In the first moments after the big bang, the universe underwent inflation — a super-fast expansion. During this time, regions of new vacuum formed inside it like bubbles in boiling water (such transitions are called first-order phase transitions). Usually they collide and disappear almost immediately. But some bubbles managed to inflate to sizes billions of times larger than the then horizon (the limit of visibility). They became invisible but did not vanish forever. When inflation ended, the horizon began to grow, and the giant bubbles came back into view. Their walls, carrying enormous energy, started colliding.
Each such collision launches gravitational waves — ripples in the fabric of curved spacetime. Why isn't the signal at ultrahigh frequencies as usual? It's all about size: large bubbles produce long waves. The bigger the bubble, the lower the frequency. For giant bubbles, the frequency drops to values accessible to LIGO (on Earth) and the future space observatory LISA. Moreover, the expansion of the universe during the prolonged collision greatly amplifies the signal — as if an echo repeatedly reflected and grew. This makes the wave noticeable even after 13.8 billion years, accounting for redshift (stretching of waves by the expanding universe).
So, giant bubbles born at the dawn of time could have left behind an audible gravitational hum. If detected, we would have direct proof of inflation and learn what the universe was like at an age less than a trillionth of a second.
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.