New observations invalidate Hawking's theory on primordial nature of dark matter

Sputnik
Fri, 05 Apr 2019 17:26 UTC

© Agence France-Presse/HO/NASA

This NASA handout image received 15 May 2007 shows dark matter ring in a galaxy center

Stephen Hawking's 1971 work looks into the so-called gravitational lensing phenomenon, which presupposes that a bunch of black holes zipping around at estimated incredible speeds would by all means bend the light of objects they pass in front of.

Although we are still in the dark about what dark matter essentially is, scientists have now ruled out one possible option, according to the research published in Nature Astronomy - that it is a bunch of minute black holes, as per a theory proposed by the ingenious Stephen Hawking back in 1971.

© Kavli IPMU

The Milky Way galaxy (left) and the Andromeda galaxy (right) are separated by 2.6 million light years. Compared with the areas where stars are clustered together, dark matter is believed to be distributed over a much larger volume.

When an international team led by researchers from Japan's Kavli Institute for the Physics and Mathematics of the Universe (IPMU) was hunting for a particular flicker of stars in a neighbouring galaxy, the way the light would make itself visible if a miniscule black hole was passing in front of it, allowed them to capture 190 consecutive images over seven straight hours, using the cutting edge Hyper Suprime Cam on the Subaru Telescope at Mauna Kea in Hawaii.

If a primordial black hole, which Hawking suggested along with others, was a result of the Big Bang, were to move between Earth and a star it's expected that the star would brighten for a few minutes to hours as the black hole's gravity magnified its light intensity.

Based on Hawking's expanded hypothesis, which was notably first proposed by scientists Yakov Zel'dovich and Igor Novikov in 1966, the team assumed that the abundance of black holes smaller than the Moon required to produce the dark matter effect would lead to at least 1,000 lensing events. However, the observations arrived at just one potential event, which means that primordial black holes can account for no more than only 0.1 percent of dark matter.

The conclusion suggests that dark matter is something more massive than a plethora of teeny black holes, as proposed by the legendary scientist. Although a great deal of further research is needed to rule out his theory altogether, one thing is clear for now - that we have to search for dark matter outside the domain of black holes.

Comment: PhysOrg adds:

[G]ravitational lensing effects are very rare events because it requires a star in the Andromeda galaxy, a primordial black hole acting as the gravitational lens, and an observer on Earth to be exactly in line with one another. So to maximize the chances of capturing an event, the researchers used the Hyper Suprime-Cam on the Subaru Telescope, which can capture the whole image of the Andromeda galaxy in one shot. Taking into account how fast primordial black holes are expected to move in interstellar space, the team took multiple images to be able to catch the flicker of a star as it brightens for a period of a few minutes to hours due to gravitational lensing.

© Niikura et al.

Data from the star which showed characteristics of being magnified by a potential gravitational lens, possibly by a primordial black hole. About 4 hours after data taking on the Subaru Telescope began, one star began to shine brighter. Less than an hour later, the star reached peak brightness before becoming dimmer.

From 190 consecutive images of the Andromeda galaxy taken over seven hours during one clear night, the team scoured the data for potential gravitational lensing events. If dark matter consists of primordial black holes of a given mass, in this case masses lighter than the moon, the researchers expected to find about 1000 events. But after careful analyses, they could only identify one case. The team's results showed primordial black holes can contribute no more than 0.1 per cent of all dark matter mass. Therefore, it is unlikely the theory is true.