By Rachna Arora
The study of astronomy is an overwhelming experience. Every time a new cosmological discovery is made, it exposes the reality of how little we know or understand the Universe. The fact that Earth is one small planet among billions, the sun is a medium-sized star among billions, that our solar system is a tiny fraction of our galaxy which again is one among billions, brings the realisation of how huge, magnificent, unexplored and sparsely understood Space is!
The universe is about 13.8 billion years old and we have not even begun to comprehend its composition despite the scientific and technological advancements. When we think of what our universe is made of, we automatically think of stars, planets and galaxies, spinning gracefully through space. But in the 1960 -70’s there were observations and discoveries that turned our understanding of the cosmos on its head. The matter that we see and experience in our everyday life, which is composed of atoms and thereby considered as ordinary, is in fact quite rare. Matter as we know it comprises less than five percent of the universe. The rest of the universe, 95 percent, is the dark universe, made of dark matter (about 25 percent) and dark energy (about 70 percent). This discovery is largely credited to astronomer Vera Ruben.
When light is incident on an object, a part of it is reflected and when reflected light enters our eye, we are able to see the object. However, when the light falls on dark matter, it simply goes through it. Light or any other electromagnetic radiation or matter does not interact with dark matter and hence it cannot be seen. (Probably, it should have been called invisible matter). At this very moment, dark matter particles are zipping right through us. What we know about dark matter is that it exists, and that’s about it. The hunt for dark matter has spanned decades. Although we cannot see, smell, taste or hear it, we can observe its gravity impacting the visible universe.
There is much evidence that supports the existence of dark matter. We know the planets closer to the sun rotate much faster than the outer planets. When we apply the same logic to the galaxies, the parts near its centre should rotate faster than the parts away from it. But measurements have conclusively proved that the outer parts are moving as fast as the inner parts of the galaxies. The logical explanation put forth for this observation is that the galaxies have huge amounts of dark matter, which is providing the additional gravitational pull to ensure that the visible matter throughout the galaxy rotates at the same rate.
The most massive bound objects in the universe are galaxy clusters, which comprise hundreds to thousands of galaxies. They too are surrounded with dark matter which is responsible for the massive velocity of gasses in the cluster, thus providing another proof of the mysterious matter.
Einstein taught us that everything, including light, is influenced by gravity. As the light from a distant galaxy travels through a galaxy cluster on its way to the earth, it bends, creating multiple images of the background galaxy, an effect christened as gravitational lensing by astronomers. Here, the dark matter in the galaxy cluster acts like a lens. If there was no dark matter, there would have been no observable gravitational lensing.
Computer simulations which create a digital universe too provide a clenching proof about the existence of the elusive dark matter. In simulations, we start with large quantities of dark matter that was born during the Big Bang, let it evolve through gravity that makes it stick to each other, transforming the perfectly homogenous space into clumps of dark matter. These eventually become the home for galaxies and also develop filamentary structures connecting different galaxies creating what is called a cosmic web. The simulations make predictions about how clusters of galaxies are to be found in the universe and the results fit beautifully with the observable data. Observation of the cosmic web and gravitational lensing is like possessing the DNA of dark matter, thus making its presence irrefutable.
Is dark matter some exotic particle yet to be discovered or simply a property of gravitational force, which we have not been able to comprehend? We don’t know. We do know what dark matter is not. Lost planets, dead stars, dark nebulae do not qualify as dark matter as they would absorb, emit or reflect light. It is also not made of subatomic particles like neutrinos, which are known to not interact with light although abundantly present, as they are as light as electrons and incapable of exerting substantial gravitational force. Maybe the answer lies somewhere in between and we need to marry the two proposed theories to solve this cosmic puzzle. Understanding dark matter is one of the biggest questions in science today and some of the greatest minds in astrophysics are trying to decipher it.
Physicists around the world have built sensitive detectors deep inside the earth and even in space to pick up some trace of dark matter. In India, the Jaduguda lab housed in an abandoned uranium mine in Jharkhand is dedicated towards unravelling the mysteries of dark matter. We are even trying to make dark matter in the lab, with CERN in Switzerland being the flagbearer of the research, by smashing the subatomic particles in the Large Hadron Collider. We have to keep looking and thinking harder to understand dark matter to unlock an entire new understanding of everything and everyone in our known universe.
Dark matter may have cloaked itself in a black shroud but it will not be able to hide for eternity from the indomitable human spirit and relentless curiosity.
(The writer is PGT- Physics at Shiv Nadar School, Noida)