Breaking the Single Clock Symmetry: measuring single-field inflation non-Gaussian features

The Universe is not just cold dark matter and dark energy, it also contains baryons, radiation and neutrinos. The presence of these components, beyond the pressure-less cold dark matter and the quasi-uniform dark energy ones, imply that the single clock assumption from inflation is no longer preserved. Here we quantify this effect and show that the single-clock symmetry is ensured only on scales where baryonic effects, neutrinos effects, or sound speed are zero. These scales depend on the cosmic epoch and the Universe composition. Hence for all use and purposes of interpreting state-of-the-art and possibly forthcoming surveys, in the accessible scales, single clock symmetry cannot be said to be satisfied. Breaking the single-clock symmetry has key consequences for the study of non-Gaussian features generated by pure single-field inflation which arise from non-linearities in the metric yielding non-Gaussianities of the local type: the $n_{s}-1$ and the relativistic $-5/3$ term.