Question

Please answer 1a and 1b.

1a. What is dark matter? How was its existence discovered, and why do astronomers think it must be present?

1b. What are quasars and how are they used to study the evolution of the Universe?

Answer 1

Dark Matter (DM) is one of the biggest mysteries as far as modern particle and astrophysics are concerned. It constitutes about 25% of the total content of our Universe whereas the visible matter which includes all matter which we can observe by telescope working at the different frequency range of electromagnetic spectrum constitutes only 5%. Its ‘invisible’ character simply means that it can neither absorb nor emit electromagnetic radiation. But it has mass so it can interact with visible matter through gravitational interaction. Its existence was discovered based on its gravitational interaction with the visible matter. Rotation curve of a star requires the mass much greater than the mass of visible matter hence proving there is something which carries significant mass but is invisible. Also, the gravitational lensing of light from a distant astronomical object also points toward the existence of invisible matter.

Quasar is a type of active galactic nuclei which are very luminous and emits extremely large amount of energy in form of jets. Because of their high luminosity they can be probed from a very large distant means that they can be usd to study remote past of our universe.