The early universe underwent rapid expansion — inflation. According to one theory, the driving force of this process (the inflaton) behaved like an axion — a particle interacting with a gauge field through a special coupling (an analog of the Chern–Simons effect). This interaction works like a pump: it amplifies quantum fluctuations of the field, turning them into real particles. Part of this energy goes into gravitational waves — ripples in spacetime.
The authors traced for the first time the entire path of wave birth: from the slow roll of the inflaton to the reheating stage (the transfer of field energy into ordinary particles) and further to the radiation-dominated era. The key moment occurs after inflation, when the inflaton starts oscillating. Its oscillations cause the instability parameter to rapidly change sign, exciting both helicities of the gauge field. This generates a burst of high-frequency gravitational waves.
Such a signal might be the strongest known at high frequencies. It can be "heard" indirectly — through its contribution to the effective number of neutrino species (a measure of how many light particles filled the early universe). Moreover, the result provides a strong impetus to build new detectors capable of capturing this ripple directly.
The oscillations of the inflaton after inflation make the field change its helicity so often that it resembles a tennis player who switches the racket to the other hand after every stroke.
In Carl Sagan's novel "Contact," a signal from space contained instructions for a machine that uses gravitational waves; here, nature itself leaves us a "whisper" of inflation.