Can Dark Matter be Composed, Even Partly, of Black Holes? : Daily Current Affairs

Relevance: GS-3: Science and Technology- developments and their applications and effects in everyday life.

Key phrases: universe, dark matter, black holes, halo, primordial black holes, LIGO, Gravitational Lensing, and extreme gravity.

Why in News?

Most ‘visible’ galaxies are like discs embedded in a dark matter halo that is much larger in size.

Context:

Astronomical observations suggest that a significant part of the universe is made up of dark matter which interacts with the rest of the universe only through the gravitational pull.
Many large lab experiments have tried to detect elementary particles that could be candidates for dark matter. However, such dark matter particles have not been detected until now.
So, the question arises – could dark matter be composed, at least partly, of compact objects such as black holes?
New research by an international team of scientists, presents a new way of addressing this question. Several astronomical observations suggest that all galaxies are embedded in a “halo” of dark matter. The “visible” galaxy is like a disc embedded in a dark matter halo that is much larger in size. “One hypothesis is that dark matter comprises a large number of compact objects such as primordial black holes,”
“While we have no conclusive evidence of spotting these objects, some of the binary black hole mergers detected by the LIGO gravitational wave detectors might be primordial black holes. The question is open,” says Prof. Ajith. From a theoretical standpoint, there is good reason to believe that primordial black holes did form in the young universe.

Primordial black holes

When the universe was very young, hot and dense – soon after the Big Bang, it must have had quantum fluctuations of its density. This, in turn, would have caused some regions to become extremely dense, and therefore, to collapse under their own gravity to form the primordial black holes.

Dark matter

Over 80% of all matter in the universe is made up of material scientists have never seen. It's called dark matter and we only assume it exists because without it, the behaviour of stars, planets and galaxies simply wouldn't make sense.
Dark matter is completely invisible. It emits no light or energy and thus cannot be detected by conventional sensors and detectors. The key to its elusive nature must lie in its composition, scientists think.
Visible matter, also called baryonic matter, consists of baryons — an overarching name for subatomic particles such as protons, neutrons and electrons. Scientists only speculate what dark matter is made of. It could be composed of baryons but it could also be non-baryonic, that means consisting of different types of particles.

How can they detect?

There are two basic ways we can detect whether there's a black hole. Black holes are, of course, dark by definition - not even light escapes them!
The first way we detect black holes is by their gravitational influence. For example, at the center of the Milky Way, we see an empty spot where all of the stars are circling around as if they were orbiting a really dense mass. That's where the black hole is.
The second way is by observing the matter falling into the black hole. As matter falls in, it settles in a disk around the black hole that can get very hot. Some of the energy liberated from falling in is turned into light, which we can then see, for example, in X-rays.
The paper explores what happens when such objects get in the way of gravitational waves travelling towards the Earth from the distance. They invoke a phenomenon called gravitational lensing that is used regularly in astronomy. When light travels through space and passes near a massive or compact body – a star, a galaxy or a black hole, for example, the intense gravity of that body may attract the light towards it, bending it from its rectilinear (straight line) path.

Gravitational Lensing

When taken to the extreme, gravity can create some intriguing visual effects that Hubble’s is well suited to observing. Einstein’s general theory of relativity describes how mass concentrations distort the space around them. A gravitational lens can occur when a huge amount of matter, like a cluster of galaxies, creates a gravitational field that distorts and magnifies the light from distant galaxies that are behind it but in the same line of sight. The effect is like looking through a giant magnifying glass. It allows researchers to study the details of early galaxies too far away to be seen with current technology and telescopes.
This phenomenon is known as gravitational lensing and was first observed by Arthur Eddington in 1919. Massive objects like galaxies can bend light significantly, producing multiple images, this is called strong lensing. Lighter objects like stars or black holes bend light less, and this is called microlensing. A similar lensing can happen to gravitational waves travelling towards the Earth, and this would leave signatures in the detected gravitational waves. This can be used to detect the presence, or the existence, of primordial black holes.

Assessing dark matter:

Until now, individual black holes have not marked out these signatures on gravitational waves detected by the LIGO-VIRGO detectors. However, if all of the dark matter is made of primordial black holes, they should have produced detectable signatures on the gravitational wave signals. The researchers use the non-observation of the lensing signatures to assess what fraction of the dark matter could be made of black holes.
This provides a new way of constraining the nature of dark matter. Our study concludes that black holes in the mass range from a hundred to a million solar masses can contribute only up to 50-80% of the dark matter in the universe. This is an upper limit and the actual fraction can be much smaller. “These upper limits will get better and better with more and more observations.”
Astronomical observations suggest that a significant part of the universe is made up of dark matter which interacts with the rest of the universe only through the gravitational pull.
Many large lab experiments have tried to detect elementary particles that could be candidates for dark matter.

Way Forward:

It’s only the presence of these massive dark matter halos, surrounding our galaxies, that allow the carbon-based life that took hold on Earth - or a planet like Earth, for that matter - to even be a possibility within our Universe.
As we’ve come to understand what makes up our Universe and how it came to be the way it is, we’re left with one inescapable conclusion: dark matter is absolutely necessary for the origin of life.
Without it, the chemistry that underlies it all - the heavy, complex elements, the ingredients necessary for biology in the first place, and the rocky planets that life takes hold on -could never have occurred at all. So that study of dark matter is important to know how the life evolve.

Source: The Hindu

Mains Question:

Q. “Despite its invisibility, dark matter has been critical to the evolution of our universe and to the emergence of stars, planets and even life.” Critically Analyse the Statement.