New research challenges black holes as an explanation for dark matter

Gravitational wave detectors LIGO and Virgo have discovered a large number of black holes whose origin is one of the great mysteries in modern astronomy. According to one hypothesis, these objects may have formed in the very early universe and may include dark matter, the mysterious substance that fills the universe.

A team of scientists from the OGLE (Optical Gravitational Lensing Experiment) from the Astronomical Observatory of the University of Warsaw have announced the results of a nearly 20-year study showing that such massive holes can contain at least a few percent of darkness. matter. Therefore, other explanations are needed for the sources of gravitational waves. The results of the study were published in the study in Nature and research in Astrophysical Journal Supplement Series.

Various astronomical observations show that ordinary matter, which we can see or touch, comprises only 5% of the total mass and energy budget of the universe. In the Milky Way, for every 1 kg of normal matter in the stars, there are 15 kg of dark matter, which does not emit any light and only interacts through its own gravity.

“The nature of dark matter remains a mystery. Many scientists think it has unknown elementary particles,” says Dr. Przemek Mr.óz from the Academy of Astronomical Observatory, University of Warsaw, lead author of both articles. “Unfortunately, despite decades of effort, no experiments (including those conducted by the Large Hadron Collider) have found new particles that could be responsible for dark matter.”

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 found that the black holes discovered by LIGO and Virgo are typically more massive (20–100 solar masses) than those 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. Mr. óz.

One possible explanation suggests that the LIGO and Virgo detectors have detected a number of primordial black holes that may have formed in the very early universe. Their existence was first suggested more than 50 years ago by the British theoretical physicist Stephen Hawking, and independently, by the Soviet physicist Yakov Zeldovich.

“We know that the early universe was not homogeneous – small changes in density gave rise to the current constellations and constellations,” says Dr. Mr. óz. “The same density changes, if they exceed the critical density difference, can collapse and form black holes.”

Since the first detection of gravitational waves, more and more scientists have been speculating that these primordial black holes may contain most, if not all, of the dark matter.

Fortunately, this hypothesis can be confirmed by astronomical observations. We see that a large amount of dark matter is present in the Milky Way. If it was formed by 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 can be bent and deflected in the gravitational field of larger objects, a phenomenon called gravitational microlensing.

“Microlensing occurs when three elements – the observer on Earth, the light source, and the lens – almost perfectly in space,” says Prof. Andrzej Udalski, chief investigator of the OGLE investigation. “During the dimming event, the source light can be deflected and magnified, and we see the temporary brightness of the source light.”

The exposure time depends on the mass of the lens object: the higher the mass, the longer the event. Mass accretion events typically last several weeks, while those of black holes that are 100 times more massive than the sun can 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 Polish space scientist Bohdan Paczyński. His idea led to the start of three major experiments: the Polish OGLE, the American MACHO, and the French EROS. The first results from these experiments showed that black holes with masses less than one solar mass may contain less than 10% of the dark matter. These observations, however, were not sensitive to very long-period lensing events, and therefore, not sensitive to supermassive black holes, similar to those recently detected by gravitational wave detectors.

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

“This data set provides the longest, largest, and most accurate survey of stars in the Large Magellanic Cloud in the history of modern astronomy,” says Prof. Udalsky.

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

“If the entire dark matter in the Milky Way was composed of black holes of 10 solar masses, we should detect 258 microlensing events,” says Dr. Mr. óz. “For 100 solar black holes, we expected 99 dimming events. For 1,000 solar black holes—27 dimming events.”

In contrast, OGLE astronomers have found only 13 events of accelerometers. Their detailed analysis shows that they can all be explained by the known number of stars in the Milky Way or the Large Magellanic Cloud itself, not by black holes.

“That suggests that supermassive black holes can compose at least a few percent of the dark matter,” says Dr. Mr. óz.

Detailed calculations show that black holes of 10 solar masses may comprise at least 1.2% of the dark matter, black holes of 100 solar masses – 3.0% of the dark matter, and black holes of 1,000 solar masses – 11% of the dark matter.

“Our observations show that primordial black holes cannot contain a large part of dark matter, and at the same time, it explains the black hole merger rates measured by LIGO and Virgo,” says Prof. Udalsky.

Therefore, another explanation is needed for the supermassive black holes discovered by LIGO and Virgo. According to one theory, they formed as a product of the evolution of massive, low-metallicity stars. Another possibility involves the merger of less massive objects into dense interstellar environments, such as globular clusters.

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