New research challenges black holes as dark matter explanation

Recent findings from the gravitational wave detectors, LIGO and Virgo, have raised doubts about the role of black holes in explaining dark matter. These detectors have detected a population of massive black holes, but their origin remains a significant mystery in the field of astronomy. One proposed hypothesis suggests that these black holes might have formed in the early stages of the universe and potentially include dark matter, a mysterious substance that pervades the cosmos. This was reported by SSPDaily.

A team of scientists from the OGLE survey, carried out by the Astronomical Observatory of the University of Warsaw, has conducted nearly two decades of observations. Their results indicate that these massive black holes might, at most, comprise only a small percentage of dark matter. Consequently, an alternative explanation is required to account for gravitational wave sources. These research findings were recently published in both Nature and The Astrophysical Journal Supplement Series.

Various astronomical observations have pointed out that ordinary matter, the matter we can perceive, accounts for a mere 5% of the total mass and energy in the universe. In the Milky Way, for every kilogram of ordinary matter in stars, there are 15 kilograms of dark matter, a substance that lacks light emissions and only interacts through gravitational forces.

Dr. Przemek Mr.óz, the lead author of the aforementioned studies, explains, "The nature of dark matter remains a mystery. Most scientists believe it consists of unknown elementary particles. However, despite many years of research, no experiment, including those conducted with the Large Hadron Collider, has yielded evidence of these particles being responsible for dark matter."

Since the initial detection of gravitational waves from merging black holes in 2015, more than 90 such events have been recorded by LIGO and Virgo. Researchers have observed that the black holes detected by these experiments are considerably more massive, ranging from 20 to 100 times the mass of the sun, compared to those previously known in the Milky Way, which typically have masses of 5 to 20 times that of the sun.

Dr. Mr.óz states, "Explaining the substantial differences between these two populations of black holes remains a fundamental enigma in modern astronomy."

One possible explanation suggests that the LIGO and Virgo detectors have unveiled a population of primordial black holes that could have formed during the early stages of the universe. This theory was initially proposed over 50 years ago by British theoretical physicist Stephen Hawking and independently by Soviet physicist Yakov Zeldovich.

Dr. Mr.óz elaborates, "The early universe was not homogenous, and small density fluctuations led to the formation of present-day galaxies and galaxy clusters. Similar fluctuations, if they surpass a critical density contrast, could collapse and give rise to black holes."

To verify this hypothesis, scientists rely on astronomical observations. The substantial presence of dark matter in the Milky Way should be detectable if it consists of black holes. Even though black holes do not emit visible light, their effect on light can be detected through a phenomenon known as gravitational microlensing.

Prof. Andrzej Udalski, the principal investigator of the OGLE survey, explains, "Microlensing occurs when the observer on Earth, a source of light, and a lens are perfectly aligned in space. This alignment causes the light from the source to be deflected and magnified, resulting in a temporary brightening of the source."

The duration of this brightening depends on the mass of the lensing object. Objects with higher mass induce longer-lasting events. While microlensing by solar mass objects typically lasts a few weeks, events caused by black holes that are 100 times more massive than the sun could endure for several years.

The idea of utilizing gravitational microlensing to study dark matter was introduced by Polish astrophysicist Bohdan Paczyński in the 1980s. His notion motivated the launch of three major experiments: the Polish OGLE, American MACHO, and French EROS projects. Initial findings from these trials indicated that black holes, which are smaller than one solar mass, may account for less than 10% of dark matter. However, these observations were not keen on detecting extremely long-timescale microlensing events associated with massive black holes like those recently detected by gravitational-wave experiments.

In their recent publication in The Astrophysical Journal Supplement Series, the researchers from the OGLE survey present results from nearly two decades of photometric monitoring on nearly 80 million stars located in the neighboring galaxy, the Large Magellanic Cloud. These studies aimed to identify gravitational microlensing events. The analyzed data covers the third and fourth phases of the OGLE project, spanning from 2001 to 2020.

Prof. Udalski highlights, "This dataset constitutes the most extensive, accurate photometric observations of stars in the Large Magellanic Cloud ever achieved in modern astronomy."

The second article, published in Nature, delves into the astrophysical implications of their findings.

Dr. Mr.óz states, "Considering black holes of 10 solar masses, if the entire Milky Way dark matter comprised such black holes, we should have detected 258 microlensing events. Likewise, we anticipated 99 events if these black holes had a mass of 100 times that of the sun. For 1,000 solar mass black holes, we would expect 27 events."

In contrast to these predictions, the astronomers from the OGLE project discovered only 13 microlensing events. Their thorough analysis reveals that all of these events can be fully explained by the known stellar populations in the Milky Way or the Large Magellanic Cloud itself, ruling out black holes as the cause.

Dr. Mr.óz concludes, "This suggests that massive black holes can constitute, at most, only a small fraction of the dark matter—around 1.2% for black holes of 10 solar masses, 3.0% for black holes of 100 solar masses, and 11% for black holes of 1,000 solar masses."

Prof. Udalski remarks, "Our observations indicate that primordial black holes cannot account for a significant portion of dark matter while simultaneously explaining the measured black hole merger rates detected by LIGO and Virgo."

As a consequence, alternative explanations must be sought out to clarify the origin of the massive black holes detected by LIGO and Virgo. One possible hypothesis involves their formation as a result of the evolution of massive, low-metallicity stars. Another proposed scenario is the merging of smaller objects in dense stellar environments like globular clusters.

Undoubtedly, these research findings will leave a lasting impact on the field of astronomy and become a valuable addition to textbooks for years to come.