A pot of water cools so slowly that it remains liquid even below freezing. One tiny disturbance, and it instantly turns to ice, releasing latent heat. In much the same way, the early Universe could have undergone a first-order phase transition: one form of matter suddenly giving way to another, with that jolt producing gravitational waves — ripples in the very fabric of spacetime.
The authors of the study considered a special hidden sector — an invisible part of the cosmos that interacts with our world almost exclusively through gravity. They described it using an extended Nambu–Jona-Lasinio model with three types of quark-like particles. This model includes interactions that violate CP symmetry (the distinction between matter and antimatter) and additional forces that stabilize the vacuum. As this sector cooled after the Big Bang, bubbles of the new phase formed, like steam bubbles in boiling water, but in reverse.
The calculations revealed an unexpected result. Usually in such models, the transition happens too quickly, and the waves come out weak. But the scientists found a region of parameter space where the transition slows down enough that the wave amplitude jumps to a level accessible to the future μAres observatory. Moreover, the key properties of the waves are determined mainly by the radial profile of the potential, a sort of "topography" of the energy landscape, while the presence of a CP-violating background has almost no effect on the final signal.
Thus, a complex hidden sector, with broken symmetries and additional interactions, acquires a surprisingly clean and loud gravitational-wave "voice." Future detectors may hear the echo of that very jolt that once turned the primordial particle "soup" into the world we know.
The μAres observatory mentioned in the article is a proposed space-based gravitational wave detector that could pick up signals in the microhertz range, inaccessible to ground-based facilities.
In Carl Sagan's novel "Contact," an alien signal arrives against a backdrop of spacetime ripples — much like how astrophysicists search for traces of phase transitions in gravitational waves.