Recent studies have dealt a major blow to the hypothesis that primordial black holes could be responsible for dark matter, shifting the focus of scientific research to alternative explanations.
Overview of the research
Two pivotal studies have explored the possibility of this original black holeswhich is believed to have formed in the early universe, could be a key component of dark matter. The research, published in Nature and the Astrophysical journal supplement seriesincluded nearly twenty years of observations by scientists from the Optical gravity lens experiment (OGLE) at the Astronomical Observatory of the University of Warsaw.
These long-term observational studies provided an extensive data set, covering the most comprehensive photometric monitoring of stars in the Large Magellanic Cloud to date. “This dataset provides the longest, largest and most accurate photometric observations of stars in the Large Magellanic Cloud in the history of modern astronomy,” said Prof. Andrzej Udalski, principal investigator of the OGLE study.
Microlensing techniques and findings
The OGLE study used gravitational microlensing to detect black holes in the Halo of the Milky Way. According to Einstein’s theory of general relativity, massive objects can bend and magnify light from distant stars, creating a microlensing effect.
The duration of these events depends on the mass of the lensing object, with black holes causing longer lasting effects. Microlensing events Through objects with solar mass usually last several weeks, while those of black holes 100 times more massive than the Sun would last a few years.
Researchers expected to find hundreds microlensing events if original black holes were an important part of it dark matter. However, only 13 events were detected, and detailed analysis showed that these could be explained by known stellar populations rather than black holes.
“If the entire dark matter in the Milky Way consisted of black holes of ten solar masses, we would have had to detect 258 microlensing events,” explains Dr. Przemek Mróz from the Astronomical Observatory of the University of Warsaw. “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.” The large difference between expected and observed events led to the conclusion that massive black holes could account for at most a few percent of the dark matter.
Implications for dark matter research
The findings significantly narrow the role of primordial black holes in the dark matter equation. Black holes of ten solar masses could account for only a small part of the total amount of dark matter in the universe. Detailed calculations show this black holes of 10 solar masses can account for a maximum of 1.2% of it dark matter, 100 solar mass black holes – 3.0%, and 1000 solar mass black holes – 11%. “Our observations indicate that ancient black holes cannot constitute a significant fraction of dark matter and at the same time explain the observed black hole merger rates measured by LIGO and Virgo,” said Prof. Udalski.
This forces scientists to investigate other potential candidates dark matter, such as unknown elementary particles or other exotic objects. Despite decades of research, not a single experiment, including the Large Hadron Collider, has been discovered new particles that could explain dark matter. “The nature of dark matter remains a mystery. Most scientists think it consists of unknown elementary particles,” noted Dr. Mróz op. This continued elusiveness underlines the need for innovative approaches and new technologies to unravel the dark matter puzzle.
Future directions and the role of gravitational waves
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. These detections showed that the black holes detected by LIGO and Virgo are generally significantly more massive (20-100 solar masses) than the black holes previously known in the Milky Way (5-20 solar masses). This discrepancy has led to speculation that these could be primordial black holes. “Explaining why these two black hole populations are so different is one of the greatest mysteries of modern astronomy,” says Dr. Mróz.
One hypothesis states that these massive black holes were created in the early universe by density fluctuations that exceeded a critical threshold, and that they collapsed to form black holes. The idea that such ancient black holes could constitute a significant portion of dark matter is the subject of intense study and debate.
However, the recent findings suggest that if primordial black holes exist, they represent only a small fraction of the dark matter, calling for other explanations for the massive black holes detected by gravitational wave observatories. Some theories propose that these black holes could result from the evolution of massive stars with low metallicity, or from mergers of less massive objects in dense stellar environments such as globular clusters.
These new insights highlight the complexity of the dark matter problem and the need for continued research using diverse astronomical instruments and methods. Advances in understanding gravitational waves and their sources offer promising opportunities for future research, potentially bringing us closer to solving one of the universe’s most profound mysteries.
