Dark Matter – The Mystery Continues!

7 minutes read

“Dark Matter is everywhere. In this room. Everywhere.” – Fabiola Gianotti

Imagine a huge tall building with a lot of apartments in it. During the night, one can see lights coming out from those apartments. It’s true that the exact architecture of those apartments cannot be seen at night. But, one can be sure that they are being supported by a ‘huge structure’ – the building. It is the very intuitive understanding of gravity that tells us that if there were no structures out there, the light states should’ve fallen to the ground. Since this is not the case, so a ‘structure’ must be there. The ‘presence’ of this structure can be interpreted just by looking at the lights coming out from it.

In the same way, what we observe in the night sky is the ‘luminous’ matter that shines or reflects light. But there is much more ‘something’ out there. And, they are not the same as the luminous matter. Without them, we cannot explain completely – the ‘huge structure’ – our observable Universe. The presence of those can only be felt by the gravitational effects (there are other evidences too). These unknown objects – or ‘something’- that do not ‘shine’ light are known as the Dark Matter.

How the name the ‘Dark Matter’ came into first place?

The name ‘Dark Matter’ is kind of misleading. Some may think of them as ‘some type’ of matter that is dark, which is not true. One of the properties that this mysterious object has is that they do not interact with photons. Hence, they do not shine lights. But, that’s not everything about Dark Matter.

The idea of dark matter is not recent. At the end of the 19^th century, scientists began to understand and model the motion of stars. One scientist, in particular, Lord Kelvin – had the idea to model the stars in the galaxies as some kind of molecules in a box of gas. Though the scale was completely different, the idea was to apply the understanding of how molecules move in the gaseous box to the idea of how stars move in the galaxies. He came to the conclusion that there might be some ‘stuff’ out there that we couldn’t see. He said, “Many of our stars perhaps a great majority of them may be dark bodies.”

Another mathematician-scientist Henri Poincare – was inspired by Lord Kelvin’s idea. He did his own version of the analysis. Although he came to a different conclusion, he introduced an important phrase – matière obscure – which is ‘Dark Matter’ in French. Therefore, the idea of the name ‘Dark Matter’ is dated back to the late 1800s.

Does the Dark Matter really exist?

The person you see in the picture with a funny looking pose – saying “OK” is of Swiss-American astronomer Fritz Zwicky. In the 1930s, he began to study the motion of galaxies in the ‘Coma cluster’, which is a few hundred light-years away from our galaxy. He found that the motion of those galaxies was such that it couldn’t be explained solely due to the ‘ingredients’ that he had observed by virtue of their light. He concluded that there had to be additional ‘dark stuff’ out there. And, they are responsible for the gravity that was pushing and pulling those galaxies.

For a long time, many scientists didn’t take Zwicky’s idea seriously. It was Vera Rubin, whose study on the motion of spiral galaxies triggered most of the scientists to consider the idea of the existence of ‘dark stuff’. In 1968, she was studying the ‘Andromeda Galaxy‘ at Kitt Peak Observatory in the mountains of southern Arizona. Particularly, Rubin was examining Andromeda’s rotation curve or the speed at which the stars around the center rotate. She realized that the stars on the outer edges moved at the exact same rate as those at the interior. It seemed like the motion of the stars was violating Newton’s laws of motion.

Why Rubin’s study on ‘Andromeda’ was so important?

Suppose, we have a wheel and some water on its circumference. When someone spins it slowly, very little water droplets will fly off. If someone gives it a faster spin, most of the water droplets will fly rid-off, obeying Newton’s law. With this idea, there is nothing to stop us from thinking that the Andromeda galaxy which is spinning at a faster rate, the stars should be flying off. Surprisingly, Vera Rubin didn’t find anything like that. She concluded that there must be “something” – dark- out there that couldn’t be seen. And, they provide a large gravitational pull that holds those stars such that they were not flying off.

The mathematical expectation was, the further one goes away from the center of the galaxy the slower the stars should be moving. But, in fact, Rubin’s observation showed the speed of the stars at the edge of the galaxy was faster than what was expected. Moreover, they were not flying off. Therefore, to account for the additional gravitational pull which was holding those stars, the idea of the dark stuff was considered seriously.

