Black Holes

There may be tens of millions of black holes in the Milky Way – regions from which, due to the force of gravity, nothing can escape. These are objects where an enormous mass is concentrated within a very small volume. They continue to fascinate not only specialists but also the general public. Thanks to the OGLE project, it has become possible to locate and study them. Researchers from the Astronomical Observatory of the University of Warsaw have investigated whether dark matter could be made up of black holes and have also attempted to find elusive examples of these phenomena – so-called isolated black holes.

Ordinary matter (called baryonic), which we can see or touch, makes up only 16% of the total mass of the Universe. The remaining 84% exists in the form of so-called dark matter, which interacts only through gravity. In recent years, the hypothesis that dark matter might consist of black holes of various masses has gained popularity. Scientists at the Astronomical Observatory of the University of Warsaw disproved this hypothesis, showing that only a tiny fraction of dark matter could be composed of black holes.

If dark matter were indeed made of black holes, we should be able to detect them in our immediate cosmic neighborhood. But this cannot be done directly – after all, black holes do not shine. Nothing, not even light, can escape from them. To test the hypothesis, researchers analyzed two decades of photometric observations collected by OGLE of nearly 80 million stars in a neighboring galaxy, the Large Magellanic Cloud. If all the dark matter in the Milky Way were in the form of black holes, they would have detected several hundred gravitational microlensing events caused by them. This effect occurs when the light of a distant star is bent and amplified in the gravitational field of a lensing object (such as a black hole). In reality, only 13 such events were found, which can be explained by the presence of ordinary cosmic objects, such as stars, rather than black holes.

Some black holes, if bound in a binary system with a normal star, can be detected through observations of the companion star. However, it is believed that most black holes are isolated and thus escape traditional methods of detection. They can, however, be revealed through the method of gravitational microlensing. Scientists from the Astronomical Observatory of the University of Warsaw contributed to the discovery of the first such object in the Milky Way. This was made possible by combining photometric data collected by OGLE with precise measurements of the star’s position in the sky obtained by the Hubble Space Telescope.