From UPSC perspective, the following things are important :
Prelims level : Dark Matter, Gravitational Lensing
Mains level : Black-hole theory and its relevance
- An international research team including researchers from IUCAA, Pune has ruled out the possibility of primordial black holes being a major constituent of dark matter.
- This finding disproves a theoretical claim of Prof Stephen Hawking.
What is Dark matter?
- In the solar system, Mercury, the planet closest to the Sun, takes just 88 days to make one revolution around the sun, while Neptune, the farthest one, takes 165 years to make one round.
- In like manner, laws of gravity expect us to see stars closer to the centre of galaxies rotating faster than the stars on the edge.
- However, in most galaxies, the stars closer to the centre and the stars at the edge of the galaxies take almost same time to make one revolution.
- This implied that something invisible and enveloping the galaxies was giving an extra push to the outer stars, speeding them up.
- This entity has remained as one of the central unresolved puzzles in cosmology since 1930s. It is, no wonder, named `Dark Matter’.
- The material is considered to be a ‘matter’ since it appears to have gravitational attraction and it is ‘dark’ because it does not seem to interact with light (or for that matter any part of the electromagnetic spectrum).
- Detailed surveys of the cosmos indicate that almost 85% of the total mass of the Universe is composed of dark matter.
Proposition by Stephen Hawking
- In 1971, Prof. Hawking showed the possibility that black holes could form very early in the universe.
- Since the actual nature of dark matter particles is a mystery, he predicted that these primordial black holes (made up of ordinary matter) could behave like dark matter.
- He computed that the mass of the primordial black holes could range from as low as one-hundredth of a milligram to as high as more than the mass of thousand Suns.
How did researchers disapprove Hawking?
- The research team used the Hyper Suprime-Cam on the Japanese Subaru Telescope located in Hawaii to look for any tell-tale evidence of primordial black holes between Earth and Andromeda galaxy using gravitational lensing technique.
- They kept observing the entire disk of the Andromeda galaxy consisting of lakhs of stars.
- Previous studies had ruled out the existence of large numbers of primordial black holes that could range in size from the mass of the Moon to about 10 solar masses.
- If the Universe is filled with invisible various primordial black holes, with masses lighter than the moon, as postulated by Hawking, then the team should have seen at least 1,000 gravitational lensing events.
- However, they were able to see at most one such event which implies Hawking’s theory that such black holes make up all of dark matter is wrong.
- Black holes are not radiant and will not be visible through any telescope.
- However, as first suggested by Albert Einstein, if by chance, a tiny primordial black hole eclipses a distant star, light rays of the star will bend around the black hole due to gravitational effect, resulting in the star appearing to be brighter than it is originally.
- Called `gravitational lensing’, this rare phenomena can occur only when the star, the black hole and the observer on the Earth are aligned in a straight line.
- When the black hole is in alignment with a distant star, due to gravitational attraction, light rays are bent inwards like a lens, making the star appear brighter.
- In extreme cases of gravity, like those produced by black holes, the star that is the source of the bent light appears magnified, an effect that can be detected by powerful telescopes.