Gravitational waves: Most massive collision of two black holes leaves us puzzled
Once again, gravitational waves have been observed whose origin should not actually exist: a collision of black holes with "forbidden" mass.
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LIGO's two gravitational wave detectors apparently observed the formation of the most massive black hole to date in the fall of 2023 – and researchers are once again faced with a puzzle. An international research group reports that two black holes, each with more than 100 solar masses, have merged to form a new object with 225 times the mass of our own star. However, at least the smaller of the two falls into a range that was previously considered impossible. One astronomer involved even calls its mass "forbidden". In addition, the two objects rotate so quickly that this also pushes the boundaries of what is possible.
Black holes with "impossible" mass
According to the research group, the gravitational waves were observed on November 23, 2023, which is why the event is known as GW231123. The signals were observed by two LIGO detectors (Laser Interferometer Gravitational-Wave Observatory) in the USA. Due to the properties of the black holes involved, this is a "unique challenge" and there is a possibility that their origin is much more complex than assumed. It will take years to analyze the complicated signal pattern and understand all the implications, predicts Gregorio Carullo from the University of Birmingham, who was involved in the work.
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To understand the excitement of the researchers, it is necessary to look at the masses of the two black holes that have probably merged here. According to our understanding of stars and their ends, we should not actually find black holes that have more than 65 and less than 120 solar masses. This is because stars in this mass range first lose so much mass at the end of their lives that they shrink to the mass at which they end up as a smaller black hole after an explosion. Instead, however, both black holes in GW231123 are located in or near this region. It is conceivable, but apparently extremely unlikely, that both were created by collisions.
Gravitational wave astronomy is based on the predictions of Albert Einstein's general theory of relativity. According to this theory, gravity is a property of space that propagates at the speed of light. If massive bodies are accelerated, they generate gravitational waves, which deform the structure of space – but only minimally, even with large masses. Einstein himself therefore assumed that these would never be detectable. However, they can be observed with modern, very powerful instruments – if the responsible event was large enough. Detectors in the USA, Italy and Japan have been detecting traces of gigantic collisions for years.
(mho)
