Effects of the speed of sound at large-N

Abstract

The implications of a smaller than unity speed of sound of the scalar perturbations during cosmic inflation is studied in terms of a model independent large-$N$ approach. We show that the $(n_s,r)$ plane is non-trivially degenerated when the effects of a different than one speed of sound $c_s$ are taken into account. We also discuss the degeneracies due to $c_s$ in the running of the spectral index, $\hat{I}{\pm }_s$. We use the bounds on $c_s$ to constrain such degeneracies.

4 figures, 5 pages. References added. Typos corrected. Improved discussion. Version to appear in PRD