New Delhi: An international research team, including two scientists from the 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. The finding also disproves a claim Stephen Hawking had made.
In the Solar System, Mercury – the innermost planet – takes just 88 days to complete one revolution around the Sun. Neptune – the outermost planet – takes 165 years for the same journey. Similarly, the laws of gravity expect us to see stars closer to the centre of a galaxy go around faster than stars at the edge. However, in most galaxies, both the innermost and outermost stars take almost the same time to complete one revolution – an observation first made by Vera Rubin in the 1970s.
It was as if some invisible material – known as dark matter – enveloped the galaxies, giving an extra push to the outer stars and accelerating them. The nature of this material has been a mystery since the 1930s, when Fritz Zwicky first proposed its existence.
Dark matter is considered to be a form of matter because it appears to exert a gravitational pull, and it is ‘dark’ because it doesn’t seem to interact with light, or for that matter with any part of the electromagnetic spectrum. Scientists have found in their detailed surveys of the cosmos that almost 85% of the universe’s total mass is just the mass of dark matter. Thus, the stars and galaxies we see all around us are just the collective tip of the matter iceberg; a lot remains out of sight and all of it is dark matter.
Enter primordial black holes
Cosmologists have pitched various ideas to explain the dark matter. Some think it could be composed of neutrinos, which are particles that have no charge (so don’t interact with light) and have a tiny mass (and interact with gravity). Some others have postulated that dark matter could be made of new kinds of elementary particles – weakly-interacting massive particles or gravitationally-interacting massive particles, either of which are yet to be detected.
After the Big Bang hypothesis was first floated, two Soviet physicists named Yakov Zel’dovich and Igor Novikov showed that at right after our universe was born, it must have contained very large amounts of energy packed so densely together that it should have created small black holes. These were called primordial black holes.
Hawking investigated them (in theory) in 1971, and calculated that the mass of each primordial black holes could range from as low as one-hundredth of a milligram to more than the mass of a thousand Suns.
Black holes are not radiant and are not visible through any telescope. However, 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 an effect called gravitational lensing. As a result, the star will appear to be brighter than it actually is for a short while. Gravitational lensing is rare and occurs only when a star, a black hole and an observer on Earth are along a straight line.
The galaxy in the foreground gravitationally lenses light from a galaxy passing behind it, making the latter seem brighter and distorted. Credit: ESO/M. Kornmesser
The research team, led by Masahiro Takada, Hiroko Niikura and Naoki Yasuda from the Kavli Institute for the Physics and Mathematics of the Universe, used the Hyper Suprime-Cam instrument on the Subaru Telescope located in Hawaii. They looked for tell-tale evidence of primordial black holes between Earth and the Andromeda Galaxy using gravitational lensing.
Looking for the flash
It would be absurd – like searching for a needle in a haystack – to keep looking at one star and hoping that, at some time or other, an invisible primordial black hole eclipses it. Instead, the research team continuously observed the entire disk of the Andromeda Galaxy, consisting of lakhs of stars. This way, there was a chance for some primordial black holes – if they did exist in sufficient numbers – to eclipse one of the stars in a noticeable way.
“The investigation is delicate and tricky. The eclipsed star may brighten up for a period of a few minutes or a few hours due to gravitational lensing, requiring the team to take multiple images to catch the flicker, if any,” Anupreeta More, a member of the team and a researcher at IUCAA, told India Science Wire.
So through a whole night, the research team took 190 images of Andromeda galaxy. “If the universe is filled with invisible, teeny-weeny primordial black holes, with masses lighter than the Moon’s … then we should have seen at least 1,000 gravitational lensing events,” More said. “We saw at most one such candidate event, if not none. This implies that Hawking’s theory – that such black holes make up all of dark matter – is wrong.”
“Previous studies had already 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,” Surhud More, another member of the research team, said. “The present study rules out the presence of primordial black holes with masses comparable to the moon and sizes of about 0.1 millimetre too. This implies that at best the lunar mass primordial black holes can contribute to 0.1 per cent of all dark matter mass.”
Their study was published in the journal Nature Astronomy on April 1, 2019.