An international team of scientists that included two scientists from Pune-based IUCAA, has shown that one of the most established hypothesis about the origins of dark matter in the universe stands on very shaky grounds.
In a paper published in Nature Astronomy, these scientists, from Japan, India and the United States, have reported the results of their observations of the Andromeda galaxy — the nearest neighbour of our own Milky Way — to claim that primordial black holes, those were formed in the early age of the universe, might not be the primary source of dark matter, as had been predicted by famous physicist Stephen Hawking.
These primordial black holes, with masses similar to or less than the moon, could not have contributed to more than one per cent of all dark matter present in the universe today.
Dark matter, though never detected, is believed to pervade the entire universe, its existence presumed because a number of observable celestial phenomena could not be possible if the universe did not have much more matter in it than is seen. Together with dark energy, another hypothesised quantity because it shows up in calculations, dark matter is believed to make up more than 95 per cent of all universe.
Attempts to detect dark matter in underground experiments or in accelerator-based facilities like the Large Hadron Collider (LHC) have so far not been successful. Hawking, a celebrated scientist best known for his work on black holes, had predicted that primordial black holes could behave like dark matter.
To test this hypothesis, the team that included Surhud More and Anupreeta More from IUCAA carried out observations on Andromeda galaxy using the Hyper Suprime-Cam on the Japanese Subaru Telescope located in Hawaii. The scientists were looking for instances of ‘gravitational lensing’ of the stars in the Andromeda galaxy.
Gravitational lensing refers to the phenomena of the bending of light by extreme gravity.
Albert Einstein had shown that the fabric of space-time itself was curved because of presence of massive objects like stars and planets, and that light travelling in a straight line will follow these curves and appear bent.
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. “Our team was looking for gravitational lensing effects to get an idea about the primordial black holes between our galaxy and our neighbour, the Andromeda galaxy,” Surhud More told The Indian Express.
The effects of gravitational lensing is extremely difficult to detect as it requires a distant star, black hole and the observer on earth to be well-aligned. Researchers were hoping to find many instances of stars in Andromeda galaxy, gravitationally lensed by intervening primordial black holes.
From 190 consecutive images of Andromeda galaxy recorded from Subaru telescope, researchers expected to find 1,000 events if all of dark matter was indeed made up of primordial black holes. However, the team could identify at most one probable candidate of this kind.
This has led the team to propose that primordial black holes, with masses similar to or less than that of the moon, could not contribute more than one per cent of all dark matter. That leaves the question of the origin of the rest of the dark matter open once again.
A statement from IUCAA said the ‘negative result’ from the experiment was still useful in advancing the knowledge about the Universe.
“This study imposes stringent constraints on the physics of the early universe and thus disfavours (or questions) Prof Hawking’s hypothesis,” it said.