The latest research shows that the largest black hole in the universe is likely not formed by the direct collapse of a single massive star, but is built "layer by layer" deep in an extremely crowded star cluster through a series of violent mergers. This study led by Cardiff University in the UK pointed out that the heaviest black holes in gravitational wave astronomical observations belong to an independent group, and their birth history is more like a "multi-generational black hole family tree" rather than the end of the evolution of ordinary stars.
The scientific research team systematically analyzed the fourth edition of the Gravitational Wave Transient Catalog (GWTC-4) released by the LIGO-Virgo-KAGRA collaboration, which included 153 highly credible black hole merger events. The researchers paid special attention to the most massive black holes in the sample to test whether they are the products of the "second generation" or even higher "generations" - that is, early black holes merged in dense star clusters to generate more massive black holes, and these black holes collided and merged again during subsequent evolution, and continued to gain weight. In this type of dense star cluster, the spatial density of stars and compact objects can be one million times higher than that in the vicinity of the sun, providing a natural stage for black hole "serial mergers".
The research results were published in the latest issue of "Nature Astronomy". The statistical characteristics given in the paper show that the heaviest black hole group observed by gravitational waves shows obvious differences in mass and spin distribution from black holes formed by the collapse of ordinary stars, and should be regarded as an independent group shaped by hierarchical mergers. In other words, gravitational waves are not only "counting" black hole collision events, but are also beginning to reveal how and where black holes grow, and inversely constrain the evolution theory of massive stars and star clusters.
Through detailed modeling and analysis of gravitational wave signals, the researchers separated two main black hole populations in the sample: one is a lower-mass black hole, whose properties are basically consistent with the traditional stellar collapse model; the other is a significantly higher-mass black hole, whose spin characteristics are fully consistent with the expectations of experiencing multiple hierarchical mergers in dense star clusters. The study of the spin of high-quality black holes is particularly critical because the size and direction of the spin record the merger history of its predecessor black holes.
The paper points out that the spins of high-quality black hole groups are not only generally faster, but also have a nearly random distribution of spin directions, which is completely different from the "orderly aligned" spin state in typical binary star evolution. This surprised the research team and greatly enhanced the credibility of the "origin of dense star clusters". Compared with previous smaller and earlier gravitational wave catalogs, the high-quality systems in this analysis "jump out" more obviously in parameter space, strengthening the judgment that they belong to an independent group.
In addition to outlining the growth path of monster black holes, this research also provides one of the strongest observational evidence to date for a long-standing prediction in astrophysics - the black hole "mass gap." The theory is that extremely massive stars will undergo a violent pair-instability process before they die, violently exploding and completely destroying themselves, leaving no more black hole remnants. This means that within a certain mass range, stars should not directly produce black holes, forming a "forbidden zone".
The research team found signs of this transition in the sample: around about 45 solar masses, the distribution of black holes changed significantly. Fabio Antonini, the paper's lead author, said they saw evidence in the data of a long-predicted "instability mass gap" - a mass range in which stars are not expected to leave behind black holes. However, gravitational wave detectors have successfully discovered black holes at or near this gap, concentrated at about 45 solar masses. This raises a key question: Are these black holes challenging existing models of stellar evolution, or are they simply not formed directly from a single star at all, but are "pieced together" through another path - hierarchical mergers?
Research shows that in the current sample, the information carried by the most massive black holes points more to the dynamic effects of star clusters, rather than just the evolution of single stars. When the mass of a black hole exceeds about 45 solar masses, its spin distribution suddenly changes significantly. This is difficult to explain through the evolution of ordinary star binary, but it can be naturally understood by "these black holes have experienced multiple rounds of mergers in dense star clusters." This further supports the idea that monster black holes are stacked and grown from generation to generation deep in star clusters.
The work also links gravitational wave astronomy to nuclear physics processes inside stars. The team used the turning point close to the mass gap to infer a key nuclear reaction involved in the helium burning of massive stars, thus providing a new way to study nuclear processes deep in the core of stars. The researchers said that with the accumulation of gravitational wave observations in the future, scientists may be able to reversely infer the complex nuclear reaction chain inside the star through the fine shape of the black hole mass distribution and mass gap.
Co-author of the paper Fani Dosopoulu and others pointed out that the so-called mass upper limit set "for instability" directly depends on the specific nuclear reactions that occur in the core of massive stars. Therefore, the continued accumulation of gravitational wave data will not only rewrite our understanding of black hole populations, but may also become a new experimental "laboratory" for studying nuclear physics. For the universe, every black hole merger is a violent and short-lived event, but with the help of gravitational wave "hearing", humans are using these momentary vibrations to reconstruct the long history of monster black holes quietly growing in the depths of the universe.