There are a lot of objects and bodies that exist in this gargantuan universe of ours. Everything in this vast abode that we call the universe, whether big or small, is said to consist of matter. Your phone, your body, your hair, dust, air and everything you see around is matter. Each and every one of these objects consists of matter and their existence can generally be perceived rather easily.
But what if I told you that most of the matter that exists in the universe cannot be perceived? What if I also told you that more than 85% of the matter in the universe has never been observed? These facts are hard to believe and are rather astounding, but, they are, indeed, facts. There is a special kind of matter called Dark matter, which constitutes about 85% of all the mass of universe and has never been observed directly.
Indeed, talking about the energy composition the universe is composed of roughly 4.6% matter, 23% dark matter and 72% dark energy (this is energy composition not to be confused with the above-mentioned mass composition). It is thought that we can neither detect nor measure dark energy but we can clearly see its implications. Let us talk about Dark matter in this blog and keep Dark energy aside for another blog.
- What is Matter?
- What tells the presence of Dark Matter?
- Types of dark matter
- Why should we find dark matter?
- What could dark matter be made of?
- How could we detect dark matter?
- Why is dark matter still a mystery?
- An Infographic On Dark Matter.
What is Matter?
To understand about Dark Matter, you have to understand about Matter first. The matter is something that has mass and occupies space. Matter can exist in any form or state. There are seven states of matter and they are:
- Ionised Plasma
- Quark-Gluon Plasma
- Bose-Einstein Condensate
- Fermionic Condensate
Matter consists of atoms, or, to be precise, the matter is made up of protons, neutrons, and electrons. This matter is called “Ordinary Matter”. The sub-atomic particles are built with some fundamental particles. These particles can be put into two groups: fermions and bosons. Fermions are the building blocks of matter. They all obey the Pauli exclusion principle. Bosons are force-carriers. They carry the electromagnetic, strong, and weak forces between fermions.
Fermions can be put into two categories: quarks and leptons. Quarks make up, amongst other things, the protons and neutrons in the nucleus. Leptons include electrons and neutrinos. The difference between quarks and leptons is that quarks interact with the strong nuclear force, whereas leptons do not.
There are four bosons in the right-hand column of the standard model. The photon carries the electromagnetic force – photons are responsible for electromagnetic radiation, electric fields and magnetic fields. The gluon carries the strong nuclear force – they ‘glue’ quarks together to make up larger non-fundamental particles. The W+, W– and Z0 bosons carry the weak nuclear force. When one quark changes into another quark, it gives off one of these bosons, which in turn decays into fermions.
All the above particles make up the Standard Model of particles and dark matter doesn’t come in this standard modelI want to know what dark matter and dark energy are comprised of. They remain a mystery, a complete mystery. No one is any closer to solving the problem than when these two things were discovered. --Neil deGrasse Tyson Click To Tweet
What tells the presence of Dark Matter?
There are many observations which strongly suggests the presence of some strange non-luminous matter or the dark matter. Let us see some of them:
- The speed of bodies located farther from the galactic centre: From Kepler’s Second Law, it is expected that the rotation velocities will decrease with increase in the the distance from the centre of the galaxy, similar to the Solar System. This is not observed and the only obvious reason we could find is the presence of Dark matter.
- Mass velocity discrepancy: Stars in bound systems must obey the Virial theorem which together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. However, some velocity dispersion estimates of elliptical galaxies do not match the predicted velocity dispersion from the observed mass distribution. This discrepancy also tells that there is some extra invisible mass out there.
- Gravitational Lensing: Galaxies and other huge interstellar objects act as a lens and bends light. Actually, these massive things distort or bend the fabric of space-time and light passing through this distortion bends. So, the bending of light clearly depends on the mass of the galaxy. Researchers have made many such observations of light coming from quasars through some galaxy clusters. The bending of that light clearly tells that there is some extra mass out there.
- Cosmic Microwave Background: The Cosmic Microwave Background radiation or CMB for short is basically electromagnetic radiation which has been travelling for these 14 billion years since the big bang. This has the temperature data also. Scientists have collected a lot of data from this radiation and created a map. This map perfectly matches with the Dark matter model and clearly tells that the universe cannot exist without Dark Matter.
