The Cosmological Phonon: Symmetries and Amplitudes on Sub-Horizon Scales

Understand

In contrast to massless spinning particles, scalars are not heavily constrained by unitarity and locality.

• Off-shell, no gauge symmetries are required to write down manifestly local theories, while on-shell consistent factorisation is trivial.
• Instead a useful classification scheme for scalars is based on the symmetries they can non-linearly realise.
• Motivated by the breaking of Lorentz boosts in cosmology, in this paper we classify the possible symmetries of a shift-symmetric scalar that is assumed to non-linearly realise Lorentz boosts as, for example, in the EFT of inflation.