Negative Running of the Spectral Index, Hemispherical Asymmetry and the Consistency of Planck with Large $r$

Abstract

Planck favours a negative running of the spectral index, with the likelihood being dominated by low multipoles $l\lesssim 50$ and no preference for running at higher $l$. A negative spectral index is also necessary for the 2-$\sigma$ Planck upper bound on the tensor-to-scalar ratio $r$ to be consistent with values significantly larger than 0.1. Planck has also observed a hemispherical asymmetry of the CMB power spectrum, again mostly at low multipoles. Here we consider whether the physics responsible for the hemispherical asymmetry could also account for the negative running of the spectral index and the consistency of Planck with a large value of $r$. A negative running of the spectral index can be generated if the hemispherical asymmetry is due to a scale- and space-dependent modulation which suppresses the CMB power spectrum at low multipoles. We show that the observed hemispherical asymmetry at low $l$ can be generated while satisfying constraints on the asymmetry at higher $l$ and generating a negative spectral index of the right magnitude to account for the Planck observation and to allow Planck to be consistent with a large value of $r$.

5 pages, 3 figures. Title altered to reflect changed status of BICEP2 result. Clarified discussion of results with new figures. Version to be published in JCAP

References in corpus (7)

• Hemispherical power asymmetry in the three-year Wilkinson Microwave Anisotropy Probe sky maps
• Power asymmetry in WMAP and Planck temperature sky maps as measured by a local variance estimator
• A joint analysis of Planck and BICEP2 B modes including dust polarization uncertainty
• Constraints on cosmic hemispherical power anomalies from quasars
• Suppressing the impact of a high tensor-to-scalar ratio on the temperature anisotropies
• Hemispherical Power Asymmetry from a Space-Dependent Component of the Adiabatic Power Spectrum
• Compensation for large tensor modes with iso-curvature perturbations in CMB anisotropies

Cited by in corpus (6)

• Joint Planck and WMAP Assessment of Low CMB Multipoles
• Expected dipole asymmetry in CMB polarization
• CMB hemispherical asymmetry from non-linear isocurvature perturbations
• Generating $\fnl$ at $\ell\lsim 60$
• Negative running prevents eternal inflation
• Ambiguity in running spectral index with an extra light field during inflation