In universe minimum 80% things are dark matter. Millions and millions of stars all the things which are visible to us are only 15% to 20% of the matter of this universe. if we cannot see this dark matter, how did we come to know this dark matter exists?
Dark Matter and Dark Energy
As an astronomer, I would like to be excused because they name some they do not think much. The word dark is present in both dark matter and dark energy and this creates confusion because many people think this is the same thing They are different, and today we will only talk about dark matter. Dark Energy is also important, but we will talk about it some other time.
What is Dark Matter
If you pick up a heavy package and you look inside it, you might not see some things inside, but it is quite heavy. This is an example of dark matter the weight of the bag tells us that something is present in the bag But when you look inside the bag, you see nothing۔ This is how astronomers came to know about the dark matter۔ The story of this dark matter started in 1930۔
Idea of Dark Matter
The story of the dark matter started in 1930. One swiss astronomer Fritz Zwicky was looking at the Galaxies. Note that the astronomers had come to know about the galaxies in the 1920’s and they realized that the smudge on the sky visible to the eye with a telescope are actually galaxies. This discovery was made in the 1920’s.
Now in 1933, Fritz Zwicky a prominent, and colorful astronomer observed that when you see the galaxies in clusters And you see their speed you realize that these galaxies should not be in those clusters meaning that the galaxies are movie so fast that they should fly away from the clusters because we do not see that much gravity in their masses which can hold them.
One idea was that we are seeing in cluster of galaxies only when these galaxies are passing through the cluster but he noted that there are many galaxy clusters in our universe so he said this was not possible. So, the question is why are galaxies a part of the cluster. So Fritz Zwicky said in 1933 that possibly there is some dark matter not visible to us because of which these galaxies are a part of these clusters.
What Fritz Zwicky said in the 30s was not taken seriously by anybody but in the 1970’s this matter came to the forefront again And discussion begun about galaxies, individual galaxies.
How Dark Matter exist in the Galaxy?
One Astronomers Vera Rubin started searching that probably dark matter is present in the galaxies and like Fritz Zwicky Vera Rubin was also not taken seriously in the beginning. After sometimes she was taken seriously and now she is called pioneer of the dark matter discovery. The question is, how did we come to know what the dark matter exist in the galaxies?
To talk about this, we have to see how gravity works in our solar system. We have our solar system in which we have the sun. The maximum mass of solar system is in the sun. Even Jupiter has 1000 times less mass from the Sun. Leave earth because it has 330,000 times less mass than the sun.
Newton and Kapler theories about orbit tell us that If you start moving away from this sun the speed with which you orbit the Sun decreases and truly when we see our solar system the nearest planet Mercury is orbiting at 48 kilometers per second After that, you see that Venus is orbiting 35 km/s our earth about 29 km/s less than 24 km/s and Jupiter 13 km/s.
And this decrease in speed is predicted according to Newton and Kepler laws. This is predicted that because maximum mass is in the Sun your speed decreases according to the distance you move away from the maximum mass.
This is what we expect in our galaxy. There are many stars inside our Milky Way galaxy and if you keep moving away from the maximum mass the speed of stars at Which they are orbiting around the center of the galaxy should decrease. Our own sun is about 30,000 ly away from the center of the galaxy and is orbiting at 220 km/s in its orbit in the galaxy. It has completed 20 orbits in our galaxy.
There are some gas clouds on the outskirts about one lakh light-years away from the 100,000 light years away from the center of our Milky Way And when we see their speed, it is 200 km/s. This is a puzzling phenomena, because according to Newton when you keep going away from the mass your speed should decrease but in the Milky Way, you see some stars and gas clouds in spite of being very far from the optical disk are moving in our galaxy at a very fast pace.
One assumption of this phenomena if your physics is correct is that maybe because of some other matter (not visible to us) the stars are orbiting so fast and in reality, astronomers can now predict the percentage of the matter in the Milky Way which cannot be seen by us. According to this theory the visible disc which consists of stars that can be seen (there are 200 billion stars in our Milky Way) Is only 10% of the mass of our galaxy. The rest of the mass is in a Halo which astronomers call Dark Matter Halo and the speed at which the stars orbit in the galaxy tell this.
