Right after the Big Bang, space cooled unevenly. In some places, folds could have formed — threads of energy as thin as a hair but incredibly dense. Physicists call them cosmic strings. They oscillate, collide, and emit gravitational waves — ripples in the fabric of space itself.
In the new work, scientists gave strings a special property: superconductivity. One of the fields making up the string behaves as a current carrier. Electric charge flows along the string without resistance, like along a perfect wire. The authors calculated on a three-dimensional lattice how such a network of strings evolves in an expanding universe and estimated the spectrum of its gravitational radiation.
The main result is that the shape of the gravitational-wave spectrum depends on the coupling strength between the string field and the current field. With weak coupling, the string emits a broad range of frequencies. But when you strengthen the interaction, the high-frequency part of the spectrum dips. This is like a guitar string that, when under high tension, stops ringing with overtones.
Such a "spectral portrait" is the key to unraveling the nature of cosmic strings. If next-generation detectors see a characteristic drop at high frequencies, it would be the first evidence that strings not only exist but also carry a superconducting current. And then a thin filament from the early universe would tell us about the forces at play in the first moments after the Big Bang.
A cosmic string as thick as a proton could weigh as much as a mountain range, and its tension is comparable to the force needed to snap a steel cable a kilometer in diameter.