Dark Matter

In News: Dark matter detector experiment named LUX-ZEPLIN (LZ) in South Dakota in the U.S. was recently in news

• As of today, this is the most sensitive dark matter detector in the world.

What is dark matter and why is it so elusive?

• 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.
• Gravitational force, besides not being fully integrated and understood by particle physicists, is extremely weak.
• For one thing, a particle that interacts so weakly becomes rather elusive to detect.
• This is because interactions from other known particles could drown out signals of dark matter particles.

Why do physicists believe strongly that dark matter exists?

• There is strong indirect evidence for dark matter, and this evidence is reflected at various levels (or distance scales).
• The discrepancy between the calculated and observed value of velocities in rotating galaxies proves that there is a certain fraction of matter which exerts a gravitational pull on the rest of the stars in the galaxy.
• This means that there is a definite amount of dark matter in the galaxy.

What are the evidences from other distance scales?

• The second evidence came from observations of the so-called 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 were 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?

• Postulated entities include the supersymmetric partner of the Z boson, a particle that mediates the electro-weak interaction.
• Yet other explanations talk about “hidden sector particles” and Axions, a boson and a condensate of dark matter.
• There are many other theories.

The search is on to find one of these candidates, for the story is one that spins together gravity.

Source: The Hindu