Steps to Reconcile Inflationary Tensor and Scalar Spectra

Abstract

The recent BICEP2 B-mode polarization determination of an inflationary tensor-scalar ratio $r=0.2_{-0.05}^{+0.07}$ is in tension with simple scale-free models of inflation due to a lack of a corresponding low multipole excess in the temperature power spectrum which places a limit of $r_{0.002}<0.11$ (95% CL) on such models. Single-field inflationary models that reconcile these two observations, even those where the tilt runs substantially, introduce a scale into the scalar power spectrum. To cancel the tensor excess, and simultaneously explain the excess already present in $\Lambda$CDM, ideally the model should introduce this scale as a relatively sharp transition in the tensor-scalar ratio around the horizon at recombination. We consider models which generate such a step in this quantity and find that they can improve the joint fit to the temperature and polarization data by up to $2\Delta \ln \mathcal{L}\approx -14$ without changing cosmological parameters. Precision E-mode polarization measurements should be able to test this explanation.

Revisions match published version with more extended analysis that leave qualitative conclusions unchanged

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