The search for dark matter

Source: The post is based on the article “The search for dark matter” published in The Hindu on 12th July 2022.

What is the News?

Physicists working with the world’s biggest dark-matter detector—in the United States known as LUX-ZEPLIN (LZ) have released their first results.

What is Dark Matter?

All interactions in the universe are a result of four fundamental forces acting on particles– strong nuclear force, weak nuclear force, electromagnetic force and gravitation. 

Dark matter is made up of particles that do not have a charge — which means they do not interact through electromagnetic interactions.

So, these are particles that are “dark”, namely because they do not emit light which is an electromagnetic phenomenon and “matter” because they possess mass like normal matter and hence interact through gravity.

Note: Many physicists strongly believe that the entire visible part of the universe forms only 5% of all matter in it. They believe the rest is made up of dark matter and dark energy. 

How do scientists know that Dark Matter exists?

There is strong indirect evidence for dark matter and this evidence is reflected at various levels:

Firstly, Rotation of Galaxies: Consider the rotation of galaxies. If one looks at stars all the way from the centre of any galaxy to its rim, the way the velocities of the observed stars change may be plotted. 

– There is a marked difference between the observed plot of star speeds and the calculated value as one moves from the inner part of the galaxy towards its rim. 

– Now if one assumes there is a certain fraction of matter which exerts a gravitational pull on the rest of the stars in the galaxy, for it cannot be seen in any other way and recalculate the plot, it fits in with the observed value. This means that there is a definite amount of dark matter in the galaxy.

Secondly, the Bullet cluster of galaxies: The Bullet cluster is formed through the merging of two galaxy clusters. Physicists found from their calculations that the way these mergers took place could not be fully explained if we believed that the visible universe was all that existed. Therefore, there should be something like dark matter as well as an estimate of how much dark matter there should be in the universe.

What are the candidates for Dark Matter particles?

The neutrino would have been an excellent candidate if it had been more massive. However, being too light, it doesn’t fit. 

Candidates for dark matter are hypothetical particles such as axions, sterile neutrinos, weakly interacting massive particles(WIMPs), supersymmetric particles, geons or primordial black holes.