Evidence of Dark Matter
You seeing Image in is a curve in which the Y-axis the speed and the X-axis is the distance from the center of the Milky Way and this curve is called rotation curve by astronomers. This rotation curve big becomes flat. The speed of stars does not diminish at the outskirts of the galaxy the visible light disappears but speed does not decrease. This is a key evidence of the dark matter.
If you see at the Andromeda Galaxy you can see the same thing happening in its rotation curve. If you see the stars and gas clouds at a distance of a 100,000 light years away from the center of the galaxy. The Rotation Curve at that point is flat that is dark matter is also present there. Up till now we have seen all the galaxies and observed the pressure of dark matter in almost all the galaxies and 80% to 90% of the mass in the galaxy is in the dark matter Halo.
How Dark Matter exist in the Galaxy Clusters?
This was pointed out by Fritz Zwicky in the 1930s that the galaxies in the clusters are moving very very fast. If they are gravitationally in the clusters it means there is some matter present there which is not visible to us. We have more precise observations of the galaxy motion and think that 80% to 90% of the matter in the clusters also in dark matter.
And not only this speed of galaxies but some x-ray gases have been seen in the Galaxy. These are gases (x-rays are a part) but when we see how much x-ray gases are present and if dark matter did not exist, these x-rays gases, which is an intergalactic medium should have dispersed by now. We also tell us consistently the dark matter is present in the clusters of the galaxies not only in the clusters of galaxies but astronomers now think that in the large scale structure of our universe visible matter follows the dark matter. Visible matter is 10% to 15% or 15% to 20%.
What is this Dark Matter
The question is what is this dark matter? We don’t know. This is the biggest mystery of astronomy and science. We only know that Dark matter is not visible.
One possibility is that these are particles or Comets or Asteroids in huge numbers around our galaxy but we cannot see them as they are very faint. But this is not possible because if you have asteroids and comets feeling up the Halo of our galaxy they can still be seen in infrared light.
Another possibility is that they are large number of black holes but even this is not possible because that distribution of the dark matter in this Halo is very very smooth.
We know that this dark matter does not interact with light neither does it emit light nor does it absorb. It only has an impact with gravity. Astronomers in theoretical prediction have called these particles “WIMP” weekly interactive massive particles.
What are WIMP
Now what is WIMP and what sort of particles are they? We don’t know. We are searching it. There are many detectors one is Antarctica trying to detect WIMP. Particles accelerators like CERN is also trying to create those particles with particles acceleration which may turn out to be the source of dark matter. But up till now, there has been no positive detection of the dark matter particles. Some claims are there that probably they are nearing the detection stage but there is no confirmation yet. It means that whoever discovers this dark matter particles rest assured he will definitely get the Noble Prize.
Idea of Modified Newtonian Dynamics
Some people think, and this is a minority view in astronomy that probably dark matter does not exist and there is a problem in our physics’ laws.
One idea is this Modified Newtonian Dynamics (MOND). This idea is that the gravity acts in a different way outside the galaxies and the Newtonian laws are not working in the same way as they work in massive or light areas. But these sort of alternative views are in a minority and they are still not accepted Paradigm and these theories create other problems which cannot be solved by astronomers now.
So most of the astronomers now think that dark matter does exist and they are particles which I have called WINPS. But the search is on and which particles are they?
In the end, I would like to summarize because we had a long conversation on dark matter. We know three things about it.
One is observations suggest that it exist from the rotation curve, from the speed in the cluster of galaxies and we know something is present there.
Secondly different from ordinary matter and only interacts with gravity.
Thirdly we know that this matter is 80% to 85% of all matter in the universe.
This is remarkable that the matter we are made up of is minority matter, we have detected 85% of the matter of the universe which we cannot see and its property is that it cannot interact with light.