Is there any more evidence?

One may think, probably we haven’t understood the force of gravity completely to explain the aforementioned observations. In some sense, this can be true. But, there are stronger pieces of evidence that indicate the presence of ‘additional-stuff’ in the sky.

Gravitational Lensing

According to General Relativity, the presence of any mass causes the space in its vicinity to curve. The curvature of space results in the bending of light rays around massive bodies. When astronomers observe distant galaxies, they often appear distorted. Such ‘distortion’ or ‘gravitational lensing effect‘ has been used to understand the existence of the ‘additional stuff’. The total mass contained in the observed region can be derived based on the gravitational lensing effect and is not in agreement with the evaluation based on the visible components. Such analysis suggests the existence of ‘extra mass’. And, they are making galaxies appear more distorted than they otherwise would be. For example, some of the best evidence that we have for the existence of dark stuff are observations of the ‘Bullet Cluster‘.

Cosmic Microwave Background (CMB)

According to the Big Bang theory, the universe was created around 13.8 billion years ago. After this ‘Big Bang’ event, it left behind ‘relic radiation’ that we can still sense today. They are also known as the ‘cosmic microwave background radiation’ (CMBR). Observations of the CMBR suggest that the universe is much denser than it appears. It also hints at ‘hotspots’ in the universe where the dark stuff may be concentrated. Moreover, it is the CMBR that strongly suggests that this dark stuff can be of ‘particle’ in nature.

It’s clear that all the evidence suggests only one thing – Dark Matter exists!

The Million Dollar question – What is it?

A simple answer to the above question doesn’t exist. In fact, we exactly don’t know what dark matter is, but we do know what it is not. We should understand that matter is not necessarily composed of atoms. It can be made of something entirely distinct.

Measurements of the CMBR help us to derive the value of the total mass-energy contained in the Universe. The ‘normal’ matter -which constitutes the stars, planets, rocks, and including us – contributes only 5% to the total mass-energy budget of the universe. The other contribution comes from entirely different Dark Energy and Dark Matter, where the dark matter contributes around 24% to the total density.

Dark matter cannot consist of baryons or normal matters. Scientists have shown that if baryons made up all the dark matter, the CMB would have looked radically different. If the dark matter is of ‘particle’ nature, it should be electromagnetically neutral. Otherwise, the electromagnetic interactions of the dark matter to the ordinary matter would have been visible to our eyes or sensitive to optical instruments.

There are theories, known as ‘MOND’, that propose a modification to Newton’s laws of motion that would apply only at very small accelerations. It explains the abnormal rotation speed of galaxies but couldn’t explain the dynamics of larger-scale galaxy clusters. It also struggles to describe the observed behavior of the Bullet Cluster. Hence, such theories are pretty much ruled out.

Galaxy rotation curves could be explained if we consider regular massive objects like brown dwarfs, burnt-out stars, and ancient black holes – the massive compact halo objects (MACHOs). But, again, they couldn’t explain the observed CMB and the Bullet Cluster properly.

So, what ‘could’ Dark Matter be?

There are currently two leading theories about dark matter. Either it may be some ‘exotic’ elementary particles or it is the plain old gravity that we haven’t understood properly. The first one is more popular and the majority of physicists believe in it.

Our knowledge of the sub-atomic world is always evolving. it’s entirely plausible that the effects that we call ‘dark matter’ could be caused by one or more undiscovered particles. There are theories that provide particle candidates for dark matter, which can explain all the observable evidence. Such types of particles are termed as the weakly interacting massive particles – WIMPs. One such theory that provides this type of candidate is ‘Supersymmetry‘. However, we haven’t found any such particles yet.

It’s true that experimentally we haven’t found any dark matter candidate yet, but the search for the same is still on. Numerous experiments such as XENON, IceCube, etc. are looking for dark matter signatures. As the experiments get more precise and telescopes get sharper, hopefully in the near future we’ll be able to unravel the ‘dark side’ of the universe.

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