What happens to cosmic ripples when spacetime symmetry breaks?

Standard cosmology assumes full diffeomorphism invariance, but some theories restrict this to a smaller group called transverse diffeomorphisms. Working out the perturbation theory for such fields reveals new interaction terms between multiple scalar fields that only appear because of the symmetry breaking, altering the effective speed of sound and conditions for stability. These changes could, in principle, show up in the cosmic microwave background or large-scale structure surveys.