A balloon is being inflated faster and faster. At first, it stretches smoothly, but at some point, the pump stutters — the speed drops, then picks up again. During this brief respite, tiny bubbles appear on the rubber. As inflation resumes, they turn into dense knots.

Something similar happened to the universe itself right after the Big Bang. Its expansion, known as inflation (the ultra-fast swelling during the first fractions of a second), slows down for a fleeting moment in some models — scientists call this the ultra-slow-roll phase. At that moment, quantum jitters, which usually remain ripples on large scales, have time to grow into dense clumps. Later, as the universe cools, such clumps can collapse into primordial black holes — objects the size of an atom but with the mass of a mountain.

The authors of the new study tested this scenario not in ordinary Einsteinian gravity but in its extended version — the teleparallel theory. In the familiar picture, gravity arises from the curvature of spacetime; here, it emerges from its torsion (roughly speaking, from how small patches "shear" when transported). This modification adds tiny corrections to the expansion rate and to the properties of gravitational waves, which could be probed in future observations.

The main result: in both studied variants of the theory, the slow-down phase indeed produces many small black holes. Their mass and abundance depend on the torsion parameter. This means that future telescopes catching gravitational waves from merging black holes could not only find such objects but also peer into the workings of gravity on the largest scales.