Whispers in the Void: The First Hints of Dark Matter Imagine looking up at the night sky and wondering what that empty space is all about! We all have wondered why most of the night sky is filled with darkness and what actually is out there. Is that emptiness in the night sky actually something? Or space is just a black sky with few shiny dots known as stars?
Just imagine passing through a playground, you stop and see 10 children playing on the merry go round. The merry go round is moving way too fast so you start wondering why no one is falling off? Then you start to think that the reason might be that the weight of the children is responsible for it. But when you calculate the weight, you realise that it is way less than the speed of the merry go round. So, you start to wonder if there is something else also present for holding the kids in place and maintaining the weight. Just like that in the early 1930s, an independent mind of Fritz Zwicky emboldened him to challenge the general assumption that the mass of the universe consists mostly of stars. In 1933, when he was investigating the great coma cluster of galaxies, he realised that there was a difference between the theory and observation regarding the movement of galaxies. He discovered that the observed speeds of galaxies moving within the coma cluster were not just based on the total mass of the stars. To his surprise, the mass of the cluster based on the speed of its galaxies was about 10 times more than the mass of the cluster. He concluded that the coma cluster contained a large amount of unseen matter, with enough gravity to keep the rapidly moving galaxies from flying apart, Zwicky in effect discovered that most of the mass in the universe was invisible. He called it “dark matter”.
"In a spiral galaxy, the ratio of dark-to-light matter is about a factor of ten. That's probably a good number for the ratio of our ignorance-to-knowledge. We're out of kindergarten, but only in about third grade."—Vera Rubin
Now imagine you are on a merry go round with some of your friends. A few of you are standing at the centre while others are on the edge, now it starts to move really fast. The kids that were standing on the edge will feel like they are pulled off, they would have to hold on to something or they might get pulled off. But for the ones in the middle they don't feel the pull as much, making it easier for them to stay on. Just like that the merry go round was the galaxy and the children were the stars. So according to this the far off stars should experience a pull towards the centre. That's what Vera Rubin ,along with Ford, thought when they began to study the orbital speeds of the spiral galaxies but to their surprise they immediately discovered something entirely unexpected. The stars far from the centres of galaxies, in the sparsely populated outer regions, were moving just as fast as those closer in. This was odd, because the visible mass of a galaxy does not have enough gravity to hold such rapidly moving stars in orbit. Her calculations showed that galaxies must contain about ten times as much “dark” mass as can be accounted for by the visible stars. In short, at least ninety percent of the mass in galaxies, and therefore in the observable universe, is invisible and unidentified. Then Rubin remembered something she learned as a graduate student about earlier evidence for unseen mass in the universe. In 1933, Fritz Zwicky had analyzed the Doppler velocities of whole galaxies within the Coma cluster. He found that the individual galaxies within the cluster are moving so fast that they would escape if the cluster were held together only by the gravity of its visible mass. Since the cluster shows no signs of flying apart, it must contain a preponderance of “dark matter”,about ten times more than the visible matter, to bind it together. Zwicky’s conclusion was correct, but his colleagues had been sceptical. Rubin realized that she had discovered compelling evidence for Zwicky’s dark matter. Most of the mass of the universe is indeed hidden from our view. Many astronomers were initially reluctant to accept this conclusion. But the observations were so unambiguous and the interpretation so straightforward that they soon realized Rubin had to be right. The luminous stars are only the visible tracers of a much larger mass that makes up a galaxy. The stars occupy only the inner regions of an enormous spherical “halo” of unseen dark matter that comprises most of a galaxy’s mass. Perhaps there are even major accumulations of dark matter in the vast spaces between galaxies, without any visible stars to trace their presence. But if so, they would be very difficult to observe.
Dark matter is like the cosmic whisperer, influencing matter across time and space with a voice that we cannot hear. Dark matter is like a secret helper working behind the scenes, by helping to hold the stars and planets together. We can't see it or touch it but we know that it is always there holding everything in place. It whispers to the galaxies telling it how to spin and where to go. Even if we can't hear it we know that it is real as it leaves clues everywhere it goes.
Written by:
Ditya Luthra
Batch 14