Through three minutes after the Big Bang, the young Universe sounded like an orchestra. It was nucleosynthesis — the birth of the first atomic nuclei. George Gamow in the 1940s first described this cosmic symphony. The most delicate instrument in it turned out to be deuterium (a light variety of hydrogen). The slightest change in the expansion tempo could drown out its sound.
Today astronomers listen to this music in two ways. Cosmic microwave background (the faint light left over from the hot youth) tells us how much matter was in the orchestra. And observations of ancient clouds reveal how much deuterium survived. But the two recordings don’t match. Adam Riess, who discovered the accelerating Universe, noticed that the discord is worsened by models of early dark energy (a mysterious force speeding up the growth). In them, even less deuterium remains.
A saving idea: the acceleration wasn’t constant but a brief spurt. This gives rise to the hypothesis of very early dark energy — a light scalar field that momentarily boosted the tempo and vanished. The key moment is when the temperature dropped to 340 million degrees. By then, helium-4 had already played out, while deuterium was still burning. Calculations with the PRIMAT code confirmed: a 9% acceleration at that instant preserves deuterium and barely touches helium. The disagreement with ancient light falls from 3.1σ to 0.7σ — almost perfect agreement.
Such precision hints at the ‘axiverse’—a family of light fields that could briefly switch on in different epochs. Perhaps the orchestra of the first minutes concealed another instrument, one that played a single note. Today scientists are testing this hypothesis against all cosmic data — from ancient light to the arrangement of galaxies. The symphony continues.
While you read this sentence, the Universe could have produced all of today’s deuterium — if the expansion in the first minutes accelerated by 9% for just a split second.