We still do not know what dark matter is, but we can draw a line with only one option. This is not, according to a theory proposed by the brilliant Stephen Hawking, a group of tiny microscopic black holes.
In the most rigorous test of the theory so far, an international team led by researchers at the Kavli Institute for Physics and Mathematics of the Universe (IPMU) in Japan has been looking for the telltale sign of these black holes as well. tiny. quite overwhelming.
The scientists were looking for a flicker of stars in a nearby galaxy, as if the light appeared to us if a black hole of less than a tenth of a millimeter passed in front of it.
A black hole of this size may sound ridiculous, but the concept stems from an elegant theory of Hawking, which was attempting to tackle our enormous dark matter problem.
The problem is this: on the basis of our observations of the gravitational forces at play in the Universe, we know that 85% of the mass is created by something that we can not see and that remains to be detected directly.
In a 1971 article, Hawking developed the theory of primordial black holes proposed by scientists Yakov Borisovich Zel & # 39; dovich and Igor Dmitriyevich Novikov in 1966.
Just after the Big Bang, according to the theory, during the creation of the Universe, there could have been regions of matter in the primordial soup denser than others, sufficiently dense for a gravitational collapse .
Since the resulting black holes did not form stars, they could be much less massive – as little as 10-8 kg.
Now, even a titchy black hole has a lot of mass. A black hole with an event horizon of 0.1 millimeter in diameter would have a mass of more than 67 quintillion tons. So, if there were a bunch of those tiny black holes out there – and they had not evaporated under the effect of Hawking radiation – they could, justifiably, represent the mass we can not see.
And, if there were a bunch of those black holes around at the incredible speed calculated by Hawking, we could see them twisting the light from the objects they are moving in front of, an effect called the gravitational lens.
It is this effect on which the team concentrated its efforts. Researchers from Japan, India and the United States used the Hyper Suprime camera on the Subaru telescope at Mauna Kea in Hawaii to observe the entire nearby Andromeda galaxy, capturing 190 consecutive images for seven hours.
If a primordial black hole were to move between us and a star, it is to be expected that the star would twinkle and brighten for a few minutes to a few hours as the gravity of the black hole would swell its light.
The team predicted that the abundance of black holes smaller than the Moon required to produce the dark matter effect would result in approximately 1,000 lens events.
But the observations produced only one potential event, which means that primordial black holes can not be responsible for more than 0.1% of the dark matter. (A study last year found that primordial black holes probably could not contain more than 40% of dark matter.)
Which actually means that dark matter is not made of tiny black holes.
This is unlikely to be the last nail in the coffin of theory – scientists like to be extremely thorough after all. But until now, primordial black holes are a non-place and we must search for dark matter elsewhere.
The search was published in Nature Astronomy.