In the first 10⁻³⁵ seconds after the big bang, the most rapid part unfolded. Space expanded faster than light — that's expansion of the universe (inflation). Almost the entire score was performed by a single field, the inflaton, conducting the tempo and key. But near the end, as inflation was winding down, other players could briefly awaken — charged Standard Model gauge fields. Their solo lasted an instant but left a mark.
That mark is a ripple in the fabric of space, gravitational waves. Usually, they are born from catastrophes like black hole mergers. Here, the mechanism is more subtle: density fluctuations left over from inflation themselves become a source of waves. Like an orchestra after a loud note: vibrations echo, just as primordial overdensities, crossing the horizon, make space ring.
The authors of the new work showed that the charged solo has a distinct signature. They introduced two dimensionless numbers: e_μ and $e_M$. The first reflects the interaction strength, the second the field energy. When these parameters change, the main peak in the wave spectrum does not just shift but jumps from one mode to another. Neutral fields lack this: their peak slides smoothly, like a musician moving a capo along the fretboard.
The effect arises from something usually ignored — the longitudinal component of the field. In a charged plasma, it becomes loud and changes the whole picture. James Peebles, who built the theory of primordial perturbations, foresaw that subtleties could tell a lot. Here, that very subtlety takes the stage.
Detectors of gigahertz waves, based on electromagnetism resonators, are still being designed. But when they start working, they will be able to catch that whisper from the past. And then in the data will emerge that very electric chord — a short, sharp solo played in the finale of the inflationary symphony. Its modulations will tell which forces conducted the birth of our world.
If a gravitational wave with a frequency of 1 GHz were detected, its wavelength would be 30 centimeters — such a wave could literally be “caught” in a resonator the size of a book.
In Carl Sagan’s novel “Contact,” the alien signal contained modulations hinting at an artificial origin. The oscillations in the gravitational wave spectrum from charged inflation — though natural — resemble such “modulation,” only they decode the laws of physics, not a message from extraterrestrial intelligence.