The LIGO and Virgo gravitational wave detectors have discovered a cluster of massive black holes, whose origins are one of the greatest mysteries in modern astronomy. One hypothesis is that these objects formed very early in the universe and may contain dark matter, the mysterious substance that fills the universe.

A team of scientists from the OGLE (Optical Gravitational Lensing Experiment) survey at the Warsaw University Observatory presented the results of nearly 20 years of observations showing that such massive black holes may contain at most a few percent of dark matter. Therefore, another explanation for the source of gravitational waves is required. The results were published in a study in the journal Nature and in a study in the Astrophysical Journal Supplement Series. According to various astronomical observations, ordinary matter that we can see and touch accounts for only 5% of the total mass and energy balance of the universe. In the Milky Way galaxy, there are 15 kilograms of dark matter for every kilogram of ordinary matter in a star. Dark matter does not emit light and interacts only through gravity. “The nature of dark matter remains a mystery. Most scientists believe that dark matter is composed of unknown elementary particles,” said Dr. Przemek Mr.óz of the Warsaw University Astronomical Observatory, lead author of both articles. “Unfortunately, despite decades of efforts, no new particles that could be responsible for dark matter have been found in any experiments (including at the Large Hadron Collider).”

Since the first detection of gravitational waves from merging black holes in 2015, the LIGO and Virgo experiments have recorded such events more than 90 times. Astronomers have found that the black holes discovered by LIGO and Virgo are typically much more massive (20 to 100 solar masses) than previously known black holes in the Milky Way (5 to 20 solar masses). “Explaining why these two black hole populations are so different is one of the greatest mysteries in modern astronomy,” says Dr. Oz. One possible explanation suggests that the LIGO and Virgo detectors have found a population of primordial black holes that may have formed very early in the universe. Their existence was first proposed more than 50 years ago by British theoretical physicist Stephen Hawking and independently by Soviet physicist Yakov Zel’dovich. “We know that the early universe was not ideally homogeneous. Small fluctuations in density led to the formation of today’s galaxies and galaxy clusters,” says Dr. Oz. “If similar density fluctuations exceeded a critical density contrast, they could collapse and form black holes.” Since gravitational waves were first discovered, more and more scientists have speculated that such primordial black holes could account for a significant portion, if not all, of dark matter. Fortunately, this hypothesis can be confirmed by astronomical observations. It has been observed that there is a large amount of dark matter in the Milky Way galaxy. If it consists of black holes, we should be able to detect them in our nearby universe. Is this possible, since black holes don’t emit detectable light?

According to Einstein’s theory of general relativity, light can be diffracted and deflected in the gravitational field of a massive object, a phenomenon called gravitational microlensing. “Microlensing occurs when three objects are aligned almost ideally in space: an observer on Earth, a light source, and a lens,” says Professor Andrey Udalsky, lead investigator of the OGLE expedition. “During microlensing, the light from the light source can be deflected and amplified, and we observe a temporary brightening of the light from the source.” The duration of the brightening depends on the mass of the lensed object. The more massive it is, the longer the phenomenon lasts. Microlensing from solar-mass objects usually lasts for a few weeks, while microlensing from a black hole with a mass 100 times that of the Sun takes several years. The idea of ​​studying dark matter with the help of gravitational microlensing is not new. It was first proposed in the 1980s by Polish astrophysicist Bohdan Paczynski. His idea inspired the launch of three major experiments: OGLE in Poland, MACHO in the United States, and EROS in France. The first results of these experiments showed that black holes with less than the solar mass could account for less than 10% of dark matter. However, these observations were not affected by very long-period microlensing and therefore not by massive black holes as recently discovered by gravitational wave detectors.

In a new paper in The Astrophysical Journal Supplement Series, OGLE astronomers present the results of nearly 20 years of photometrically monitoring about 80 million stars in the Large Magellanic Cloud, a nearby galaxy, and searching for gravitational microlensing. The data analyzed were collected during the third and fourth phases of the OGLE project, from 2001 to 2020.

The second paper, published in Nature, discusses the astrophysical implications of the results. “If all the dark matter in the Milky Way consisted of 10 solar-mass black holes, we would have had to detect 258 microlensing events,” says Dr. Oz. “For 100 solar-mass black holes, we expected 99 microlensing events. For 1,000 solar-mass black holes – 27 microlensing events.” In contrast, OGLE astronomers found only 13 microlensing events. Their detailed analysis shows that they can all be explained not by black holes, but by known populations of stars in the Milky Way or the Large Magellanic Cloud itself. “This suggests that massive black holes could account for at most a few percent of dark matter,” says Dr. Oz. Detailed calculations suggest that a black hole with 10 solar masses could account for up to 1.2% of dark matter, a black hole with 100 solar masses could account for 3.0% of dark matter, and a black hole with 1,000 solar masses could account for 11% of dark matter. dark matter. “Our observations show that primordial black holes cannot account for a significant fraction of dark matter, and at the same time explain the observed black hole merger rates measured by LIGO and Virgo,” says Professor Udalsky. Therefore, another explanation is needed for the massive black holes discovered by LIGO and Virgo. According to one hypothesis, they formed as a product of the evolution of massive stars with low metallicity. Another possibility is the merger of less massive objects in dense stellar environments such as globular clusters. “Our results will be published in astronomy textbooks for decades to come,” Prof. Udalsky added.

More information: Przemek Mróz et al, No massive black holes in the Milky Way halo, Nature (2024). DOI: 10.1038/s41586-024-07704-6. www.nature.com/articles/s41586-024-07704-6. On arXiv: DOI: 10.48550/arxiv.2403.02386

Przemek Mróz et al, Microlensing Optical Depth and Event Rate toward the Large Magellanic Cloud Based on 20 yr of OGLE Observations, The Astrophysical Journal Supplement Series (2024). DOI: 10.3847/1538-4365/ad452e