Dark matter is not comprised of tiny black holes new research suggests

Tiny primordial black holes may have been eliminated as candidates for dark matter by a team of international scientists using the Subaru Telescope.

In the ongoing search to discover what composes dark matter — the substance that makes up 70–90% of the observable Universe — the elimination of candidates is of utmost importance.

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 (Kavli IPMU)

That is what a team of researchers from across the world have used the Subaru Telescope to do — eliminate the possibility that dark matter is comprised of tiny primordial black holes.

Primordial black holes smaller than a tenth of a millimetre — were first suggested by the late Stephen Hawking to compose dark matter — the substance which gravitationally prevents galaxies from flying apart — in a 1974 theory. These tiny black holes — born shortly after the big bang as the name ‘primordial’ suggests — may well still exist.

The researchers used the gravitational lensing effect — the fact that strong gravitational influence from massive objects can ‘bend’ light rays from distant objects — to look for primordial black holes between Earth and the Andromeda galaxy. In extreme cases, such light bending causes the background star to appear much brighter than it originally is.

However, gravitational 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.

As the Subaru Telescope on Earth looks at the Andromeda galaxy, a star in Andromeda will become significantly brighter if a primordial black hole passes in front of the star. As the primordial black hole continues to move out of alignment, the star will also turn dimmer (go back to its original brightness). (Kavli IPMU)

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.

Over seven hours on one clear night, the team took 190 consecutive images of the Andromeda galaxy. Using these images they hunted 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 such 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.

The team — led by Kavli Institute for the Physics and Mathematics of the Universe Principal Investigator Masahiro Takada, PhD candidate student Hiroko Niikura, Professor Naoki Yasuda, and including researchers from Japan, India and the US — now plans to further develop their analysis of the Andromeda galaxy by investigating a new theory that suggests binary black holes discovered by gravitational wave detector LIGO are in fact primordial black holes.

Original research https://www.nature.com/articles/s41550-019-0723-1