How Baby Black Holes Build Giants express
The James Webb Space Telescope has spotted ancient galaxies harboring supermassive black holes — far too massive for their young age after the Big Bang. There wasn't enough time for normal growth, so they must have formed differently.
Perhaps primordial black holes — hypothetical crumbs dreamed up by Stephen Hawking — served as the building material. These 'bricks', mixed with dark matter, quickly assemble into cosmic heavyweights. In clusters, they merge like raindrops on a windshield — the same law governs an avalanche of mergers. Heavier objects sink to the center, collide more often, and grow explosively.
An unexpected twist: each such crumb, the size of an atom, weighs as much as a mountain, and their collective behavior follows the equation of raindrops. Simulations confirm that mere moments after the Big Bang, a swarm of these babies could give birth to a giant black hole that we see today as a red dot.
Original abstract
as written by the authors arXiv:2604.01684
In arXiv:2507.07171, we demonstrate that the high-redshift supermassive black holes in the so-called "little red dots" discovered by James Webb Space Telescope (JWST) can be explained by the primordial black hole (PBH) clustering on small scales. In this paper, we present a comprehensive simulation of the successive PBH mergers within a cluster by solving the Smoluchowski coagulation equation. We derive the coagulation kernel considering both cases with and without the effects of mass segregation. Then we employ the Monte Carlo method to solve the equation, implementing the full-conditioning scheme using the discrete inverse transformation method. Our simulations determine the runaway timescales of clusters and the mass population evolution of PBHs across a wide range of cosmic redshifts, depending on the number of PBHs within the cluster and the associated density.
Primordial black holes have never been directly detected, but they could make up a significant chunk of dark matter — the invisible stuff that keeps galaxies from flying apart.