Dark matter around a galaxy

( Illustration: NASA/CXC/M.Weiss)

Human are always curious to understand the nature and its phenomenon. When we look towards the space, we find that the space contains large number of massive objects like asteroids, planets, stars, clusters, galaxies. The size of any galaxy is enormously large that we can’t imagine. Even our sun is very large although it is an average star. But you will be surprised to know that all these objects like stars and galaxies taken together make just 5 % of total content of the universe. Are you curious to know where the remaining part of the universe is? The remaining content of the universe is distributed in the form of Dark matter and Dark energy which are 25 % and 70 % of total content respectively. Now the question arises what is Dark matter?

Human are always curious to understand the nature and its phenomenon. When we look towards the space, we find that the space contains large number of massive objects like asteroids, planets, stars, clusters, galaxies. The size of any galaxy is enormously large that we can’t imagine. Even our sun is very large although it is an average star. But you will be surprised to know that all these objects like stars and galaxies taken together make just 5 % of total content of the universe. Are you curious to know where the remaining part of the universe is? The remaining content of the universe is distributed in the form ofandwhich are 25 % and 70 % of total content respectively. Now the question arises





The Dark matter is not an object which is black in colour. In simple words we can say that it is the matter which does not interact with any other form of matter and even with itself. The Dark matter also does not interact with light. Now you must be thinking if the Dark matter does not interact with anything then how do we know that it is present in the universe? is not an object which is black in colour. In simple words we can say that it is the matter which does not interact with any other form of matter and even with itself. The Dark matter also does not interact with light. Now you must be thinking if the Dark matter does not interact with anything then how do we know that it is present in the universe?





The answer lies in the gravity, although it does not interact via electromagnetic forces but due to its mass it can attract normal matter with gravitational force.





What is Dark matter?





The concept of Dark matter was proposed to explain the anomalies in the rotation curve of galaxies.





To understand this lets first focus on our solar system which contains the sun and the planets. The planets revolve around the sun with different velocities. The planet closer to the sun revolves with largest speed in all the planet of the solar system. As you go to further planets you will find that the velocity of revolution keeps on decreasing. So the planet Uranus revolves with the slowest speed and the planet Mercury with largest speed.





The speed of planets decreases with the increase in the distance from the sun.







This behaviour is due to 1/r2 dependence of gravitational force. The force of gravitation decreases with the increase in the separation between two bodies. This results in the decrease of velocity of revolution with the increase in the radius of the orbit. Actually the velocity decrease as 1/sq(r) where r is the distance.





Applying the same analogy to the galaxies we expect that the stars present near the center should rotate with larger speed as compared to the stars which are present at the larger distance from the center. The stars present at the edge of galaxy should move with slowest speed. So the velocity of revolution should decrease with the increase in the distance from the center.





The observation of velocity of revolution of stars which are present at various distances from the center is totally surprising. The velocity of revolution does not decrease with the increase in the distance from the center of galaxy. It is observed that all the stars moves with nearly same velocity in spite of being present at different distances from the center. The velocity becomes constant and does not decrease with distance.



Rotation curve of a galaxy: predicted (A) and observed (B). The velocity

does not decreases with the increase in distance from the center. (Source: Wikipedia)







The velocity of revolution of star is very large. What I mean is that the whole mass of the stars, planets, gases taken together cannot produce such a large velocity of revolution.





The constant and high velocity of revolution directly indicates towards another source of gravity which we are not familiar with. The calculations indicate that there must be present some invisible matter spread up to distance of million light year around the galaxy where the size of galaxy is just 100 thousand light year.





Dark matter ”. This matter is present in the spherical shape and spreads throughout the galaxy. This matter provides the gravity required to produce observed velocities of stars. It does not interact with light that’s why it is invisible and named as “”.





Properties of Dark matter





The Dark matter does not have any electric charge which makes it neutral for all the electromagnetic interactions. The light can pass directly through the Dark matter without being reflected and absorbed because of absence of electric charge. The Dark matter does not interact with anything and even with itself.does not have any electric charge which makes it neutral for all the electromagnetic interactions. The light can pass directly through the Dark matter without being reflected and absorbed because of absence of electric charge.



You must be wondering that if the Dark matter is present all around us then why can’t we feel it? To answer this question we can look at some example. When we press the surface of table, our hand does not pass through the table because when we press the surface the separation between the molecules of surface decreases which results in the electrostatic force of repulsion. This is the reason why can’t two objects just pass through each other. But in the case of Dark matter, the absence of electric charge ensures that no force of repulsion will be observed when Dark matter will come in contact with normal matter. It will just pass through the normal matter. In this way, the presence of the Dark matter does not interfere with the motion of the planets and stars.





The life time of Dark matter should be very large more than 13.8 billion years, that’s why it is still present in the huge amount. The Dark matter is very stable which indicates that the creation of dark matter requires huge amount of energy which must be present at the time of big-bang. So the Dark matter must be formed very early after the big-bang.



Evidence for Dark matter





The presence of insufficient gravity of galaxies is one of the strongest evidence of Dark matter. Most of the evidences of Dark matter are based on the gravitational force.



Presence of Dark matter causes the light to bend and results in gravitational lensing ( Source: NASA/ESA).







The presence of Dark matter is also indicated by the gravitational lensing. Whenever a light passes near a massive object, it bends due to gravity. The same phenomenon occurs when you are trying to observe a distant galaxy and some other galaxy or cloud of gas comes in front. The bending of light resembles the optical lensing, the image of galaxy present behind gets stretched and distorted. The mass of object present in the path can be calculated by analysing the amount of distortion and stretching.



Distortion in image due to gravitational lensing ( Source: ESA/Hubble, NASA)







Suppose you are observing some galaxy and the images are much distorted. But you can’t see any big object in the path. This is the direct indication of Dark matter which is invisible but causing the gravitational lensing. The gravitational lensing caused by galaxies are also indicating that there must have larger mass then that of visible matter.





Detection of Dark matter Particle





There are many theories which predict different type of particle of Dark matter but none of them has been detected yet. Many theories say that the particle of Dark matter would be light enough to be produced in the LHC. Even if they were created at the LHC, they would escape through detectors because of no interaction. However, they would carry momentum and energy which could be the possible way to detect them. So the physicists could infer their existence from the missing momentum and energy.



Conclusion





Dark matter. The Dark matter is present around the galaxies and is providing the stability to the galaxy. In the absence of Dark matter the galaxy will be just torn apart because of centrifugal force. The Dark matter is also supposed to be responsible for providing huge gravity for formation of galaxies and stars. Although today, we know very little about the nature of dark matter but in future it is going to explain many mysteries of universe. The universe contains 25 % content in the form of. The Dark matter is present around the galaxies and is providing the stability to the galaxy. In the absence of Dark matter the galaxy will be just torn apart because of centrifugal force. The Dark matter is also supposed to be responsible for providing huge gravity for formation of galaxies and stars. Although today, we know very little about the nature of dark matter but in future it is going to explain many mysteries of universe.



