In the first moments, the world resembled a boiling sea of fields. As it cooled, it underwent a Phase transition into 'bubbles' — regions where the field froze into different low-energy states, like water turning into ice with different crystal orientations. The boundaries between these bubbles are topological defect films — ultra-thin and under colossal tension energy. Their thickness is less than an atom. Their mass, however, is billions of tons per square meter.

If all vacuum types were perfectly symmetric, the network of films would grow and grow. By the time the first stars appeared, it would have filled the visible Universe, breaking the law of expansion of the universe. But nature abhors perfect equality. The slightest initial imbalance — one kind occurs a bit more often — and the film begins to tear. Just as a soap bubble pops at a touch, cosmic walls collapse, emitting gravitational waves.

Scientists have traced this process for the first time in a numerical simulation. They made the Universe expand on screen and watched how the imbalance grows until all walls vanish. The resulting gravitational waves have a spectrum that barely depends on the magnitude of the initial imbalance: a steep rise at low frequencies, a fall at high frequencies, and a sharp peak. This broken power-law profile is a universal signature of the process.

Today, this signal is being searched for. Ground-based LIGO interferometers, pioneered by Rainer Weiss, and pulsar timing arrays (PTAs) — essentially vast detectors that use the rotation of dead stars — can pick up this stochastic hum. The universality of the spectrum allows testing many fundamental theories, including axion models proposed by Roberto D. Peccei and Helen Quinn. There, such a population imbalance is almost the only way to remove the walls.

The cosmic soap film, having torn, left behind a faint but audible rustle. Its shape is already known. We just need to listen for it.