The Universe's Expansion Amplified Gravitational Waves ✛
In the first moments after the Big Bang, the cosmos was filled with hot plasma. As it cooled, this plasma underwent phase transitions — akin to water freezing, but on a cosmic scale. This spawned bubbles of a new phase that collided, producing shock-like sound waves. These sound oscillations, in turn, gave rise to gravitational waves — ripples in the very fabric of space. Until now, nearly all calculations overlooked one key factor: the Universe was rapidly expanding at the time, and this should have had a significant impact.
To test this, scientists conducted a computer simulation — a 3D simulation of a phase transition in an expanding metric, first described by Georges Lemaître and Edwin Hubble. It turned out that expansion doesn't muffle gravitational waves; on the contrary, it accelerates them to astonishing levels — the amplitude grows by tens to hundreds of times. These findings directly concern the operation of laser interferometers and pulsar timing arrays, as well as radio astronomy observations. Perhaps phase transitions will explain those mysterious signals that don't fit within the Standard Model, extended thanks to Peter Higgs.
If the Universe weren't expanding, the gravitational echo from a phase transition would gradually fade, like a dying bell toll. But expansion turns it into a growing rumble, as if the bell rings ever louder.
In Greg Egan's novel Schild's Ladder, vacuum phase transitions are depicted as capable of redrawing reality. Our story isn't as catastrophic, but it shows that even a gentle reshaping of space during expansion yields a powerful burst of gravitational waves, an echo of cosmic reforging.