THE DARK MATTER SERIES

EP-1 How did we detect Dark Matter?

In 1937, a scientist named “Zwicky” published a paper in which he mentioned that there was a “Mass Discrepancy” problem which he found after studying the “Coma Cluster”. This Coma cluster is a cluster of galaxies, and using the motion of some galaxies in the cluster as a tracer of the gravitational field that binds the entire cluster together, he discovered that the average velocity had a shockingly high value.

Larger gravitational forces induce higher velocities in the objects they attract for that reason, he estimated a very high mass for the cluster. As a reality check on that estimate, you can sum up the masses of each member galaxy that you see. And even though Coma ranks among the largest and most massive clusters in the universe, it does not contain enough visible galaxies to account for the observed speeds Zwicky measured.

His conclusion was:

The missing mass to account for the gravity of the “Coma Cluster” does exist but in some unknown invisible form.

This observation did not arouse a sense of curiosity among the scientific community until in the 1970s American Astrophysicist “Vera Rubin” observed that :

In the outer parts of the galaxies, in particular, the visible matter falls far short of what is required to explain the flat rotation curves.

This was observed not for just one or two galaxies, but almost every galaxy out there more or less followed the same trend. Then it was realized that there is something that is seriously going wrong somewhere.

Within the visible disk of each galaxy, the stars farther from the center move at greater speeds than stars close in. The farther stars have more matter (stars and gas) between themselves and the galaxy center, enabling their higher orbital speeds.

This led to the proposal of the existence of some unknown, matter which we now call “Dark Matter”(DM). This was not something like the proposal of the existence of “Ether medium in space” which was to explain how light travels in space.

The fact that there is something of the sort which we call Dark Matter is cemented by many proofs:

1. It causes the Bulk of gravitation that arises in our universe.

2. It is observed that Cosmic DM has about six times the gravity of all visible matter.

3. GRAVITY FROM ONLY ORDINARY MATTER WAS NOT ENOUGH FOR IT TO WIN AGAINST THE EXPANSION. IT NEEDED HELP FROM DM.

HOW MUCH MORE GRAVITY WAS NEEDED?

SIX TIMES THAT PROVIDED BY ORDINARY MATTER WHICH IS WHAT WE DETECT IN THE UNIVERSE.

Thus all these facts point in the direction that as observed from the effects, DM is real and it exists although we can only detect it gravitationally. It does not interact with gravity or with Electromagnetic radiation(which includes light as well). It also does not interact with strong and weak nuclear forces.

If DM was not present there would be more He atoms relative the H atoms in the universe.

How? you ask, well there is one helium atom for 10 hydrogen atoms in the universe due to nuclear fusion in the first few minutes after the big bang. If DM was also undergoing nuclear fusion, there would be a lot more helium atoms relative to hydrogen in the universe.

So now the question arises “What is this Dark Matter ?” and “How is it so different from ordinary matter ?” “Why is it that most of the matter in the universe is DM?”

More on this in the next episode of this series. STAY TUNED!!!

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Just trying to use Science to explain concepts of Astrophysics to the common folk.

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Manas Karnik

Manas Karnik

Just trying to use Science to explain concepts of Astrophysics to the common folk.

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