New research challenges black holes as an explanation for dark matter

The LIGO and Virgo gravitational wave detectors have detected a population of massive black holes whose origin constitutes one of the greatest mysteries of modern astronomy. One hypothesis is that these objects may have formed at the very beginning of the universe and may contain dark matter, a mysterious substance that fills the universe.

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

Various astronomical observations indicate that ordinary matter, which we can see or touch, represents only 5% of the total mass and energy budget of the universe. In the Milky Way, for every 1 kg of ordinary matter in stars, there are 15 kg of dark matter, which emits no light and only interacts through its gravitational attraction.

“The nature of dark matter remains a mystery. Most scientists believe that it is composed of unknown elementary particles,” explains Dr. Przemek Mr.óz of the Astronomical Observatory of the University of Warsaw, lead author of both items. “Unfortunately, despite decades of effort, no experiments (including experiments 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 pair of merging 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 are generally significantly more massive (20 to 100 solar masses) than those previously known in the Milky Way (5 to 20 solar masses).

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

One possible explanation posits that the LIGO and Virgo detectors discovered a population of primordial black holes that may have formed in the very beginning of the universe. Their existence was first proposed 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 ideally homogeneous: small fluctuations in density gave rise to today’s galaxies and galaxy clusters,” explains 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 these primordial black holes could contain a significant fraction, if not all, of dark matter.

Fortunately, this hypothesis can be verified through astronomical observations. We observe that there are large amounts of dark matter in the Milky Way. If it were composed of black holes, we could detect them in our cosmic neighborhood. Is this possible, given that black holes emit no 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) align in a virtually ideal way in space,” explains Professor Andrzej Udalski, principal investigator of the OGLE investigation. “During a microlensing event, the light from the source can be deflected 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 caused by solar-mass objects typically last several weeks, while those caused by 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 launch of three major experiments: the Polish OGLE, the American MACHO and the French EROS. The first results of these experiments demonstrated that black holes less massive than a solar mass could contain less than 10% dark matter. These observations were, however, not sensitive to very long-scale microlensing events and, hence, massive black holes, similar to those recently detected with gravitational wave detectors.

In the new article of The Astrophysical Journal Supplement Series, OGLE astronomers present the results of a nearly 20-year photometric monitoring of nearly 80 million stars located in a nearby galaxy, called the Large Magellanic Cloud, and the search for microlensing events gravitational. 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 precise photometric observations of the Large Magellanic Cloud stars in the history of modern astronomy,” explains Professor Udalski.

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

“If all the dark matter in the Milky Way was composed of black holes of 10 solar masses, we should have detected 258 microlensing events,” says Dr. Mr.óz. “For 100 solar-mass black holes, we expected 99 microlensing events. For 1,000 solar-mass black holes, 27 microlensing events.”

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

“This indicates that massive black holes can only constitute a few percent of dark matter,” explains Dr. Mr.óz.

Detailed calculations demonstrate that black holes of 10 solar masses can contain at most 1.2% dark matter, 100 black holes of solar mass – 3.0% dark matter and 1000 black holes of solar mass – 11% dark matter. black matter.

“Our observations indicate that primordial black holes cannot constitute a significant fraction of dark matter and simultaneously explain the observed black hole merger rates measured by LIGO and Virgo,” explains Professor Udalski.

Therefore, other explanations are needed for the massive black holes detected by LIGO and Virgo. According to one hypothesis, they were formed following the evolution of massive stars with low metallicity. Another possibility involves mergers of less massive objects in dense stellar environments, such as globular clusters.

“Our results will remain in astronomy textbooks for decades,” adds Professor Udalski.