Topics Covered:
- Awareness in space.
Hawking’s Theory for Source of Dark Matter ruled out
What to study?
For Prelims and Mains: What was Hawking’s theory, what are the recent findings and their significance? Gravitational lensing.
Context: A group of scientists have ruled out Stephen Hawking’s theory for mysterious dark matter.
What did Hawking propose?
Stephen Hawking proposed a theory that primordial black holes are a source of 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 a thousand Suns.
What are primordial black holes?
Two Soviet physicists, Yakov Borisovich Zel’dovich and Igor Dmitriyevich Novikov, showed that at the initial instant of the big bang, the densities would have been very high at many points, resulting in the formation of small black holes. They were named `primordial black holes’.
Are they visible?
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 originally is for a short while. 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.
Latest findings:
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.
For one whole night, the research team took 190 consecutive images of Andromeda galaxy. If the Universe is filled with invisible teeny weeny primordial black holes, with masses lighter than the moon, as postulated by Stephen Hawking, then the team should have seen at least 1,000 gravitational lensing events. However, they were able to see at most one such candidate event, if not none. This implies Prof Stephen Hawking’s theory that such black holes make up all of dark matter is wrong.
What is dark matter?
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 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).
Composition: Almost 85% of the total mass of the Universe is composed of dark matter.
Sources: the hindu.
