Modified gravity and its cosmology

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In addition to discussing general properties of modified gravity theories, our research in FY2017 focused particularly on clarifying possible observational signatures predicted by an inflationary massive gravity model we proposed in FY2016. The following are the main results:1. We pointed out the possible scale dependence of CMB non-Gaussianity arising from a large tensor-scalar-scalar coupling during inflation, and that this non-Gaussian feature may be already seen in the observational data obtained by WMAP and Planck.2.We found a class of minimally modified gravity theories, of which graviton has only two dynamical degrees of freedom, challenging the singular role of general relativity as the unique non-linear theory of massless spin-2 particles.3. We studied the evolution of gravitational waves in the inflationary massive gravity model in which graviton undergoes a parametric resonance after inflation. We found a sharp peak in the energy density spectrum of stochastic gravitational wave background due to the parametric resonance. This peak may be detected in the near future either by ground-based or space-based gravitational wave detectors.