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Supersymmetric DBI inflation

We discuss a supersymmetric version of DBI (Dirac-Born-Infeld) inflation, which is a typical inflation model in string cosmology. The supersymmetric DBI action together with a superpotential always leads to correction terms associated with the potential into the kinetic term, which drastically change the dynamics of DBI inflation. We find two significant features of supersymmetric DBI inflation. The first one is that ultra-relativistic motion is prohibited to cause inflation, which leads to order of unity sound velocity squared and hence small non-Gaussianities of primordial curvature perturbations. The second one is that the relation between the tensor-to-scalar ratio and the field variation is modified. Then, significant tensor-to-scalar ratio $r \gtrsim 0.01$ is possible even for sub-Planck variation of the field. These new features are in sharp contrast with those of the standard non-supersymmetric DBI inflation and hence have a lot of interest implications on upcoming observations of cosmic microwave background (CMB) anisotropies by the Planck satellite as well as direct detection experiments of gravitational waves like DECIGO and BBO.