New research challenges black holes as an explanation for dark matter

The gravitational wave detectors LIGO and Virgo have detected 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 contain dark matter, a mysterious substance that fills the universe.

A team of scientists from the OGLE (Optical Gravitational Lensing Experiment) study at the University of Warsaw Astronomical Observatory has released the results of nearly two decades of observations that indicate that such massive black holes represent at most a few percent of dark holes can determine. matter. Another explanation is therefore needed for gravitational wave sources. The results of the research have been published in a study in Nature and a study in it The Astrophysical Journal Supplement Series.

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 Way, for every kg of ordinary matter in stars, there is 15 kg of dark matter, which does not emit light and interacts only through its gravity.

‘The nature of dark matter remains a mystery. Most scientists think it is made up of unknown elementary particles,” says Dr. Przemek Mr.óz from the Astronomical Observatory of the University of Warsaw, 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.”

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 tend to be significantly more massive (20-100 solar masses) than the black holes previously known in the Milky Way (5-20 solar masses).

“Explaining why these two black hole populations are so different is one of the greatest mysteries of modern astronomy,” says Dr. Mr.óz.

One possible explanation holds that LIGO and Virgo detectors have revealed a population of primordial black holes that may have formed in the very early universe. Their existence was first proposed over fifty 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 gave rise to current galaxies and galaxy clusters,” says Dr. Mr.óz. “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.

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,” says 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 objects with the mass of the Sun typically last several weeks, while those from black holes 100 times 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 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 a mass less than one solar mass can consist of less than 10% dark matter. However, these observations were not sensitive to microlensing events on extremely long timescales, and therefore not sensitive to massive black holes, similar to those recently detected with gravitational wave detectors.

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.

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

“If the entire dark matter in the Milky Way consisted of black holes with a mass of 10 solar masses, we would have had to detect 258 microlensing events,” says Dr. Mr.óz. “For 100 black holes with the mass of the Sun, we expected 99 microlensing events. For 1000 black holes with the mass of the Sun – 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 can only account for a few percent of dark matter at most,” says Dr. Mr.óz.

The detailed calculations show that black holes with a mass of 10 solar masses can comprise up to 1.2% of the dark matter, 100 black holes of the solar mass – 3.0% of the dark matter, and 1,000 black holes of the solar mass – 11% of dark matter.

“Our observations indicate that primordial black holes cannot constitute a significant part of the dark matter, and at the same time can 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 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.