A statistical analysis of 740 supernova explosions showed that black holes can account for no more than 40 percent of the volume of dark matter in the Universe, which in turn drives another nail into the coffin of the theory of massive astrophysical compact halo objects. According to this theory, primordial black holes may be the source of dark matter. The observation of two American scientists from the University of California at Berkeley casts doubt on this theory.
In February 2016, scientists at the Laser Interferometric Gravitational Wave Observatory (LIGO) announced a new era in astronomy. Researchers have discovered for the first time predicted gravitational waves created by a pair of colliding black holes. Aside from the amazing nature of the discovery itself, the discovery of gravitational waves has revived the old theory that dark matter is a derivative of massive astrophysical compact halo objects (MACHOs), ultra-dense objects that do not emit light.
According to modern assumptions, dark matter can account for up to 85 percent of the volume of all matter in the Universe, but physicists have not yet discovered this matter, so they do not know what it is. The topic of the existence of dark matter attracted active discussion around itself after the American astronomer Vera Rubin in the 70s, studying the rotation curves of galaxies, revealed discrepancies between the predicted circular motion of galaxies and the observed motion (stars at the edge of galaxies should rotate slower than those that are closer to the galactic center, but observation showed that the speed of rotation of the outer and inner stars was actually the same). This fact, known as the "galaxy rotation problem", has become one of the main pieces of evidence for the existence of dark matter. However, the question of whetherwhat dark matter is still remained and remains open.
Over the next several decades, many candidates have been proposed for the role of dark matter. Today, the most popular are particles such as axions or weakly interacting particles. However, objects (in particular black holes) proposed several decades earlier by the MACHO theory were considered as the main source of dark matter. According to this theory, dark matter is actually made up of baryonic particles (particles of ordinary matter that can be seen) moving in interstellar space, being not associated with any planetary system and practically (or completely) not emitting any energy. According to the theory, MACHO may represent neutron stars, brown dwarfs, orphan planets and primordial black holes that appeared shortly after the Big Bang.
In the 90s, the theory of MACHO objects went out of fashion. Scientists have focused their search for the source of dark matter in particles, but the recent discovery of LIGO has reignited interest in black holes as a possible explanation for invisible dark matter.
Since MACHO objects, according to the theory, do not emit any energy, for the observer these objects will be "dark", that is, invisible. Based on this, the researchers expected to detect them using the effect of gravitational microlensing. This is the phenomenon of curvature of light waves of the observed object in relation to the observer due to the very powerful gravitational field of very dense and massive objects located between the observed object and the observer. This effect can significantly increase the brightness of stars very distant from us and allow us to see those objects that cannot be seen by ordinary traditional methods of observation. The role of gravitational lenses can be played, for example, by galaxies, galactic clusters, and also black holes.
Physicists Miguel Tsumalakraregi and Urosh Selyak of the University of California, Berkeley have performed sophisticated data analyzes from 740 supernova explosions - extremely bright explosions of stars - to trace the contribution of primordial black holes to the curvature and amplification of supernova light. Supernova explosions are often used by astronomers to measure distances in the universe, because these objects have incredible brightness, which decreases very slowly, allowing calculations to be made. The research is published in the journal Physical Review Letters.
Scientists assumed that a deviation in brightness of several tenths of a percent, indicating the effect of microlensing on black holes and explained by the mass of invisible dark matter, would be found in at least 8 of the 740 observed supernovae. However, scientists have never found a single deviation indicating microlensing on a black hole.
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The findings of the study do not exclude black holes as sources of dark matter, but significantly limit their contribution to its volume inside the Universe. It is estimated that even if black holes do contribute to the phenomena that are associated with dark matter, then it is no more than 40 percent. According to the authors, they already have and have not yet published the results of a more complete analysis, which covered more than 1,000 supernovae and forces them to further lower this figure - to a maximum of 23 percent.
“We are back to normal discussions again. What is dark matter? It looks like we're out of good options. This is a challenge for the next generations,”says Professor Urog Selyak.