Black holes and dark revelations: Gravitational waves provide new clues to the composition of dark matter

A population of massive black holes whose origins are one of the greatest mysteries in modern astronomy has been discovered by the LIGO and Virgo gravitational wave detectors.

According to one hypothesis, these objects may have formed in the very early universe and could form dark matter, a mysterious substance that fills the universe. A team of scientists has released the results of nearly two decades of observations that indicate such massive black holes could account for at most a few percent of dark matter. Therefore, another explanation is needed for gravitational wave sources.

The results of the research have been published in two articles, in Nature and the Astrophysical Journal Supplement series. The research was conducted by scientists from the OGLE (Optical Gravitational Lensing Experiment) study at the University of Warsaw Astronomical Observatory.

Insight into the composition of dark matter in the universe

Several astronomical observations indicate that ordinary matter, which we can see or touch, makes up only 5% of the universe’s total mass and energy budget. In the Milky WayFor every pound of ordinary matter in stars, there are 15 pounds of “dark matter,” which does not emit any light and interacts only through its gravity.

“The nature of dark matter remains a mystery. Most scientists think it consists of unknown elementary particles,” said Dr. Przemek Mróz from the University of Warsaw Astronomical Observatory, the lead author of both papers. “Unfortunately, despite decades of effort, no experiment (including experiments conducted at the Large Hadron Collider) has found new particles that could be responsible for dark matter.”

The mystery and potential of primordial black holes

Since the first detection of gravitational waves from a merging pair of black holes in 2015, the LIGO and Virgo experiments have detected more than 90 such events. Astronomers have noticed that the black holes detected by LIGO and Virgo are typically significantly more massive (20-100 solar masses) than black holes previously known in the Milky Way (5-20 solar masses).

“Explaining why these two populations of black holes are so different is one of the great mysteries of modern astronomy,” says Dr. Mróz.

One possible explanation postulates that LIGO and Virgo detectors have uncovered a population of primordial black holes that may have formed in the very early universe. Its existence was first proposed over fifty years ago by the famous British theoretical physicist Stephen Hawking and, independently, by the Soviet physicist Yakov Zeldovich.

“We know that the early universe was not ideally homogeneous – small fluctuations in density led to the formation of current galaxies and galaxy clusters,” says Dr. Mróz. “Similar density fluctuations, if they exceed a critical density contrast, can collapse to form black holes.”

Since the first detection of gravitational waves, more and more scientists have speculated that such primordial black holes could contain a significant portion, if not all, of dark matter.

Research into dark matter with microlensing techniques

Fortunately, this hypothesis can be verified with astronomical observations. We see that there are large amounts of dark matter in the Milky Way. If it were made up of black holes, we should be able to detect them in our cosmic environment. Is this possible, since black holes emit no observable light?

According to Einstein’s general theory of relativity, light can be bent and deflected in the gravitational field of massive objects, a phenomenon called gravitational microlensing.

“Microlensing occurs when three objects – an observer on Earth, a light source and a lens – are almost ideally aligned in space,” said Prof. Andrzej Udalski, the principal investigator of the OGLE study. “During a microlensing event, the light from the source can be diffracted and magnified, and we observe a temporary brightening of the light from the source.”

The duration of the brightening depends on the mass of the lens object: the higher the mass, the longer the event. Microlensing events by Sun-mass objects typically last several weeks, while those from black holes 100 more massive than the Sun would last a few years.

The idea of ​​using gravitational microlensing to study dark matter is not new. It was first proposed in the 1980s by the famous Polish astrophysicist Bohdan Paczyński. His idea inspired the start of three major experiments: the Polish OGLE, the American MACHO and the French EROS. Initial results from these experiments showed that black holes less than one solar mass could make up less than 10 percent of dark matter. However, these observations were not sensitive to microlensing events on extremely long time scales and therefore not sensitive to massive black holes, similar to those recently detected with gravitational wave detectors.

Long-term observational studies by OGLE

In the new article in the Astrophysical journal supplement seriesOGLE astronomers present the results of nearly two decades of photometric monitoring of nearly 80 million stars in a nearby galaxy, the Large Magellanic Cloud, and the search for gravitational microlensing events. The analyzed data were collected during the third and fourth phases of the OGLE project from 2001 to 2020.

“This dataset provides the longest, largest and most accurate photometric observations of stars in the Large Magellanic Cloud in the history of modern astronomy,” says Prof. Udalski.

Implications of recent findings on dark matter

The second article, published in Naturediscusses the astrophysical implications of the findings.

“If all the dark matter in the Milky Way consisted of ten solar mass black holes, we should have detected 258 microlensing events,” says Dr. Mróz. “For 100 solar mass black holes, we expected 99 microlensing events. For 1000 solar mass black holes – 27 microlensing events.”

In contrast, the OGLE astronomers found only 13 microlensing events. Their detailed analysis shows that they can all be explained by the known star populations in the Milky Way or the Large Magellanic Cloud itself, and not by black holes.

“That indicates that massive black holes could account for at most a few percent of dark matter,” says Dr. Mróz.

The detailed calculations show that black holes with a mass of 10 solar masses can contain a maximum of 1.2% dark matter, 100 black holes with our mass – 3.0% dark matter, and 1000 black holes with our mass – 11% dark matter.

“Our observations indicate that primordial black holes cannot constitute a significant portion of the dark matter at the same time as the observed one black hole “merger rates measured by LIGO and Virgo,” says Prof. Udalski.

Therefore, other explanations are needed for the massive black holes detected by LIGO and Virgo. According to one hypothesis, they formed as a product of the evolution of massive stars with low metal content. Another possibility involves the merger of less massive objects in dense stellar environments, such as globular clusters.

“Our results will remain in astronomy books for decades to come,” adds Prof. Udalski.

Reference:

“No huge black holes in the halo of the Milky Way” by Przemek Mróz, Andrzej Udalski, Michał K. Szymański, Igor Soszyński, Łukasz Wyrzykowski, Paweł Pietrukowicz, Szymon Kozłowski, Radosław Poleski, Jan Skowron, Dorota Skowron, Krzysztof Ulaczyk, Mariusz Groma dzki , Krzysztof Rybicki, Patryk Iwanek, Marcin Wrona and Milena Ratajczak, June 24, 2024, Nature.
DOI: 10.1038/s41586-024-07704-6

Reference: “Microlensing optical depth and frequency of events toward the Large Magellanic Cloud based on 20 years of OGLE observations” by Przemek Mróz, Andrzej Udalski, Michał K. Szymański, Mateusz Kapusta, Igor Soszyński, Łukasz Wyrzykowski, Paweł Pietrukowicz , Szymon Kozłowski, Radosław Poleski, Jan Skowron, Dorota Skowron, Krzysztof Ulaczyk, Mariusz Gromadzki, Krzysztof Rybicki, Patryk Iwanek, Marcin Wrona and Milena Ratajczak, June 24, 2024, The Astrophysical Journal Supplement Series.
DOI: 10.3847/1538-4365/ad452e