Which corners of a Lorentz-violating gravity theory survive cosmology

Type 3 New General Relativity modifies gravity by relaxing Lorentz-boost invariance while keeping diffeomorphism symmetry, leaving two free parameters. Working around a flat expanding universe, the authors derive 'background-hierarchy bounds' — conditions under which the standard linear perturbation expansion doesn't break down because background evolution swamps the kinetic terms. Scalar, vector, and tensor modes are each analyzed, and the viable region of parameter space is mapped out. The work also clarifies which gauge choices are most natural for this class of metric-affine theories, tidying up an inconsistency in earlier treatments.