Dark Radiation and Inflationary Freedom after Planck 2015

TL;DR

This paper investigates how potential deviations from the standard power-law primordial power spectrum, including dark radiation and inflationary parameters, are constrained by Planck 2015 data, emphasizing the importance of polarization measurements.

Contribution

It analyzes degeneracies between non-standard primordial spectra and neutrino-related parameters, demonstrating the impact of polarization data on constraining inflationary models.

Findings

Polarization data reduces degeneracies in parameter estimation.

No significant deviation from standard ΛCDM with a power-law PPS.

Constraints on dark radiation and sterile neutrinos are less strict without polarization.

Abstract

The simplest inflationary models predict a primordial power spectrum (PPS) of the curvature fluctuations that can be described by a power-law function that is nearly scale-invariant. It has been shown, however, that the low-multipole spectrum of the CMB anisotropies may hint the presence of some features in the shape of the scalar PPS, which could deviate from its canonical power-law form. We study the possible degeneracies of this non-standard PPS with the neutrino anisotropies, the neutrino masses, the effective number of relativistic species and a sterile neutrino or a thermal axion mass. The limits on these additional parameters are less constraining in a model with a non-standard PPS when only including the temperature auto-correlation spectrum measurements in the data analyses. The inclusion of the polarization spectra noticeably helps in reducing the degeneracies, leading to results that typically show no deviation from the $\Lambda$CDM model with a standard power-law PPS.