The universe in its first moments is like an inflating balloon. Cosmologists are trying to figure out what makes it puff up, and there's no shortage of ideas. One idea is to tweak Einstein's gravity slightly — adding an 'extra ingredient' that only kicks in at mind-boggling energies.
In this work, for the first time, sixteen such tweaks were tested properly: their predictions were compared with what the Planck satellite and a telescope in the Atacama Desert actually saw. Each tweak is its own recipe for how an invisible field (a kind of 'conductor') switches on an extra force. The recipes came in four types: the field could ramp up smoothly, following a power law; sharply, following an exponential; a mixed scenario; or very slowly, logarithmically.
All sixteen models unanimously produced the same number — about 0.97. This is the spectral index (a measure of how the ripples in the microwave background change from large patches to small ones). That's exactly the value telescopes see. And another detail: the key parameter μ (mu) froze at about 0.1 in all cases, as if it were a built-in cog in the mechanism that keeps the theory from falling apart.
The parameter μ≈0.1, which turned out to be common to all models, governs the 'ghostliness' of the theory: it ensures that no particles with negative energy appear in the equations, which would destroy the universe.
In Greg Egan's novel 'Schild's Ladder,' physicists tinker with corrections to gravity to create a new universe from scratch — not unlike the recipe-hunting in this work!