New research questions explanation for black holes as dark matter

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

A team of scientists from the OGLE (Optical Gravitational Lensing Experiment) survey of the University of Warsaw Astronomical Observatory has announced the results of nearly 20 years of observations, which indicate that such massive black holes can contain at most a few percent of dark matter. Therefore, another explanation for gravitational wave sources is needed. The results of the research were published in a study in Nature and a study in The Astrophysical Journal supplement series.

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

“The nature of dark matter remains a mystery. Most scientists believe that it consists of unknown elementary particles,” says Dr. Przemek Mr.óz from the Astronomical Observatory of the University of Warsaw, the lead author of both articles. “Unfortunately, despite decades of efforts, no experiment (including the Large Hadron Collider experiments) has 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 recorded more than 90 such events. Astronomers noticed that the black holes discovered by LIGO and Virgo are typically much 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 greatest mysteries in modern astronomy,” says Dr. Mr. óz.

One possible explanation is that the LIGO and Virgo detectors have discovered a population of primordial black holes that may have formed in the 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 density fluctuations led to the formation of today’s 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 discovery of gravitational waves, more and more scientists have speculated that such primordial black holes could make up a significant part, if not all, of dark matter.

Fortunately, this hypothesis can be confirmed by astronomical observations. We observe that the Milky Way contains large amounts of dark matter. If it were made up of black holes, we should be able to detect them in our cosmic neighborhood. Is this possible, since black holes do not emit any perceptible 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,” says Prof. Andrzej Udalski, the lead researcher of the OGLE survey. “During a microlensing, the light from the 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 lensing object: the higher the mass, the longer the event lasts. Microlensing events of solar-mass objects typically last several weeks, while those of black holes 100 times more massive than the Sun would last several 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 of these experiments showed that black holes with less than one solar mass can account for less than 10% of dark matter. However, these observations were not sensitive to microlensing with extremely long timescales and thus not to massive black holes such as those recently discovered with gravitational wave detectors.

In the new article in The Astrophysical Journal supplement seriesOGLE astronomers present the results of almost 20 years of photometric monitoring of nearly 80 million stars in a nearby galaxy, the Large Magellanic Cloud, and searching 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, most extensive and most precise photometric observations of stars in the Large Magellanic Cloud in the history of modern astronomy,” says Prof. Udalski.

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

“If all the dark matter in the Milky Way consisted of black holes with 10 solar masses, we would have detected 258 microlensing events,” says Dr. Mr. óz. “With 100 solar-mass black holes, we would expect 99 microlensing events. With 1,000 solar-mass black holes, we would expect 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 stellar populations in the Milky Way or Large Magellanic Cloud themselves, and not by black holes.

“This suggests that massive black holes can only account for a few percent of dark matter at most,” says Dr. Mr. óz.

The detailed calculations show that black holes with 10 solar masses can make up a maximum of 1.2% dark matter, 100 black holes with solar masses can make up 3.0% dark matter, and 1,000 black holes with solar masses can make up 11% dark matter.

“Our observations suggest 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 Prof. Udalski.

Therefore, other explanations are needed for the massive black holes discovered by LIGO and Virgo. One hypothesis is that they were formed as a product of the evolution of massive stars with low metallicity. Another possibility is mergers of less massive objects in dense stellar environments such as globular clusters.

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