Dark Matter and Dark Energy

Science is about predictions, making predictions and testing predictions. We don’t know what is going to happen to all the planets of the solar system. There might be a chance that one day some planet may collapse or two planets may collide or some explosion on Pluto will destroy the earth. Well the last one seems very much impossible but other two may be possible. We know that our solar system is stable since some billion years. But beyond that we genuinely don’t know. And the Sun, we think it will last 5 billion years. So it’s fair to say that we don’t have any idea that whether the solar system is actually stable. And that raise many questions about the past of the universe or its future.

And also things like “dark matter” or “dark energy”. Inside our solar system, as we know that the Sun pulls the planets and prevents them to fly away. This gravity is also pulling the planets towards each other. If you look up at the dark sky with your naked eyes, you may see many stars. But if you see the same portion of the sky with a powerful telescope, you will see many bright objects there. Are these all stars? For .e.g. if you look at the “Andromeda” constellation, then as well as stars you might see a faint mere of light and that faint mere of light is nothing but the another galaxy which is called “Andromeda” galaxy and that has also millions and billions of stars. Now if you’ve given a telescope let’s say “Hubble Space” telescope and asked to take the pictures of the blank sky. That will be impossible because every time you’ll find millions of galaxies and billions of stars in that picture even in the one tenth part of the sky. Now suppose we go to one the galaxies that you’ve seen through that telescope and we try to find how much stuff or matter are there in it. There are many ways:

 

Fig.n. Andromeda Galaxy

First method is that we will find “How bright is that galaxy?” and from that we will find “How many stars are there in that galaxy?” but there is another method. As we know that the galaxies are continuously spinning as the same way the earth is spinning. If we calculate “How fast is the galaxy spinning?” and from that we can calculate “How strong is its gravity?” and then we can find “How much stuff is there in it?” because gravity is possessed by only stuffs or objects. But practically these approaches give us the numbers far more different that the actual value. If we use the spinning method, we will end up with a number which is 5 times more than the number of matter that we can see. So there must be some matter which we can’t see but has mass. We named that matter “Dark matter”.

 

Fig.n. Dark Matter

In a cluster of galaxies, when many galaxies are very close to each other, the gravity becomes extremely strong which actually bends the light. And when we apply the above mentioned methods to find that how much stuff is there in the galaxy, like “How bright is it?” and from that we can calculate “How much stuff is there in it?” and then “How much the light is bend in those images?” and from that we can find out “Strength of the gravity?” and from that we can find out “How much stuff is there in it?”. And surprisingly we end up with a number which will be exactly same as mentioned above i.e. 5 times more. So these things convince us that dark matter does exist. But at the same time it also raises many questions. For e.g. “What is dark matter made of?” or “Where it comes from?"

We know that every matter is made of particles i.e. smallest elements that can’t further be divided into smaller chunks. Then we assume that dark matter is made up of particles too as it also a matter having some mass. Then we want to know how these particles behave? Now the main thing we know is about particles is that they feel forces. For instance, why a man is attracted towards the Earth i.e. because all the particles that are making him up are pulled down by the force of gravity possessed by the Earth. Now the question is why doesn’t the man sink into the floor? So there must be some another force acting on the man’s body acting opposite to the gravity. And that is electromagnetic force which is pushing him up. And electromagnetic force is also the reason that why we can see things e.g. light itself is the aspect of the force. Now if we talk about “dark matter”, they do have gravity but on the other hand we can’t see them. So that means that they don’t feel the force of electromagnetism.

Fig.n. Electromagnetic Waves
 

Now the question is from where does these matters come from? Let’s try to find out how much dark matter is on the Earth. Now suppose that dark matter is coming towards the Earth, it’ll pass through the earth because it doesn’t feel
force of electromagnetism. So you can’t count them. But if somehow you freeze
the time and search all the dark matter, there will be around trillions of particles. But they are very tiny i.e. millionth of the millionth of the millionth of the kilogram. And now how can we find that particle which is of this size and invisible and untouchable. Dealing with the question that how on the Earth do we know that where these are coming from and from which direction. We can do this by using computers. Now suppose if we drop a blog of dark matter inside a virtual galaxy inside our computer (32-bit) and record the motion of that blog, we can see that as the gravity is strongest at the center of the galaxy, the dark matter moves towards it and gets thrown away outwards. It repeats the same actions in random direction. And every time the motion is unpredictable. And if we use a more powerful computer say 128 bit or more, it may give more accurate motion but it’s still unpredictable. So we can’t predict the exact motion of that blob but what we can do is we can predict the motion using the pattern formed by that blob i.e. by finding average.

Now what is “Dark Energy”? It is based on the following facts: 

Universe is expanding and we all know that i.e. all galaxies are getting further away from each other. Not only it’s expanding but it is expanding at an accelerating rate, and that is due to this energy called dark energy. It has a strange property i.e. it expands as the volume of space increases to keep its energy density constant. So as the universe expands, there will be more and more dark energy. So the future universe will be more and more dominated by dark energy and it will become colder.

 

Fig.n. Dark Energy

 

We still don’t know about the death of our solar system or the death of the universe because we can’t exactly predict. Like in the case of dark matter we reach for what we know about the particles and we came up with ideas and in the case of dark energy we did something similar i.e. we take ideas and we calculate average because it is all about patterns not specifics.