Exploring the Invisible: Searching for Primordial Black Holes in the Milky Way

Astronomers have long debated the existence and role of primordial black holes (PBHs)—black holes potentially formed shortly after the Big Bang—in explaining dark matter and gravitational wave events. A study led by Przemek Mróz and his team used the OGLE (Optical Gravitational Lensing Experiment) survey to investigate whether PBHs could constitute a significant portion of the Milky Way's dark matter halo.

The Search for Microlensing Events

The OGLE survey monitored the Large Magellanic Cloud (LMC) for 20 years, observing nearly 80 million stars. Researchers searched for a phenomenon called gravitational microlensing, where the gravity of a compact object, like a PBH, temporarily magnifies the light of a background star. The team developed new methods to analyze the data, aiming to detect long-timescale microlensing events that would indicate the presence of massive PBHs.

Results: No Long-Timescale Events

Despite their thorough analysis, the researchers did not find any microlensing events lasting longer than a year. The shorter events detected could all be explained by known stellar populations, such as brown dwarfs and remnants of stars within the LMC and the Milky Way.

Implications for Dark Matter

The study placed strict limits on the fraction of dark matter that PBHs could represent. For PBHs in the mass range of 1.8 × 10⁻⁴ to 6.3 solar masses, they cannot make up more than 1% of dark matter. This finding significantly challenges theories suggesting that PBHs are a major component of dark matter or are responsible for the black hole merger rates observed by gravitational wave detectors.

Broader Impact

This research provides valuable constraints not only on PBHs but also on other compact objects as dark matter candidates. The methods developed could be applied to future studies, enhancing our understanding of dark matter and the nature of the cosmos.

Source: Mróz