Effect of transitions in the Planck mass during inflation on primordial power spectra

Abstract

We study the effect of sudden transitions in the effective Planck mass during inflation on primordial power spectra. Specifically, we consider models in which this variation results from the nonminimal coupling of a Brans-Dicke type scalar field. We find that the scalar power spectra develop features at the scales corresponding to those leaving the horizon during the transition. In addition, we observe that the tensor perturbations are largely unaffected, so long as the variation of the Planck mass is below the percent level. Otherwise, the tensor power spectra exhibit damped oscillations over the same scales. Due to significant features in the scalar power spectra, the tensor-to-scalar ratio $r$ shows variation over the corresponding scales. Thus, by studying the spectra of both scalar and tensor perturbations, one can constrain sudden but small variations of the Planck mass during inflation. We illustrate these effects with a number of benchmark single- and two-field models. In addition, we comment on their implications and the possibility to alleviate the tension between the observations of the tensor-to-scalar ratio performed by the Planck and BICEP2 experiments.