Gravitational Waves Provide New Clues to the Composition of Dark Matter

Exploring Dark Matter With Microlensing Techniques

Fortunately, this hypothesis can be verified with astronomical observations. We observe that copious amounts of dark matter exist in the Milky Way. If it were composed of black holes, we should be able to detect them in our cosmic neighborhood. Is this possible, given that black holes do not emit any detectable light?

According to Einstein’s theory of general relativity, light may 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 source of light, and a lens – virtually ideally align in space,” says Prof. Andrzej Udalski, the principal investigator of the OGLE survey. “During a microlensing event, the source’s light may be deflected and magnified, and we observe a temporary brightening of the source’s light.”

The duration of the brightening depends on the mass of the lensing object: the higher the mass, the longer the event. Microlensing events by solar mass objects typically last several weeks, whereas those by black holes that are 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: Polish OGLE, American MACHO, and French EROS. The first results from these experiments demonstrated that black holes less massive than one solar mass may comprise less than 10 percent of dark matter. These observations were not, however, sensitive to extremely long-timescale microlensing events and, therefore, not sensitive to massive black holes, similar to those recently detected with gravitational-wave detectors.

Night over the Las Campanas Observatory in Chile (operated by the Carnegie Institution for Science). The OGLE project observing station and the Large and Small Magellanic Clouds. Credit: Krzysztof Ulaczyk

Long-Term Observational Studies by OGLE

In the new article in the Astrophysical Journal Supplement Series, OGLE astronomers present the results of nearly 20-year-long photometric monitoring of almost 80 million stars located in a nearby galaxy, called the Large Magellanic Cloud, and the searches for gravitational microlensing events. The analyzed data were collected during the third and fourth phases of the OGLE project from 2001 to 2020.

“This data set 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 Nature, discusses the astrophysical consequences of the findings.

“If the entire dark matter in the Milky Way was composed of black holes of 10 solar masses, 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 have found only 13 microlensing events. Their detailed analysis demonstrates that all of them can be explained by the known stellar populations in the Milky Way or the Large Magellanic Cloud itself, not by black holes.

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

The detailed calculations demonstrate that black holes of 10 solar masses may comprise at most 1.2% of dark matter, 100 solar mass black holes – 3.0% of dark matter, and 1000 solar mass black holes – 11% of dark matter.

“Our observations indicate that primordial black holes cannot comprise a significant fraction of the dark matter and, simultaneously, explain the observed DOI: 10.1038/s41586-024-07704-6

Reference: “Microlensing Optical Depth and Event Rate toward the Large Magellanic Cloud Based on 20 yr 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, 24 June 2024, The Astrophysical Journal Supplement Series.
DOI: 10.3847/1538-4365/ad452e