A runner on the middle of a track suddenly switches to a walk — not speeding up or slowing down, but maintaining a constant pace. Something similar may have happened to the universe itself shortly after the Big Bang. This phase has been called "stasis," from the Greek word for "standing still." In 2022, the group of Georges Lemaître (yes, the very same one who first described the expansion of the universe) showed that such a pause is theoretically possible: the expansion freezes for a while, and the equation of state, the quantity describing how "stiffly" the cosmic medium behaves, remains constant.
In the new work, physicists have shown that if stasis occurred, it must have left an imprint in gravitational waves — oscillations of space itself, generated during inflation (the super-fast expansion) right after the Big Bang. This imprint turns out to be surprisingly simple. All microscopic details of the era are erased, leaving a spectrum, the distribution of wave strength across frequencies, described by a template with just two parameters: that same constant equation of state and the duration of the pause.
The template predicts two independent measurements: the slope of the spectrum in the frequency range corresponding to stasis, and the jump in amplitude at the boundaries of that range. By eliminating the equation of state, one obtains a curve — a consistency line on which the data must lie if the pause with a constant equation truly occurred. If future measurements yield a point off this curve, the stasis hypothesis with a constant equation will be refuted, and this does not even require knowing in advance what the equation of state was.
The name "stasis" comes from medicine, where it means stagnation of blood, and from biology, where it refers to a long pause in the evolution of a species. In cosmology, it has taken on a new meaning — a temporary halt in the changing properties of the expanding universe.
In Isaac Asimov's novel "The End of Eternity," characters manipulate time by creating "stasis fields" within which time stops — much like the cosmological era where expansion itself freezes.