Tuning a radio is a delicate business: one knob turns easily, another resists. In the physics of the early Universe, there are also 'knobs'—parameters that determine what we see today. One of them is the strength of the coupling between a special field (the one that drove the rapid expansion of the Universe) and the geometry of space. But this geometry is not described by the usual curvature; instead, it involves a quantity called 'nonmetricity.'

The coupling is denoted by the letter ξ (xi), and it acts like a volume control for two key observables: the tilt of the primordial perturbation spectrum $n_s$ and the fraction of gravitational waves r. When ξ is too small, the space after inflation ends up too 'choppy' with gravitational waves, like a sea in a storm. When it is too large, the perturbation spectrum becomes unnaturally blue, which disagrees with data. The working range turned out to be very narrow: from 0.001 to 0.01. For comparison, that is like the allowable play in a car's steering wheel being fractions of a millimeter.

The authors also considered a model where the field potential changes smoothly, like a gentle hill (the so-called Cosh type). Here everything fell into place particularly beautifully. With 60 'breaths' of expansion (e-folds), the model predicts $n_s$ ≈ 0.965–0.967 and r ≈ 0.017–0.018 — numbers that almost perfectly match observations. It turns out that a tiny but nonzero coupling between the field and nonmetricity is not just permissible: it is necessary for the picture of the world not to fall apart.

Thus, like the fine-tuning of a receiver, choosing the right coupling strength turns the chaos of the first moments into an ordered Universe with galaxies, stars, and planets. Turn this 'knob' a bit differently, and our world could have been completely different or might never have appeared at all.