector in Jharkhand called the Jaduguda Underground Science Laboratory (JUSL), and a gamma ray telescope at Hanle in Ladakh which are looking for signs.
Both of these sites are searching for dark matter based on the set of properties of particles scientists know. “These two experiments represent an opening up of additional signals that we think we can get from the dark matter if there is some self-interaction. If they do, we are likely to see these gamma rays.”
“Wouldn’t Want To Go Near Black Holes”
“Black holes are enigmatic objects in the universe. They are a place in the universe where the gravity is so intense that not even light can escape. Any matter that gets close to a black hole will get completely swallowed by the black hole. Even light doesn’t make its way out,” explained Professor Natarajan.
Explaining how “dangerous” black holes are, Professor Natarajan shared an example of what would happen if she went near one of the black holes. “Suppose I went in head first. The difference in the intensity of gravity between my head and my feet falling in would be so strong that I would be pulled out and made into a little spaghetti. I would lose my hair.”
It was believed that black holes are made from the end states of massive stars, probably 10 times the size of the sun. When these stars have lived their life, they explode and leave behind a little black hole, said the astrophysicist.
“We were starting to find black holes that were a billion times the mass of the sun, in place in the centre of galaxies, actively gobbling matter. We don’t see the black hole itself, but the matter that is being pulled in by the gravity of the black hole. At some level, we see the dying gasps of matter. And that glows in the X-rays typically. So that’s how you see black holes,” she added.
Around 20 years ago, Professor Natarajan worked on the theory of “direct collapse black holes”. She found that black holes could form directly from the collapse of huge gas clouds. These black holes will be 10,000 times or even 100,000 times the mass of the sun. Last year, she figured that the oldest black hole in the galaxy UHZ1 satisfies all properties of direct collapse black holes.
Telescope at Hanle in Ladakh can also detect black holes if they emit gamma rays, she said. “Black holes are poor eaters. They dribble out. The matter pushed out from a black hole has a lot of gamma rays, and this phenomenon can be detected by this telescope at Hanle. Ground and space data can give us a fuller picture of this phenomenology.”
Gravitational Waves
Gravitational waves are tremors in space-time that result when two black holes collide with each other and merge.
Scientists globally are working on the Laser Interferometer Gravitational-Wave Observatory (LIGO) project, which includes the LIGO detectors in the United States, the Virgo detector in Italy, and now, with recent approval, a new LIGO detector in India. They have been detecting gravitational waves from merging stars nearby.
“LIGO detectors can help us pinpoint where those two black holes merged, which is the origin point of the gravitational waves that are now detected.
For India to help unravel the “unknown” of the universe, Professor Natarajan suggests international collaborations. “It would be great if India was much more actively participating in these large surveys that are done now. So, the Dark Energy Survey, for example, and the Sloan Digital Sky Survey, and its various incarnations.”