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Understanding black holes: young star clusters filling up gaps

Since their groundbreaking discovery of gravitational waves from a pair of in-spiralling black holes back in 2015, the LIGO-Virgo-KAGRA collaboration has detected nearly 70 candidates of such events, 50 being confirmed and published until now. A tight binary comprising black holes or neutron stars would always spiral in and become tighter with time by losing energy in the form of gravitational waves until they plunge onto each other and merge. The final phase (typically, some ten seconds long) of this merger process would generate the strongest “chirp” of gravitational waves which the mammoth laser interferometers of LIGOVirgo can register as an “event.” Each of such event, apart from being fascinating by its own right, also provides us with valuable information such as how heavy the black holes are, how fast they are spinning, and whether the spins are tilted. Such information is the key to understanding how black holes are formed out of stars and how they end up in binaries.

Most, if not all, of the stars in the Universe form in densely-packed spherical groups. Such a fresh hatch of stars, called a young cluster, is held together by the stars’ mutual gravitational pull. Black holes are formed when the most massive and shortest living of the stars, some tens to hundreds of times heavier than the Sun, run out of their nuclear fuel and collapse under their own gravity. Once formed, these black holes become by far the heaviest members of the cluster. Therefore, they sink up to the stomach of the cluster where they are free to interact and exchange energy with each other via gravitational attraction. That way, they often end up pairing.

The interaction chain doesn’t stop there, though. The binary black holes continue to interact with neighbouring black holes, forming triples of black holes (rarely, even up to quadruples and quintuplets). Such a triple, by itself, is a highly active system undergoing extreme internal oscillations (known as the Kozai-Lidov oscillation), a process which causes its constituent black holes to periodically zip by, approaching each other’s event horizon: this is when things become relativistic. This is how, mediated purely by gravitational interactions, binary black holes form and go all the way up to in-spiral and merge inside star clusters.

I recently conducted a computer simulation of this whole picture that has explicitly reproduced all the key properties of the merging binary black holes that LIGOVirgo have derived from their observations. These are extensive and ab initio simulations of star clusters, comprising tens of thousands of stars, with practically no inherent simplifying assumptions. No compromise is made in these simulations: the latest knowledge of black hole formation from stars and Einstein’s general relativity are knit together with a rigorous mechanism for tracking all sorts of encounters that the stars and the black holes would go through. In terms of physical ingredients, these are the most advanced and realistic computer simulations of star clusters to date which successfully tackle a long-awaited problem in computational and multi-disciplinary astrophysics.

The binary black hole mergers from these calculations reproduce not only the overall trends in masses and spins of the observed LIGO-Virgo merger events but also the oddest ones among them—. Events, for example, like the recently revealed GW190521, involving a black hole of 80 solar mass that is “forbidden” by stellar evolution theory, occur naturally in these simulations. Current understanding of stars and binaries tells us that black holes cannot have masses in between 60 and 120 solar masses which is why GW190521 is, so far, the oddest among the odds. Of course, in my own simulations, black holes are never born within this forbidden range. However, as time passes, some of the black holes jump up in mass to enter the forbidden zone. The mass jump happens either by merging with another black hole (which is itself a “normal” event) or by eating up a star. Inside a cluster, such a massive object becomes a mighty attractor enabling it to participate in a merger again and thus create an apparently impossible gravitational-wave event. In these simulations, black holes of up to 100 solar mass undergo mergers, forming intermediate-mass black holes as in GW190521.

The computed star cluster models also make mergers resembling other remarkable LIGO-Virgo events such as GW170729 (formation of an 80 solar mass, unusually spinning black hole) and GW190412 (merger of two black holes that are unexpectedly dissimilar in mass). By virtue of their high internal activity and ambience, young, moderate-sized star clusters have the potential to explain both the trend and the oddities of the observed gravitational-wave merger events. The importance of these apparently humble clusters in generating the most energetic events in the Universe is just beginning to be realised.

Featured image by Brett Ritchie

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