Hidden Sector Prepares a Gravitational Concert ✎ express

The universe is like a layered cake, and our layer (stars, planets, and people) is just one of many. In a neighboring layer, which physicists call the hidden sector, processes are boiling away. This work describes one such process: a first-order phase transition, where matter abruptly changes state, like water turning into ice. But here, the transition occurs in a supercooled liquid: it remains liquid well below its freezing point, then suddenly solidifies.

The authors considered a hidden sector with a special symmetry (denoted U(1)) that is spontaneously broken. Along a certain direction, the field potential becomes almost flat, and the barrier between the old and new states grows not by itself but due to radiative corrections, as if sculpted by an invisible hand. A key role is played by the mass of the interaction carrier particle, called the gaugino. It sets the depth of the barrier, while the mass of its scalar partner helps the new state be stable. As the temperature drops, bubbles of the new phase nucleate in the old vacuum, just like steam bubbles in boiling water. Their walls collide, and this turbulence generates gravitational waves — ripples in spacetime.

Calculations show that for gaugino masses between 5 and 23 percent of the symmetry-breaking scale, the signal reaches a level detectable by future observatories — the Einstein Telescope and the Cosmic Explorer. But everything depends on the thermal contact between the hidden and visible sectors. If the hidden sector is initially cold, the signal is barely noticeable for weak coupling and strengthens by hundreds of times as coupling grows. If the hidden sector is initially hot, a strong signal emerges even with weak contact. The scientists traced this evolution using a system of 11 equations that separately describes the cold external region and the heated interior of the bubbles.