Spacetime is not empty: it shivers. After the Big Bang, the universe was filled with a hum of gravitational waves, like the low rumble of a giant bell. In this ripple, as it turns out, particles of dark matter, the invisible substance that holds galaxies together, can be born.
But the previous idea had a snag. Gravitational waves created massless particles, and making them massive required extra fields and assumptions. The new work removes that crutch. The authors built on spacetime geometry with torsion (a possibility that Elie Cartan already entertained) and showed: when space is not just curved but also twisted, the born particle automatically acquires mass. It is not a free parameter but a child of the universe's expansion rate at that moment.
The calculation elegantly reduces everything to a single quantity — the Hubble expansion rate at the epoch of birth. The amount of dark matter in the universe now depends only on how fast it was expanding then. Fewer guesses, more geometry.
Torsion of spacetime is not exotic: it is absent in Einstein's general relativity, but it arises naturally when one tries to unify gravity with quantum theory.
In Alastair Reynolds' novel "Gravity", gravitational waves are used as a weapon capable of distorting matter — here, they act as a creator.