Primordial gravitational waves in DHOST inflation

We consider DHOST inflationary models with a shift symmetry leading to a de Sitter space-time at the background cosmological level. Deviations from scale invariance of the scalar and tensor perturbations follow from the breaking of the shift symmetry by quadratic and quartic operators. These models show a strong violation of the consistency relation of single-field inflationary models with a very flat spectrum of tensor perturbations. This opens up the prospects of future detection of primordial gravitational waves by mHz experiments.