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| Dark matter around a galaxy ( Illustration: NASA/CXC/M.Weiss) |
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?
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.
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| 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.
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| 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.
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 “Dark matter”.
Properties of Dark matter
The Dark matter does not interact with anything and even
with itself. 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.
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.
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| 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.
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| 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
The universe contains 25 % content in the form of 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.
If you want to explore more, you can visit this video which
provides nice and elaborate information related to Dark matter. You can also comment
if you have ideas and doubts related to what
is Dark matter?