Planck 2013 Results. XXIV. Constraints on primordial non-Gaussianity

TL;DR

The Planck 2013 results provide the most precise constraints to date on primordial non-Gaussianity in the CMB, testing various inflationary and early-Universe models with multiple estimators and comprehensive validation.

Contribution

This paper introduces a comprehensive analysis of primordial non-Gaussianity using three optimal bispectrum estimators on Planck data, providing new constraints and model-independent reconstructions.

Findings

Primordial local fNL= 2.7±5.8, equilateral fNL= -42±75, orthogonal fNL= -25±39.

Residual point sources and ISW-lensing bispectrum detected, consistent with LambdaCDM.

Constraints on inflationary models, including bounds on sound speed and curvaton decay fraction.

Abstract

The Planck nominal mission cosmic microwave background (CMB) maps yield unprecedented constraints on primordial non-Gaussianity (NG). Using three optimal bispectrum estimators, separable template-fitting (KSW), binned, and modal, we obtain consistent values for the primordial local, equilateral, and orthogonal bispectrum amplitudes, quoting as our final result fNL^local= 2.7+/-5.8, fNL^equil= -42+/-75, and fNL^ortho= -25+\-39 (68% CL statistical). NG is detected in the data; using skew-C_l statistics we find a nonzero bispectrum from residual point sources, and the ISW-lensing bispectrum at a level expected in the LambdaCDM scenario. The results are based on comprehensive cross-validation of these estimators on Gaussian and non-Gaussian simulations, are stable across component separation techniques, pass an extensive suite of tests, and are confirmed by skew-C_l, wavelet bispectrum and Minkowski functional estimators. Beyond estimates of individual shape amplitudes, we present model-independent, 3-dimensional reconstructions of the Planck CMB bispectrum and thus derive constraints on early-Universe scenarios that generate primordial NG, including general single-field models of inflation, excited initial states (non-Bunch-Davies vacua), and directionally-dependent vector models. We provide an initial survey of scale-dependent feature and resonance models. These results bound both general single-field and multi-field model parameter ranges, such as the speed of sound, c_s \geq 0.02 (95% CL), in an effective field theory parametrization, and the curvaton decay fraction r_D \geq 0.15 (95% CL). The Planck data significantly limit the viable parameter space of the ekpyrotic/cyclic scenarios. The amplitude of the 4-point function in the local model tauNL < 2800 (95% CL). These constraints represent the highest precision tests to date of physical mechanisms for the origin of cosmic structure.