LIGO Has Detected Unusual Black Holes Merging, But they Probably Don’t Explain Dark Matter

January 13, 2025 by Mark Thompson

The phenomenon of black holes merging has been made visible by the Laser Interferometer Gravitational-Wave Observatory (LIGO). These events challenge traditional theories of black hole formation, particularly their relation to dark matter. A recent study involving the Large Magellanic Cloud (LMC) found nearly negligible microlensing events attributed to these black holes, suggesting they might not be the key to understanding the elusive nature of dark matter.

The Process of Black Hole Formation

Traditionally, black holes are formed from the remnants of massive stars that have exhausted their nuclear fuel. These stars, typically more than 20 times the mass of the Sun, end their life cycle in a supernova explosion, ejecting most of their outer layers into space. What remains is a core that collapses under its own gravity, creating a singularity endowed with intense gravitational pull.

This is the classic view of black hole formation; however, recent observations have revealed massive black holes that are not easily accounted for by this model. Instead, these massive black holes may have originated from density fluctuations in the early universe, leading to what are referred to as primordial black holes (PBHs).

Primordial Black Holes: A Possible Explanation for Dark Matter?

Some hypotheses suggest that these primordial black holes could account for a significant proportion of dark matter. However, to be considered viable candidates, their numbers must be sufficient enough to match the observed effects of dark matter. Observing PBHs within the Milky Way's dark matter halo is challenging, thus the reliance on gravitational microlensing events, which should be detectable if they exist in significant numbers.

OGLE Survey Findings on Microlensing Events

For this purpose, researchers utilized data from the Optical Gravitational Lensing Experiment (OGLE), which has been running since 1992 from the Las Campanas Observatory in Chile. Over a span of nearly two decades, the survey aimed to detect long-timescale microlensing events, conducting thorough analysis on variable phenomena, including variable stars and the occurrence of supernovae.

Long-term Results: Absence of Microlensing Events

The analysis yielded no significant long-duration microlensing events within the desired observational periods of longer than one year. Shorter anomalies were detected, which are believed to relate more closely to stellar activity rather than the presence of supermassive primordial black holes.

Conclusion: Implications for Dark Matter Hypotheses

The data suggests that primordial black holes, especially those exceeding 6.3 million solar masses, cannot account for more than 1% of the dark matter content in our galaxy. Additionally, the findings regarding larger PBHs—those approaching 860 million solar masses—indicate that they may constitute at most 10% of dark matter.

As research continues and further methodologies are incorporated, our understanding of both primordial black holes and dark matter will evolve. The conclusions draw from OGLE's observations clarify the limits of primordial black holes as feasible candidates for dark matter, potentially steering investigations in other directions.

Next Steps for Research

Future explorations may delve deeper into alternative dark matter candidates and refine observational techniques to better identify potential primordial black holes within our galaxy. The cosmic discussions around dark matter remain critical as astrophysicists continue testing the validity of existing hypotheses.

For more information, check out:

• New insights into black hole formation
• No massive black holes in the Milky Way halo
• LIGO updates and findings
• Black hole research trends

References:

Source: Universe Today