We still don’t know what exactly dark matter is, but we can gradually rule out what it is not. Now new research disproves the theory put forward by brilliant physicist Stephen Hawking that dark matter is made up of a bunch of tiny microscopic black holes.
A team of astronomers including Surhud More and Anupreeta More from the Interuniversity Center for Astronomy and Astrophysics (IUCAA) has just ruled out the possibility that primordial black holes are one of the main components of dark matter.
Cosmological dilemmas
In the solar system, it takes only 88 days for Mercury, the closest planet to the Sun, to go around the Sun, while Neptune, the furthest, takes 165 years to do this round. In the same way, the laws of gravity expect us to see stars closer to the center of galaxies that rotate faster than the stars on the edge. However, in most galaxies, the stars closest to the center and those on the edge of the galaxies take about the same time to complete their orbit. This implied that something invisible and enveloping the galaxies was giving the outer stars an extra boost, speeding them up. This entity has remained one of the central unsolved puzzles in cosmology since the 1930s. It is what we know as dark matter.
It is considered to be “matter” since it appears to have a gravitational attraction, and it is “dark” because it does not appear to interact with light (or, actually, any part of the electromagnetic spectrum).
Detailed surveys of the cosmos indicate that almost 85% of the total mass of the universe is made up of this dark matter. So the stars, galaxies, and atoms we see around us are just the tip of the iceberg, and dark, elusive matter makes up most of the universe.
Primordial black holes
Cosmologists have proposed various hypotheses and theories to explain dark matter. Some postulate that it is composed of neutrinos, which are particles that have no charge but have a tiny mass and, therefore, do not interact with the electromagnetic spectrum, but rather interact gravitationally. Others have postulated that they may be some new type of elementary particles: “weakly interacting massive particles (WIMP)”, or “gravitationally interacting massive particles” (GIMP), which have not yet been detected.
When the Big Bang hypothesis was proposed, two Soviet physicists, Yakov Borisovich Zel’dovich and Igor Dmitriyevich Novikov, showed that at the initial instant of the Big Bang, the densities would have been very high at many points, resulting in the formation of small black holes. They were called “primordial black holes”. Stephen Hawking investigated them in 1971 and calculated that the mass of primordial black holes could range from a hundredth of a milligram to a mass of a thousand suns.
Black holes are not radiant and are not visible through any telescope. However, as suggested by Albert Einstein, if by chance a small primordial black hole overshadows a distant star, the light rays from the star will bend around the black hole due to the gravitational effect, making the star appear more bright than it originally is for a short time. It’s what we call “gravitational lensing ,” and this rare phenomenon can occur only when the star, the black hole, and the observer on Earth are aligned in a straight line.
The research team, led by Masahiro Takada, Hiroko Niikura and Naoki Yasuda of the Kavli Institute for Physics and Mathematics of the Universe, used the Hyper Suprime-Cam on the Japanese Subaru Telescope located in Hawaii to search for any evidence of primordial black holes between Earth. and the Andromeda galaxy using the gravitational lensing technique.
Looking for the needle
Experts continued to observe the entire disk of the Andromeda galaxy, which is made up of hundreds of thousands of stars.
“The investigation is delicate and complicated. The eclipsed star can be illuminated for a period of a few minutes or a few hours due to gravitational lensing, which requires the team to take multiple images to capture the flicker, if applicable, ”comments Anupreeta More, a member of the team.
During an entire night, the research team made 190 consecutive images of the Andromeda galaxy. “If the universe is full of small, invisible primordial black holes, with masses lighter than the Moon, as Stephen Hawking postulates , we should have seen at least 1,000 gravitational lensing events. We saw at most one such candidate event, if not none. This implies that Professor Stephen Hawking’s theory that such black holes make up all dark matter is wrong, “Anupreeta concludes.
“ Previous studies had already ruled out the existence of a large number of primordial black holes that could vary in size from the mass of the Moon to about 10 solar masses. The present study rules out the presence of primordial black holes with masses comparable to the Moon and sizes of about 0.1 millimeters as well. This implies that, in the best of cases, primordial lunar-mass black holes can contribute 0.1% of the entire dark matter mass “, explains Surhud More, co-author of the work collected in the journal Nature Astronomy.
Referencia: Microlensing constraints on primordial black holes with Subaru/HSC Andromeda observations. Nature Astronomy (2019) DOI: https://doi.org/10.1038/s41550-019-0723-1
