In the first moments after the Big Bang, cosmic symmetry breaking divided the hot plasma into regions with different vacuums — the lowest energy states of a field. The world resembled a giant soap foam: each bubble-universe had its own vacuum, and between the bubbles stretched domain walls — spatial defects akin to films with enormous surface energy. A wall is thinner than an atom. The energy locked inside is colossal.
If all bubbles were perfectly identical, the wall network would grow indefinitely. By the time the first stars appeared, it would have filled the entire visible universe, grossly violating Hubble's law of expansion. Fortunately, nature is rarely perfectly symmetric. Even a tiny initial imbalance (a population bias, where one vacuum type occurs slightly more often) makes the film tear. Just as a stretched soap bubble pops at a touch, domain walls annihilate, releasing energy into gravitational waves and other fields.
The authors of the work, for the first time, traced the fate of a population-biased network using numerical simulations on a lattice. The dynamics of a scalar field were tracked in an expanding universe with the CosmoLattice code, augmented by a “stretching” technique — artificially maintaining the physical wall width to avoid losing resolution. The simulation showed that the bias grows as a power law B ~ $B_s$ (η/$η_s$)^p, with p ≈ 1.37, and the annihilation time depends inversely on the initial bias: $η_{ann}$ ∝ $B_s^{–0.8}$. The smaller the initial asymmetry, the longer the walls live, but they inevitably disappear.
When walls collapse, they emit a powerful gravitational-wave spectrum with a characteristic broken shape: a steep infrared rise (∝ $f^3$), a gentle ultraviolet fall (∝ $f^{-1}$), and a peak at roughly twice the Hubble frequency at the time of generation. The integrated conversion efficiency of energy into gravitational waves reaches 6%—only half that of the mechanism with explicit symmetry breaking in the potential. Notably, the spectral shape is almost independent of the initial bias magnitude, making the prediction robust.
This signal is already searchable. Pulsar timing arrays (PTAs) and ground-based interferometers LIGO-Virgo-KAGRA, pioneered by Rainer Weiss, can detect such a stochastic background. The universality of the spectrum opens a path to testing a wide class of theories, including models with gauge symmetries where explicit potential bias is forbidden. A prominent example is the Peccei–Quinn mechanism (Roberto Peccei and Helen Quinn), which solves the strong CP problem with axions; here, population bias becomes almost the only way to eliminate the walls.
Thus, a cosmic soap film tearing from one side leaves behind a faint but detectable trace — a background hum of gravitational waves. Its shape is already calculated. It remains to tune the antennas.
If domain walls stretched across the visible Universe, crossing one would create a density jump capable of tearing galaxies apart. Fortunately, a tiny initial imbalance — just 0.01% — makes them vanish long before the first stars form.
The notion of separated vacua echoes the worlds of Roger Zelazny's 'Chronicles of Amber,' where Shadows exist between realities and crossing them takes effort. Here, however, the transitions between vacua turn out to be unstable and collapse on their own.