Planck 2015 results. XX. Constraints on inflation

TL;DR

The Planck 2015 data tightly constrain inflationary models, favoring those with low tensor-to-scalar ratios and disfavoring models like quadratic and natural inflation, while confirming the adiabatic nature of primordial perturbations.

Contribution

This paper provides the most precise constraints on inflationary models using full Planck CMB data, including temperature and polarization, and explores primordial power spectrum reconstructions.

Findings

Spectral index n_s = 0.968 ± 0.006

Tensor-to-scalar ratio r_{0.002} < 0.11 (95% CL)

Consistent with adiabatic primordial perturbations

Abstract

We present the implications for cosmic inflation of the Planck measurements of the cosmic microwave background (CMB) anisotropies in both temperature and polarization based on the full Planck survey. The Planck full mission temperature data and a first release of polarization data on large angular scales measure the spectral index of curvature perturbations to be $n_\mathrm{s} = 0.968 \pm 0.006$ and tightly constrain its scale dependence to $d n_s/d \ln k =-0.003 \pm 0.007$ when combined with the Planck lensing likelihood. When the high-$\ell$ polarization data is included, the results are consistent and uncertainties are reduced. The upper bound on the tensor-to-scalar ratio is $r_{0.002} < 0.11$ (95% CL), consistent with the B-mode polarization constraint $r< 0.12$ (95% CL) obtained from a joint BICEP2/Keck Array and Planck analysis. These results imply that $V(\phi) \propto \phi^2$ and natural inflation are now disfavoured compared to models predicting a smaller tensor-to-scalar ratio, such as $R^2$ inflation. Three independent methods reconstructing the primordial power spectrum are investigated. The Planck data are consistent with adiabatic primordial perturbations. We investigate inflationary models producing an anisotropic modulation of the primordial curvature power spectrum as well as generalized models of inflation not governed by a scalar field with a canonical kinetic term. The 2015 results are consistent with the 2013 analysis based on the nominal mission data.