The first moments after the big bang: space expands rapidly — physicists call this period inflation. Inside this swelling fabric, like soap bubbles in a stream of air, regions of new vacuum begin to form — a first-order phase transition. Most bubbles pop or merge as soon as they appear. But some manage to inflate to monstrous sizes — far beyond what could then be seen.
The radius of such a giant bubble during inflation becomes billions of times larger than the Hubble radius. It vanishes from sight, like a balloon drifting beyond the boundaries of a vast hall. Only when inflation ends and expansion slows does the horizon begin to grow faster. The giants reappear in view, and now their walls, carrying immense energy, collide with each other. Each collision is a tremor of spacetime curvature itself, generating gravitational waves — ripples spreading in all directions at the speed of light.
Typically, phase transitions produce a squeak at megahertz frequencies, but giants rewrite the rules: their enormous size shifts the frequency downward — into the range where detectors like LIGO (hundreds of hertz) and the future space interferometer LISA (millihertz) operate. After accounting for redshift (the stretching of waves by the expanding universe), the spectral peak can land on any frequency from billionths of a hertz to hundreds of hertz — also covering the sensitivity bands of pulsar timing arrays.
Fortunately, constraints from primordial nucleosynthesis and the cosmic microwave background do not forbid this scenario: they set an upper amplitude bound around 10⁻⁶, which leaves a window for detection. Inflation models like the alpha-attractor of Alan Guth naturally predict the required parameters. The expansion of the universe, described by the equations of Friedmann and Lemaître, acts here as a co-author: it stretches the collision time and greatly amplifies the signal.
And now, 13.8 billion years later, the echo of those collisions may reach Earth. The giant bubbles, once lost beyond the horizon, have left a trace — a low hum that our instruments can hear. If detectors pick up this low-frequency gravitational buzz, we will not just confirm inflation theory: we will peer into an era when the very laws of physics were born.
If such a superhorizon bubble were to somehow pop up today, its radius would be on the order of $10^{28}$ cm, a million times larger than the observable Universe.
The colliding-bubbles idea echoes the plot of Liu Cixin's novel *The Three-Body Problem*, where aliens weaponize vacuum decay. But in our Universe, it's a natural cosmological process, not someone's scheme.