Observational signatures of gravitational couplings in DBI inflation

In scalar-tensor theories the scalar fields generically couple nontrivially to gravity. We study the observable properties of inflationary models with nonminimally coupled inflaton and Dirac-Born-Infeld (DBI) kinetic term. Within the assumptions of the priors of our Monte-Carlo simulations we find these models can generate new interesting observable signatures. Our discussion focuses on string theory inspired phenomenological models of relativistic D-brane inflation. While successful string theory constructions of ultraviolet DBI brane inflation remain elusive, we show that in suitable regions of the parameter space it is possible to use cosmological observables to probe the nonminimal coupling. Fortunately, the most observationally promising range of parameters include models yielding intermediate levels of non-Gaussianity in the range consistent with WMAP 5-year data, and to be constrained further by the Planck satellite.