The first three minutes after the birth of the world are a cosmic kitchen where nature cooked the lightest atomic nuclei. The recipe is set by gravity: it controls the cooling rate of the hot 'broth' of particles. If gravity worked differently than we are used to, the dish would have turned out different — with different amounts of helium and deuterium.
The authors took a fresh theory of spacetime curvature: it is called $f(R,G,\mathcal{T})$ and is more complex than Einstein's. In this theory, curvature depends not only on matter but also on its internal pressure, as well as on a special combination of curvatures — the so-called Gauss–Bonnet invariant. Four variants of such gravity were stress-tested against data from the 'kitchen': how much helium was cooked in the first minutes.
The constraints turned out to be no tighter than for standard cosmology. For each of the four models, wide parameter regions exist where the calculated helium abundance falls within the allowed corridor. In other words, the new gravity does not spoil the proven recipe for light nuclei: it just adds new, still permissible ingredients.
Thus, the first minutes of the universe become not only the cradle of matter but also a testing ground for the boldest theories of gravity. The same cosmic kitchen that gave us hydrogen and helium now helps verify the geometry of space itself.
Almost all the helium that fills party balloons was not born in the Big Bang but in the interiors of stars, yet primordial helium still makes up about a quarter of all helium in the universe.