Inflationary gravitational waves in the effective field theory of
  modified gravity
  In the approach of the effective field theory of modified gravity, we derive
the second-order action and the equation of motion for tensor perturbations on
the flat isotropic cosmological background. This analysis accommodates a wide
range of gravitational theories including Horndeski theories, its
generalization, and the theories with spatial derivatives higher than second
order (e.g., Horava-Lifshitz gravity). We obtain the inflationary power
spectrum of tensor modes by taking into account corrections induced by
higher-order spatial derivatives and slow-roll corrections to the de Sitter
background. We also show that the leading-order spectrum in concrete modified
gravitational theories can be mapped on to that in General Relativity under a
disformal transformation. Our general formula will be useful to constrain
inflationary models from the future precise measurement of the B-mode
polarization in the cosmic microwave background.
