New DelhiAn international research team, including two scientists at the Interuniversity Center for Astronomy and Astrophysics (IUCAA), Pune, ruled out the possibility that primordial black holes are one of the main constituents of dark matter. The discovery also refutes an assertion Stephen Hawking made.
In the Solar System, Mercury – the innermost planet – takes only 88 days to complete a revolution around the Sun. Neptune – the outermost planet – takes 165 years for the same journey. Likewise, the laws of gravity await us to see stars closer to the center of a galaxy, going faster than stars at the edge. However, in most galaxies, both the innermost and outermost stars take almost the same time to complete a 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 boost 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 a form of matter because it seems to exert a gravitational pull, and is "dark" because it does not seem to interact with light, or with any part of the electromagnetic spectrum. Scientists have discovered in their detailed research on the cosmos that almost 85% of the total mass of the universe is just the mass of dark matter. Thus the stars and galaxies that we see around us are only the collective point of the iceberg of matter; much remains out of sight and all of this is dark matter.
Get into primordial black holes
Cosmologists have come up with several ideas to explain dark matter. Some think that it can be composed of neutrinos, which are particles that have no charge (so they do not interact with light) and have a small mass (and interact with gravity). Some others postulated that dark matter could be made of new types of elementary particles – massive particles of weak interaction or massive particles that interact gravitationally, and which have not yet been detected.
After the Big Bang hypothesis was launched, two Soviet physicists named Yakov Zel? Dovich and Igor Novikov showed that shortly after the birth of our universe he must have contained large amounts of energy so densely together that he should have created small black particles. holes. These were called primordial black holes.
Hawking investigated them (in theory) in 1971, and calculated that the mass of each primordial black hole could range from a hundredth of a milligram 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, the star's rays of light will curl around the black hole due to an effect called a gravitational lens. As a result, the star will look brighter than it really is for a short time. The gravitational lens 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 illuminates the light of a galaxy passing behind it, making the latter appear brighter and more distorted. Credit: ESO / M. Kornmesser
The research team, led by Masahiro Takada, Hiroko Niikura and Naoki Yasuda of the Kavli Institute of Physics and Mathematics of the Universe, used the Hyper Suprime-Cam instrument on the Subaru Telescope located in Hawaii. They searched for evidence of primordial black holes between Earth and the Andromeda Galaxy using gravitational lenses.
Looking for flash
It would be absurd – like looking for a needle in a haystack – staring at a star and hoping that at some point or another an invisible primordial black hole would eclipse it. Instead, the research team continuously observed the entire disk of the Andromeda galaxy, made up of thousands of stars. In this way, there was a chance that some primordial black holes – if they existed in sufficient numbers – eclipsed one of the stars in a perceptible way.
"The investigation is delicate and complicated. The eclipsed star may light up for a few minutes or a few hours due to gravitational lenses, requiring the team to take multiple images to catch the flicker, if any, "Anupreeta More, a member of the IUCAA team and researcher, said. India Science Wire.
So all night long, the research team took 190 images of the Andromeda galaxy. "If the universe is full of invisible, tiny, black holes with lighter masses than the Moon … then we should have seen at least a thousand gravitational lens events," said More. "We saw at most one of these candidate events, if not one. This implies that Hawking's theory – that these black holes make up all dark matter – is wrong. "
"Previous studies have ruled out a large number of primordial black holes that can range in size from the mass of the Moon to about 10 solar masses," said Surhud More, another member of the research team. "The present study excludes the presence of primordial black holes with masses comparable to the moon and sizes of about 0.1 millimeter as well. This implies that, at best, the primordial black holes of the lunar mass may contribute 0.1% of the entire mass of dark matter. "
His study was published in the journal Astronomy of Nature on April 1, 2019.
T.V. Venkateswaran writes for India Science Wire and tweets on @TVVen.