New research challenges black holes as an explanation for dark matter

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

A team of scientists from the OGLE survey (Optic Gravitational Lensing Experiment) from the Astronomical Observatory of the University of Warsaw have announced the results of nearly 20 years of observations that show that such massive black holes can make up at most a few percent of darkness. matter. Another explanation is therefore needed for the sources of gravitational waves. The results of the research were published in a study in Nature and a study in Astrophysical Journal Supplement Series.

Various astronomical observations show that ordinary matter, which we can see or touch, makes up only 5% of the total mass and energy budget of the universe. In the Milky Way, for every 1 kg of ordinary matter in the star, there is 15 kg of dark matter, which emits no light and interacts only by its gravitational pull.

“The nature of dark matter remains a mystery. Most scientists think it consists of unknown elementary particles,” says Dr. Przemek Mr.óz from the Astronomical Observatory, University of Warsaw, lead author of both articles. “Unfortunately, despite decades of effort, no experiments (including experiments conducted with 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 noted that the black holes detected by LIGO and Virgo are typically significantly 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 postulates that the LIGO and Virgo detectors have detected a population of primordial black holes that may have formed in the very early universe. Their existence was first proposed over 50 years ago by British theoretical physicist Stephen Hawking, and independently, by Soviet physicist Yakov Zeldovich.

“We know that the early universe was not ideally homogeneous – small fluctuations in density created the present galaxies and galaxy clusters,” says Dr. Mr. óz. “Similar density fluctuations, if they exceed a critical density contrast, can collapse and form black holes.”

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

Fortunately, this hypothesis can be verified by astronomical observations. We observe that large 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 detectable light?

According to Einstein’s theory of general 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,” says Prof. Andrzej Udalski, principal investigator of the OGLE survey. “During a microlensing event, the source light can be deflected and magnified, and we observe a temporary brightening of the source light.”

The duration of the illumination depends on the mass of the lensed object: the higher the mass, the longer the event. Microlensing events from solar-mass objects typically last a few weeks, while those from black holes 100 times more massive than the Sun will 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 astrophysicist Bohdan Paczyński. His idea inspired 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 less massive than a solar mass may make up less than 10% of the dark matter. However, these observations were not sensitive to extremely long microlensing events, and therefore, were not sensitive to massive black holes, similar to those recently discovered with gravitational wave detectors.

In the new article in Astrophysical Journal Supplement Series, OGLE astronomers present the results of nearly 20 years of photometric monitoring of almost 80 million stars located in a nearby galaxy, called the Large Magellanic Cloud, and searches for gravitational microlensing events. The data analyzed 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. Udalsky.

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

“If all the dark matter in the Milky Way was composed of 10-solar-mass black holes, we should have detected 258 microlensing events,” says Dr. Mr. óz. “For 100 solar-mass black holes, we expected 99 microlensing events. For 1000 solar-mass black holes – 27 microlensing events.”

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

“This indicates that massive black holes may make up at most a few percent of dark matter,” says Dr. Mr. óz.

Detailed calculations show that black holes with 10 solar masses can make up 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 show that primordial black holes cannot constitute a significant fraction of dark matter and, at the same time, explain the observed black hole merger rates measured by LIGO and Virgo,” says Prof. Udalsky.

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, low-metallicity stars. Another possibility involves merging less massive objects into dense stellar environments, such as globular clusters.

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