Mimetic gravity as DHOST theories

Understand

We show that theories of mimetic gravity can be viewed as degenerate higher-order scalar-tensor (DHOST) theories that admit an extra local (gauge) symmetry in addition to the usual diffeomorphism invariance.

• We reformulate and classify mimetic theories in this perspective.
• Using the effective theory of dark energy, recently extended to include DHOST theories, we then investigate the linear perturbations about a homogeneous and isotropic background for all mimetic theories.
• We also include matter, in the form of a $k$-essence scalar field, and we derive the quadratic action for linear perturbations in this case.