Inflation in Motion: Unitarity Constraints in Effective Field Theories
  with Broken Lorentz Symmetry
  During inflation, there is a preferred reference frame in which the expansion
of the background spacetime is spatially isotropic. In contrast to Minkowski
spacetime, observables can depend on the velocity of the system with respect to
this cosmic rest frame. We derive new constraints from radiative stability and
unitarity on effective field theories with such spontaneously broken Lorentz
symmetry. In addition to a maximum energy scale, there is now also a critical
velocity at which the theory breaks down. The theory therefore has different
resolving power in time and in space, and we show that these can only coincide
if cubic Lorentz-violating interactions are absent. Applying these bounds to
the Effective Field Theory of Inflation, we identify the region of parameter
space in which inflation can be both single-field and weakly coupled on
subhorizon scales. This can be implemented as a theoretical prior, and we
illustrate this explicitly using Planck observational constraints on the
primordial bispectrum.
