Solid inflation

We develop a cosmological model where primordial inflation is driven by a ‘solid’, defined as a system of three derivatively coupled scalar fields obeying certain symme-tries and spontaneously breaking a certain subgroup of these. The symmetry breaking pattern differs drastically from that of standard inflationary models: time translations are unbroken. This prevents our model from fitting into the standard effective field the-ory description of adiabatic perturbations, with crucial consequences for the dynamics of cosmological perturbations. Most notably, non-gaussianities in the curvature per-turbations are unusually large, with fNL ∼ 1/(c2s), and have a novel shape: peaked in the squeezed limit, with anisotropic dependence on how the limit is approached. Other unusual features include the absence of adiabatic fluctuation modes during inflation— which does not impair their presence and near scale-invariance after inflation—and a slightly blue tilt for the tensor modes.