What’s the matter with Dark Matter?
For almost a decade now, we’ve been on a search for something that we can’t see. Today we’ll glance at what dark matter exactly is and why it has been startling astrophysicists for well over 90 years.
What is Dark Matter?
Dark matter is a form of material that neither emits, reflects nor absorbs any spectrum of Electromagnetic radiation. Dark matter is entirely invisible, which makes it altogether harder to detect.
What could Dark Matter be?
Truly speaking, we don’t know! It’s a mystery that leaves our scientists perplexed. But we have our assumptions, we have hypothesised what Dark Matter could be. They are followed by why we think they cannot be Dark Matter.
1. Could Dark Matter be dead stars, rogue planets, or nebulae?
Before we name a few more, one mustn’t fail to notice that all these objects absorb, emit, or reflect some type of Electromagnetic Radiation.
2. Could it be Neutrinos then?
Neutrinos are sub-atomic particles that interact with Gravitational and weak interaction. Neutrinos for a long time were undetected because they are electrically neutral, and their rest mass is extremely small. For this reason, they were termed as a “Ghost Particle”. This had motivated scientists to think of why Dark matter could possibly be another flavour of Neutrinos.
3. Maybe a new elementary particle?
When Physicists can’t explain something, they tend to theorise a new particle. Jokes apart few physicists think that Dark Matter is another elementary particle waiting to be discovered. Particle Physicists already have an idea of another subatomic particle that solves charge-parity problems in Quantum Chromodynamics, called axions. They have never been detected but have the same property as Dark Matter does. They don’t interact with light very well; they have a very small mass and above all, they don’t interact with normal matter at all. These properties make axions very similar to Dark Matter, but are they the same? Well, that remains to be discovered.
Difference between Normal Matter and Dark Matter:
Normal Matter as we know it interacts with all the fundamental forces, it also interacts with light, by either absorbing, reflecting, or emitting light.
Unlike Normal Matter, Dark Matter does not interact with light or any Electromagnetic Wave for that matter. Dark Matter also does not have any charges.
Evidence for Dark Matter:
We’ve talked so much about Dark Matter, but do we even know that Dark Matter exists?
In this part of the blog, we shall go through the various and abundant amount of evidence provided by Physicists to confirm the fact that Dark Matter does indeed exist.
1. Rotation of Galaxies:
In the 1960s-70s, Vera Rubin, an American Astronomer was researching spiraling Galaxies. She was interested in galaxies that rotate because there was something odd about them.
Owing to Johannes Kepler and Isaac Newton, we know that the further we move away from a gravitationally dense object that bends the fabric of Space-time, the slower we revolve around it. Keeping this in mind Rubin started studying Rotating galaxies and found out that the objects at the edge of the galaxy were moving at almost the same speeds as an object near the centre of the galaxy. This is surprising because there is not a lot of “visible matter” at the edge of the galaxy to produce such large velocities. The anticipation of velocity constantly decreasing was broken by the fact of velocity remaining the same across the galaxy.
Here is a graph depicting this unusual phenomenon:
So, Rubin concluded that there must be “invisible matter” surrounding the galaxy like a halo. She also concluded that this “invisible matter” must at least be 5–6 times more than visible matter to cause this effect. In the 1930s, Fritz Zwicky, also an astronomer came down to the same conclusion with a lot of error in his observation and called the matter we cannot see as “Dark Matter”. And since then, the name stayed on.
2. Gravitational Lensing:
Having learnt from the General Theory of Relativity, we know that the space-time curvature can even bend light itself. So, light from the other galaxies gets bent around due to high gravitational force causing high space-time curvature. This lensing makes light from distant galaxies appear in the form of a ring or an arc.
The image below beautifully describes this spectacle:
So how does dark matter play its part here, it’s a natural process? Well, it turns out there isn’t enough mass to cause this phenomenon in the image shown above. And turns out we exactly need 5–6 times more of this matter than normal matter to cause this effect. So, we find out the total mass share between matter, dark matter, and dark energy. Turns out dark matter takes up about 24% of mass density in the total density state.
3. Cosmic Web:
When sometimes conducting an experiment is tedious and not possible, we turn to simulations. In this part of the blog, we come across a simulation manifested on a supercomputer. We conduct a simulation where we only place dark matter with some gravity in the universe. As time moves on, we observe that dark matter starts clumping, forming a very intricate and beautiful network.
When we render a picture of all the galaxies now, we deduce that these galaxies also settle in a similar intricately produced network. This shows that dark matter is the backbone structure of galaxy clusters.
These pieces of evidence are enough to provide satisfactory proof of the existence of Dark Matter.
Possible ways of detection of dark matter:
So, now that we know that dark matter exists, let’s find out how we can detect it.
Below is an image of what Physicists call a Feynman Diagram. This Feynman Diagram depicts the possible ways of interaction of Dark Matter with Normal Matter. Here DM represents Dark Matter and NM represents Normal Matter.
There are 4 ways of how we can detect or produce Dark Matter:
1. Laboratory Direct Detection:
As we move from left to right in the Feynman Diagram we realise, how we can Detect Dark matter directly.
When dark matter passes through an atom, it ionises the atom and releases an electron. This indicates that something has entered the detector. Physicists make sure that all other interferences are counted for and this experiment is conducted under low temperatures to make sure that there is no thermal excitation. Physicists also make sure that these detectors are shielded from Cosmic Rays.
An example of this experiment is the SuperCDMS, which stands for Super Cryogenic Dark Matter search.
2. Indirect Detection:
Physicists believe that dark matter near the centre of the galaxy, collide with each other to form Normal matter. However, there could be other astrophysical objects that could cause this. This phenomenon could be caused by pulsars, anti-matter, gamma Rays or even Neutrinos. So how do you detect that these particles were formed from Dark Matter and not Pulsars or objects? We have different types of detectors that can detect such particles.
We have detectors in space, on the ISS that can detect anti-matter and Gamma rays. We also have detectors here on the surface of Earth that detect protons and Gamma rays. Lastly, we have detectors underwater or in ice that can detect Neutrinos.
Few examples of experiments on Indirect detection of Dark Matter include-
Fermi/Glast satellite that searches for Gamma Rays, HESS Cherenkov telescope that searches for atmospheric showers and AMS that looks for Anti Matter on the ISS.
3. Making Dark Matter on Earth:
You could also make Dark Matter here on Earth. You could either collide protons and anti-protons to form Normal Matter and Dark Matter, or you could shoot a beam of protons onto a fixed target which would also yield Normal Matter and Dark Matter.
So how do you detect this Dark Matter with the collider? Well, we directly don’t. Since Dark Matter carries some momentum, we will have some momentum missing from the end products, due to conservation of momentum. This is when we realise that we have detected Dark Matter.
We conduct these types of experiments at the LHC (Large Hadron Collider) at CERN.
What will we learn through Dark Matter?
In the field of Particle Physics, we will find out more properties of Dark Matter, like their stability, lifespan, etc. By the detection of Dark Matter, we would get a more complete Particle Theory.
In the field of Cosmology, we will deduce the meaning of Dark Matter and its necessity. We will also find out more about the early universe.
In the field of Astronomy, we will finally understand the role of Dark Matter and why it is distributed as such.
But we would’ve finally determined the existence of Dark Matter, a question that had been unanswered for 90 years. Physicists will not stop until they prove the existence of Dark Matter or prove that Dark Matter does not exist. But if not Dark Matter, what is it? I personally think that there is some light of hope in these very Dark Matters.
