In the scorching plasma of the infant Universe, right after inflation, conceived by Alan Guth, the kinetic dominance epoch reigned. In this churning cauldron, even tiny magnetic fields — a billion times weaker than Earth's — played the role of a cosmic bow, causing a resonance that birthed primordial black holes. These holes, as first proposed by Stephen Hawking, could make up the elusive dark matter.

Envision: a bow endlessly drawn across a single string, producing a steady, almost inaudible hum. But the instrument's resonant soundboard is designed so that this monotonous sound nonlinearly generates piercing harmonics — the very ones capable of shattering glass. That's exactly how vector modes of magnetic fields — frozen ripples in spacetime — during the kinetic epoch, when the energy density of the expanding Universe obeys a stiff equation of state, induce scalar curvature perturbations. Their secret lies in constancy: vector modes neither decay nor grow, creating an ideal foundation for the secondary birth of inhomogeneities.

The power spectrum of the resulting perturbations is sharply amplified on large scales, following a universal law with a slope of minus five — a hallmark of the kinetic epoch, as unique as a fingerprint. In the Ratra magnetogenesis model, these perturbations reach a critical amplitude, and gravitational collapse gives birth to black holes with the mass of an asteroid yet the size of a proton. Such objects do not conflict with either big bang nucleosynthesis data, studied by George Gamow long ago, or observations of the cosmic microwave background.

The uncovered mechanism is a bridge to understanding the nature of dark matter and the origin of cosmic magnetism. The next step is to search for a gravitational-wave background with a unique spectral “signature” of –5. Experiments like pulsar timing arrays are already targeting this echo from the first moments. Thus, the quiet hum of magnetic fields transforms into a symphony that deciphers the mysteries of the cosmos.