In the first moments after the Big Bang, the Universe hummed like a bell. Inflation, the epoch of rapid expansion confirmed by measurements of the cosmic microwave background, ended, and the inflaton field began oscillating around the minimum of its potential. Like a clapper striking a dome, these oscillations transferred energy to spacetime. This stage is called reheating: the inflaton spawned particles, filling the world with matter. The idea of reheating was developed by Andrei Linde, building on early inflation work by Alan Guth. But the potential might contain a tiny defect. Like a microcrack in a bell, it changes the character of the oscillations.

Physicists considered a concrete realization — the α-attractor T-model of inflation. They added a heavy scalar field that, through virtual processes, creates a quantum field correction known as the Coleman–Weinberg correction — a microscopic defect at the bottom of the potential. The correction's shape depends on a parameter κ. At κ = 1, the quadratic term vanishes, and the potential near the minimum becomes nearly quartic. To see the consequences, scientists ran a numerical simulation using the CosmoLattice code. On a three-dimensional lattice, they evolved the inflaton field and metric, obeying the Einstein field equations and the Friedmann equations. This gives rise to a phase transition known as parametric self-resonance: individual fluctuation modes grow exponentially, driving each other. As if a strike produces not just a ring in the bell, but a vibration that amplifies itself, creating a complex pattern.

The spectrum of gravitational waves generated by parametric self-resonance at κ = 1 turned out to be unusual. Instead of a smooth energy distribution across frequencies, several sharp maxima appeared. The main peak reaches an amplitude h²ΩGW ∼ 10⁻¹⁰ and occurs at frequencies around 10⁹ Hz — roughly the same frequency as microwave ovens. This is an ultra-high-frequency range, far beyond the capabilities of current gravitational wave detectors. If κ is slightly greater than one, the resonance weakens and the spectrum smooths out. Reducing the scale M broadens the window for peaks, but at too small M, higher-order terms in the potential smear the resonance. Such signals could serve as a probe for quantum gravity — the physics at the Planck scale, where spacetime froths with quantum fluctuations.

The cosmic bell rang only an instant after the Universe's birth, but its sound, a faint rustle of gravitational waves, could have survived to this day. The microcrack, a quantum correction, left a characteristic imprint on this hum, as predicted by calculations. If future detectors pick it up, we will for the first time hear the voice of physics hidden beyond the Standard Model. Meanwhile, numerical simulations refine the picture, adding interactions with other fields and studying dissipative effects. Each new stroke brings the answer closer.