New research questions black holes as an explanation for dark matter

The gravitational wave detectors LIGO and Virgo have detected a population of massive black holes whose origin is one of the greatest mysteries of 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) survey at the Astronomical Observatory of the University of Warsaw reported the results of nearly 20 years of observations, which suggest that such massive black holes can contain at most a few percent of darkness. mass. A different explanation is therefore needed for gravitational wave sources. The research results were published in a study in Nature and study in The Astrophysical Journal Supplement Series.

Various astronomical observations suggest that ordinary matter that 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 normal matter in the stars, there are 15 kg of dark matter, which emits no light and interacts only through its gravity.

“The nature of dark matter remains a mystery. Most scientists think it consists of unknown elementary particles,” says Dr. Mr. Przemek from the Astronomical Observatory of the University of Warsaw, the main author of both articles. “Unfortunately, despite decades of effort, no experiment (including those performed 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 note 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. Walrus.

One possible explanation posits 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 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 present-day galaxies and galaxy clusters,” says Dr. Walrus. “Such 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 may contain a significant portion, if not all, of dark matter.

Fortunately, this hypothesis can be verified using astronomical observations. We observe that there is a large amount of dark matter in the Milky Way. If it consisted of black holes, we should be able to 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.

“A microlens occurs when three objects – the observer on Earth, the light source and the lens – are practically perfectly aligned in space,” says Prof. Andrzej Udalski, principal investigator of the OGLE survey. “During microlensing, the source light can be deflected and magnified, and we observe a temporary brightening of the source light.”

The brightening time depends on the weight of the lens: the higher the weight, the longer the event. Microlensing for solar-mass objects typically takes a few weeks, while black holes that are 100 times more massive than the Sun would take several years.

The idea of ​​using gravitational microlensing to study dark matter is not new. It was first proposed in the 1980s by the Polish astrophysicist Bohdan Paczyński. His idea inspired the beginning of three big experiments: the Polish OGLE, the American MACHO and the French EROS. The first results from these experiments showed that black holes less massive than one solar mass may contain less than 10% dark matter. However, these observations were not sensitive to extremely long microlensing events and thus were not sensitive to massive black holes, similar to those recently detected with gravitational wave detectors.

In a new article in The Astrophysical Journal Supplement SeriesOGLE astronomers present the results of nearly 20 years of photometric monitoring of nearly 80 million stars located in a nearby galaxy called the Large Magellanic Cloud and the search for gravitational microlenses. The data analyzed were collected during the third and fourth phases of the OGLE project between 2001 and 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 black holes with 10 solar masses, we should detect 258 microlensing events,” says Dr. Walrus. “For 100 solar-mass black holes, we expected 99 microlensing events. For 1,000 solar-mass black holes – 27 microlensing events.”

In contrast, the OGLE astronomers found only 13 microlenses. A detailed analysis of them shows that they can all be explained by known stellar populations in the Milky Way or the Large Magellanic Cloud itself, rather than black holes.

“This suggests that massive black holes can make up at most a few percent of dark matter,” says Dr. Walrus.

Detailed calculations show that 10 solar-mass black holes can contain at most 1.2% of dark matter, 100 solar-mass black holes – 3.0% of dark matter, and 1,000 solar-mass black holes – 11% of dark matter.

“Our observations suggest that primordial black holes cannot form a significant part 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 arose as a product of the evolution of massive, low-metallicity stars. Another possibility is the merger of less massive objects in dense stellar environments such as globular clusters.

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