The James Webb Space Telescope has been revealing a bewildering menagerie of compact, red objects in the first billion years of cosmic history. At the same time, theoretical work led by researchers at the University of California, Los Angeles (UCLA) suggests that the early universe was a bustling arena where stars collided far more often than they do today. By reproducing those chaotic conditions on supercomputers, the team uncovered a plausible pathway from chaotic stellar crowds to the massive black-hole behemoths that power modern galaxies.
Star collisions in crowded early clusters
In the simulations, nascent galaxies are packed with stars whose typical separations are tiny compared with today’s calm galactic disks. This high density forces stars onto intersecting orbits, so that “slam-into-each-other” events become routine. The researchers found two regimes: high-speed impacts that shred the participants, and slower encounters that allow two stars to coalesce. Repeated mergers can eventually produce a single, gargantuan star whose mass rivals that of the entire original cluster. As Claire Williams of UCLA put it, “Perhaps the strangest outcome of our simulations is that stars can merge repeatedly within the cluster until they form one gigantic star with a mass equal to that of an entire cluster.”
From massive stars to intermediate-mass black holes
When such an over-massive star exhausts its nuclear fuel, its core can no longer support its own weight and it collapses. In the modern universe, a typical stellar-mass black hole weighs between 10 and 100 times the mass of the Sun. The simulations predict that the ultra-massive progenitors formed in dense clusters would instead leave behind black holes in the intermediate range—between 100 and 100,000 solar masses. These intermediate-mass black holes provide the missing “seed” that can rapidly grow, via accretion and mergers, into the supermassive black holes observed when the universe was less than a 1 billion-year old. Williams added, “Dense star clusters provide a natural birthplace for massive black-hole seeds in our model.”
Little Red Dots: clues from JWST
Among the most puzzling JWST discoveries are the so-called Little Red Dots—tiny, intensely red sources that appear abundant in the early cosmos but seem to vanish once the universe approaches a 2-billion-year age. Smadar Naoz, also of UCLA, likened the community’s attempts to describe them to “blind people describing an elephant.” The new simulations imply that many of these dots could be exactly the dense star clusters described above. Repeated stellar collisions inside them would generate the massive star that eventually collapses into an intermediate-mass black hole, potentially powering the observed red emission. As the universe expands and cools, such extreme environments become rare, explaining why the Little Red Dots fade from view in later epochs.
Why the objects disappear after two billion years
In a younger universe, gas and stars are still plentiful, allowing clusters to remain tightly bound and metal-poor—a condition that limits mass loss from stellar winds and favors runaway collisions. Over time, the cosmic expansion dilutes matter, galaxies settle into more ordered shapes, and metallicities rise as successive generations of stars enrich the interstellar medium. These changes lengthen the typical collision timescale and reduce the likelihood of forming the monstrous stars required for heavy black-hole seeds. Consequently, the environments that gave rise to Little Red Dots and massive seeds become increasingly uncommon, leading to their apparent disappearance after the universe reaches roughly a 2-billion-year milestone.
Altogether, the UCLA simulations tie together several long-standing mysteries: they offer a natural explanation for the rapid emergence of supermassive black holes less than a 1 billion-year after the big bang, and they provide a physical identity for JWST’s enigmatic Little Red Dots. As observational capabilities improve, astronomers will be able to test whether these dense, early clusters truly serve as the factories that forged the heavy black-hole seeds shaping the cosmic landscape we see today.
