Context

• An international research team from Inter-University Centre for Astronomy and Astrophysics (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 Professor Stephen Hawking.

About

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.
• 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, known as `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 with any part of the electromagnetic spectrum).
• Almost 85 per cent of the total mass of the Universe is composed of dark matter.
• Cosmologists have come up with various hypothesis and theories to explain the dark matter. Some postulate it to be composed of neutrinos — particles that have no charge but have tiny mass and, therefore, do not interact with the electromagnetic spectrum, but are interacting gravitationally.
• Others have postulated they may be some new kind of elementary particles — “weakly-interacting massive particles (WIMPs)”, or “gravitationally-interacting massive particles (GIMPs)”, which are yet to be detected.

Primordial black holes

• When the big bang hypothesis was proposed, two Soviet physicists, YakovBorisovichZel'dovich and Igor DmitriyevichNovikov, proposed 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. These were named “primordial black holes”.
• Stephen Hawking investigated them in 1971 and 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.

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.

Gravitational lensing

• 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 the gravitational force, 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.