Like this, there are many other proofs but these four are the most prominent proofs for the existence of some unknown and invisible matter out there.
Types of dark matter
The classification of dark matter is based on its velocities. Free streaming length (FSL) is used to describe the distance objects would travel due to the random motions in the early universe. The size of a protogalaxy is used for determining the category of dark matter.
- Cold dark matter: Dark matter whose constituents have an FSL less than the size of a protogalaxy.
- Warm dark matter: Dark matter whose constituents have an FSL comparable to the size of a protogalaxy.
- Hot dark matter: Dark matter whose constituents have an FSL greater than the size of a protogalaxy.
Why should we find dark matter?
Dark matter constitutes 85% of the Universe’s Mass and it is present in really huge quantity and a lot of it might be present here on earth as well. If detected, we could probably use it for energy production and many other unbelievable applications might come up.
Other than applications, dark matter could unveil some of the dark secrets of the universe which are lying unanswered for centuries.
What could dark matter be made of?
There are several theories about what dark matter could be made of and some of them are:
- WIMPs(Weakly interacting massive particles): WIMPs are hypothetical particles that are thought to make the dark matter. These are totally new particles interacting through weak forces which are probably weaker than the weak nuclear force. These particles are not included in the above-mentioned standard model. Researchers are trying and developing a lot of experiments to detect such particles.
- Axions: Axion is another hypothetical elementary particle. It was actually postulated to solve the strong CP problem in quantum chromodynamics. Scientists believe that if they axions exist and have some specific properties then they can be a possible component of dark matter.
Like this, there are many proposed things and to understand all these hypothetical particles, we need a deeper understanding of physics. There are theories also saying that the current understanding of gravity itself is wrong and should be modified according to the observations but there are limitations to this also.
How could we detect dark matter?
We can locate the places in the universe where dark matter is present using techniques like Gravitational Lensing and we can even create the model of galaxies including dark matter. But we are not yet able to detect the particles which make this dark matter. So, how could we detect dark matter? Let us discuss the possible approaches. Basically, there are three approaches and they are:
- Make it here: Physicists have been bombarding particles in accelerators like LHC and there is a hope that someday we create dark matter particles and hopefully detect them.
- Direct Detection: Considering the amount of dark matter present in the universe, there is a possibility that dark matter is present here on earth as well and there is a possibility that some sensitive detector could detect it. So, scientists have been building extremely sensitive detectors to detect dark matter. One such detector is The Large Underground Xenon experiment (LUX) aimed to directly detect weakly interacting massive particle (WIMP) interactions with the ordinary matter on Earth
- Dark matter collisions: Scientists believe that collisions of dark matter could probably release something which we could detect. So, researchers are trying to use this approach as well.
The Pauli's Exclusion principle states that two electrons in same orbitals have:
The Pauli Exclusion Principle states that, in an atom or molecule, no two electrons can have the same four electronic quantum numbers. As an orbital can contain a maximum of only two electrons, the two electrons must have opposing spins.
Why is dark matter still a mystery?
While dark matter is the simplest explanation for the extra gravity and mass that exists, it is not necessarily the correct explanation. There are several theories that claim to explain this extra gravity and mass in the universe. Nobody really knows for sure if the existence of dark matter is a sufficient enough explanation for the existence of the extra mass. Dark matter does not give off light and as I have mentioned, does not interact with particles. Without any interactions, it is extremely hard to derive any conclusions on its nature and properties.
A recent paper in the physical review journals gave the maths claiming that dark matter might be created before the big bang itself which ads another mystery to the already existing mysteries around dark matter
Dark matter may be considered as the universe’s biggest mystery. It is known that something makes objects faster than they should but we do not actually know what it is and where it came from. The origins of dark matter might be even more peculiar than it is known.
Jonathon Swift, an Anglo-Irish poet once said, “Vision is the art of seeing what is invisible to the others”. Dark matter may be invisible, but it has served to solve a lot of mysteries in this astonishingly mysterious universe. Without the invisible phenomenon of dark matter, there would still be a lot of perplexity regarding the formation of galaxies and their movements. Despite all the information we possess about the universe, nobody can say with certainty that dark matter exists. Perhaps, that is where the magnificence of physics lies, in its mystery, and this mystery is what makes the search for the truth worthwhile.
An Infographic On Dark Matter