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

The traditional theory of black hole formation seems to struggle to explain how black holes can merge into larger more massive black holes yet they have been seen with LIGO. It’s possible that they may have formed at the beginning of time and if so, then they may be a worthy candidate to explain dark matter but only if there are enough of them. A team of researchers recently searched for microlensing events from black holes in the Large Magellanic Cloud but didn’t find enough to account for more than a fraction of dark matter. 

Classical black hole formation theory explains how they from the remnants of massive stars that have reached the end of their life and exhausted their fuel. When a star with a mass greater than about 20 times that of the Sun reaches the end of its life, it undergoes a supernova explosion, ejecting most of its outer layers into space. 

The core that is left behind is no longer supported by the pressure from nuclear fusion so it collapses under its own gravity. If the core’s mass is sufficient, typically several times the mass of the Sun, it will continue to collapse into a singularity—an infinitely dense point with an extremely strong gravitational pull. This process creates a black hole, characterised by the event horizon, a boundary beyond which nothing, not even light, can escape its gravity.

That’s a widely accepted description of the formation of black holes. However a recent set of observations using gravity wave detectors has identified some massive black holes. When compared to those that can be seen in the Milky Way they bare little resemblance. One possible explanation suggests that they may have instead formed from fluctuations in density during an earlier part of the universe’s history. These are known as primordial black holes and some theories suggest that they may account for dark matter. Possibly even up to 100% of the dark matter to account for the observed black hole merger rates. If they exist in the dark matter halo of the Milky Way then they should be observable by gravitational microlensing events. 

Previous studies have failed to identify such events but the team believe the observations were not sensitive enough. The paper published by Przemek Mroz from the University of Warsaw and team offer their findings of long-timescale microlensing events (events that occur over extended periods of times from weeks sometimes even years) in the Large Magellanic Cloud over the 20 years of the OGLE (Optical Gravitational Lensing Experiment) survey. The survey began in 1992 and is a long term study to detect microlensing events and observe variable phenomenon such as variable stars and supernova. It’s based at the Las Campanas Observatory in Chile and using the 1.3 metre telescope to monitor sections of sky. 

Having analysed the 20 years of data they found no events within the timescales longer than a year. Other shorter period events were identified but these are more likely down to stellar events than supermassive primordial black holes (PMB.) They find therefore, that PMB’s up to 6.3 million solar masses cannot make up more than 1% of dark matter. Those in the larger category up to 860 million solar masses cannot compose any more than 10% of dark matter. The unmistakable conclusion is that PMBs, based on the observations in the Large Magellanic Cloud, cannot account for a significant fraction of dark matter.

Source : No massive black holes in the Milky Way halo

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