The Knotted Sky II: Does BICEP2 require a nontrivial primordial power spectrum?
Abstract
An inflationary gravitational wave background consistent with BICEP2 is difficult to reconcile with a simple power-law spectrum of primordial scalar perturbations. Tensor modes contribute to the temperature anisotropies at multipoles with , and this effect --- together with a prior on the form of the scalar perturbations --- was the source of previous bounds on the tensor-to-scalar ratio. We compute Bayesian evidence for combined fits to BICEP2 and Planck for three nontrivial primordial spectra: a) a running spectral index, b) a cutoff at fixed wavenumber, and c) a spectrum described by a linear spline with a single internal knot. We find no evidence for a cutoff, weak evidence for a running index, and significant evidence for a "broken" spectrum. Taken at face-value, the BICEP2 results require two new inflationary parameters in order to describe both the broken scale invariance in the perturbation spectrum and the observed tensor-to-scalar ratio. Alternatively, this tension may be resolved by additional data and more detailed analyses.
Keywords
Cite
@article{arxiv.1403.5922,
title = {The Knotted Sky II: Does BICEP2 require a nontrivial primordial power spectrum?},
author = {Kevork N. Abazajian and Grigor Aslanyan and Richard Easther and Layne C. Price},
journal= {arXiv preprint arXiv:1403.5922},
year = {2014}
}
Comments
14 pages, 5 figures, 4 tables; v2: references added, discussion updated, matches published version