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Phenomenology of D-Brane Inflation with General Speed of Sound

Hiranya Peiris (University of Chicago)
03 July 2007, 14:30 - 16:15

We investigate the phenomenology of D-brane inflation using a general numerical algorithm. In this inflationary model based on string theory, fluctuations in the inflaton field can propagate at a speed significantly smaller than the speed of light, in contrast to standard single field slow-roll inflation, yielding observable effects that are distinct from the latter. We extend the inflationary flow formalism now routinely discussed for standard slow-roll models to warped brane inflation, describing both the Hubble parameter and the Lorentz factor of brane motion through a Monte Carlo approach based on the Hamilton-Jacobi formalism for the modified equations of motion for the inflaton. We use our numerical results to make qualitative predictions for the inflationary observables expected for D-brane inflation with general sound speed, such as the amplitude of the gravitational wave background, non-Gaussianity of primordial perturbations, and the shape of the primordial power spectrum of scalar perturbations. We discuss various consistency relations for D-brane inflation proposed in the literature, and compare the qualitative shapes of the warp factor we derive from numerical models with the analytical warp factors considered in the literature. We derive and apply a generalized bound on the field variation during inflation. While we can generate a large number of DBI models generally consistent with existing cosmological constraints, almost all these models violate microphysical constraints on the field range in four-dimensional Planck units. We discuss the perdictions of models that survive the microphysical constraints for the stochastic gravitational wave background. If the field range bound is to hold, then the tensor component predicted by D-brane inflationary models is far below the amplitude that can be reasonably detected with any observational program.
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