Where did it all start?
The concept of black holes has intrigued both scientists and the public for centuries, evolving from theoretical considerations to fundamental phenomena in astrophysics. The journey began in the 18th century, when philosophers and scientists such as John Michell and Pierre-Simon Laplace pondered the existence of objects with gravitational fields so strong that light alone could not escape them. However, these early ideas remained largely speculation and did not gain significant popularity until the advent of Albert Einstein’s general theory of relativity in the early 20th century. [1]
Einstein’s groundbreaking equations suggested that a sufficiently compact mass could warp space-time to such an extent that it would create a region from which nothing could escape, thus laying the theoretical foundation for what we now know as black holes. It was Karl Schwarzschild who, in 1916, provided the first exact solution to Einstein’s equations, describing the geometry of space-time around a spherical mass – a solution that was later recognized as characterizing a black hole.
Despite these significant theoretical advances, the term “black hole” and a full understanding of its implications did not appear until much later. In 1958, physicist David Finkelstein published a paper interpreting black holes as regions in space where the escape velocity exceeds the speed of light, making them true “traps” in the structure of space-time [2]. Even then, however, black holes were often considered mathematical curiosities rather than physical entities.
The 1960s marked a turning point, as theoretical work began to solidify the understanding that black holes were not only possible, but were a natural consequence of the general theory of relativity. This period also saw the discovery of neutron stars, compact objects formed from the remnants of supernova explosions, which increased interest in the study of gravitational collapse.
Research development
The first empirical evidence for the existence of a black hole appeared in 1971 with the identification of Cygnus X-1. This X-ray source, located in the constellation Cygnus, exhibited properties that strongly suggested the presence of a black hole, attracting the attention of the scientific community and the world at large. The discovery was a watershed moment, transforming black holes from abstract concepts into tangible astrophysical objects.
Today, black holes are at the forefront of astronomical research and continue to capture our imagination. They are studied not only as endpoints of stellar evolution, but also as laboratories for testing the laws of physics under extreme conditions. Modern observations, including those from the Event Horizon Telescope, have provided stunning images and valuable data, deepening our understanding of these mysterious entities. The study of black holes covers everything from Hawking radiation emission, predicted by quantum field theory, to their role in the dynamics of galaxies and the behavior of matter in extreme gravitational fields.
OGLE as the largest modern survey of the sky – a Polish achievement?
In a landmark study entitled “Microlensing Optical Depth and Event Rate toward the Large Magellanic Cloud Based on 20 Years of OGLE Observations,” a team of astronomers from the University of Warsaw and other institutions presented the results of an extensive 20-year study. The study uses data from the Optical Gravitational Lensing Experiment (OGLE) to investigate the optical depth of microlensing and the event rate toward the Large Magellanic Cloud (LMC).
source: Microlensing Optical Depth and Event Rate toward the Large Magellanic Cloud Based on 20yr of OGLE Observations, The Astrophysical Journal Supplement Series, July 2024, https://doi.org/10.3847/1538-4365/ad452e
The microlensing phenomenon, in which a massive object (such as a black hole or star) passes between an observer and a distant star, causing the star’s light to bend and magnify, is a powerful tool for detecting dark matter and other compact objects. Earlier studies, such as those by Alcock et al. (2000) and Bennett (2005), suggested that the optical depth toward the LMC was much smaller than expected if the Milky Way’s dark matter halo consisted entirely of compact objects such as primordial black holes. However, the study was limited in its ability to detect long-period events that indicate the existence of more massive black holes.
The study identified 16 microlensing events over a 20-year observation period. These events were caused by stars in the Milky Way and the Large Magellanic Cloud (LMC), rather than by compact dark matter objects.
The research team concluded that massive and intermediate-mass black holes do not account for a significant portion of dark matter.
- Measurements of the optical depth of microlensing and event rates toward the Large Magellanic Cloud (LMC) can be used to study the distribution and mass function of compact objects toward this galaxy – the Milky Way disk, the Milky Way dark matter halo and the LMC itself. Previous measurements, based on small samples of statistical events, have shown that the optical depth is an order of magnitude smaller than that expected from the entire dark matter halo of compact objects, as quoted in the publication [3].
These findings are important because they help narrow down the number of possible dark matter candidates, suggesting that dark matter probably consists of non-luminous and non-baryonic particles rather than massive astrophysical objects.
- Our main motivation for conducting this work and combining the OGLE-III and OGLE-IV data sets was the fact that all previous microlensing experiments were not sensitive to events with time scales longer than tE ≈ 2.5-3 years, raising the possibility that such long time scale events were not included in the optical microlensing depth calculations [3].
The results of this study have significant implications for our understanding of dark matter. By excluding the large population of massive and moderately massive black holes in the Milky Way’s halo, the results narrow the number of possible dark matter candidates. The research supports the hypothesis that dark matter probably consists of non-luminous and non-baryonic particles, rather than massive astrophysical objects.
The research team includes: Przemek Mroz, Andrzej Udalski, Michał K. Szymański, Igor Soszyński, Łukasz Wyrzykowski, Paweł Pietrukowicz, Szymon Kozłowski, Radosław Poleski, Jan Skowron, Dorota Skowron, Krzysztof Ulaczyk, Mariusz Gromadzki, Krzysztof Rybicki, Patryk Iwanek, Marcin Wrona, Milena Ratajczak, Mateusz Kapusta.
Bibliography:
[1] Nola Taylor Tillman, Daisy Dobrijevic, Black holes: Everything you need to know, 19.05.2023, https://www.space.com/15421-black-holes-facts-formation-discovery-sdcmp.html
[2] BLACK HOLES ARE THE KEY, https://www.davidritzfinkelstein.com/blackholes.html
[3] Przemek Mróz, Andrzej Udalski, Michał K. Szymański, Igor Soszyński, Łukasz Wyrzykowski, Paweł Pietrukowicz, Szymon Kozłowski, Radosław Poleski, Jan Skowron, Dorota Skowron, Krzysztof Ulaczyk, Mariusz Gromadzki, Krzysztof Rybicki, Patryk Iwanek, Marcin Wrona, Milena Ratajczak, Mateusz Kapusta, Microlensing Optical Depth and Event Rate toward the Large Magellanic Cloud Based on 20 yr of OGLE Observations, 24. 06.2024, https://iopscience.iop.org/article/10.3847/1538-4365/ad452e
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