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 $l\lesssim 100$, 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.

14 pages, 5 figures, 4 tables; v2: references added, discussion updated, matches published version

References in corpus (23)

• BICEP2 I: Detection Of B-mode Polarization at Degree Angular Scales
• The Cosmic Linear Anisotropy Solving System (CLASS) I: Overview
• A Measurement of the Cosmic Microwave Background B-Mode Polarization Power Spectrum at Sub-Degree Scales with POLARBEAR
• Inflation after WMAP3: Confronting the Slow-Roll and Exact Power Spectra with CMB Data
• BICEP2 II: Experiment and Three-Year Data Set
• Inflation model constraints from the Wilkinson Microwave Anisotropy Probe three-year data
• A joint analysis of Planck and BICEP2 B modes including dust polarization uncertainty
• Minimally Parametric Power Spectrum Reconstruction from the Lyman-alpha Forest
• Hunting Down the Best Model of Inflation with Bayesian Evidence
• Model selection applied to reconstruction of the Primordial Power Spectrum
• B-modes from Cosmic Strings
• A Nearly Scale Invariant Spectrum of Gravitational Radiation from Global Phase Transitions
• Suppressing the impact of a high tensor-to-scalar ratio on the temperature anisotropies
• The Knotted Sky I: Planck constraints on the primordial power spectrum
• WMAP 3-year primordial power spectrum
• Running Spectral Index from Large-field Inflation with Modulations Revisited
• New constraints on the observable inflaton potential from WMAP and SDSS
• Latest inflation model constraints from cosmic microwave background measurements
• Steps to Reconcile Inflationary Tensor and Scalar Spectra
• What do WMAP and SDSS really tell about inflation?
• Constraining Inflation
• How to constrain inflationary parameter space with minimal priors
• A note of clarification: BICEP2 and Planck are not in tension

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