Planck 2013 Results. XXIV. Constraints on primordial non-Gaussianity (1303.5084v2)

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.

• The paper uses optimal bispectrum estimators on Planck CMB data to derive precise constraints on primordial non-Gaussianity parameters.
• It reports fnl_local = 2.7 ± 5.8, fnl_equil = -42 ± 75, and fnl_ortho = -25 ± 39, supporting the simplest inflationary models.
• The study effectively mitigates foreground and systematic contamination through simulations and component separation techniques to ensure robust results.

Overview of the Planck 2013 Results on Primordial Non-Gaussianity

The research paper titled "Planck 2013 Results. XXIV. Constraints on Primordial Non-Gaussianity" provides a comprehensive analysis of the data obtained from the Planck satellite mission, focusing on the constraints of primordial non-Gaussianity (NG) using Cosmic Microwave Background (CMB) maps. The paper explores the methodologies for analyzing the bispectrum of the CMB and presents findings on the primordial local, equilateral, and orthogonal bispectrum amplitudes. The key goal is to probe the physics of the early universe, examining various inflationary models and their potential signatures.

Summary of Results

1. Constraint on Primordial Non-Gaussianity: The analysis results in specific values for the primordial NG parameters, with $\fnllocal = 2.7 \pm 5.8$, $f_\textrm{NL}^\textrm{equil}= -42 \pm 75$, and $f_\textrm{NL}^\textrm{ortho}= -25 \pm 39$. These constraints imply no significant deviations from Gaussianity in the primordial fluctuations, consistent with the predictions of the simplest inflationary models.
2. Detection of Secondary Non-Gaussian Signals: The ISW-lensing bispectrum, a secondary effect within the CMB, is detected at levels consistent with expectations under the standard cosmological model, demonstrating Planck’s sensitivity to subtle CMB features.
3. Methodology: The paper applies various optimal bispectrum estimators to analyze the CMB data, such as KSW, binned, and modal estimators. These techniques allow efficient computation and robust cross-checking of results, ensuring the reliability of the derived constraints.
4. Foreground and Systematic Contamination: The research addresses potential sources of contamination, including foreground emissions and systematic errors, utilizing simulations and component separation methods to mitigate these effects.

Implications for Cosmology and Inflation

• Single-field Inflation Models: The results significantly limit the parameter space for models like DBI inflation, characterized by a small sound speed $c_s$. The findings effectively rule out high non-Gaussianity levels that some theories predict, particularly influencing models with higher-derivative interactions or non-canonical kinetic terms.
• Multi-field and Curvaton Models: The bounds on $\fnllocal$ provide constraints on the curvaton scenario, implying that any secondary scalar fields present during inflation must be tightly coupled to the inflaton.
• Feature and Resonance Models: Analyses of features and resonant non-Gaussianity suggest that signatures seen in the CMB bispectrum align with the lack of significant anomalies, reinforcing the conventional inflationary paradigm.

Overall, this detailed exploration of the Planck data fortifies the case for scalar fields producing nearly Gaussian primordial fluctuations during inflation, presenting a formidable challenge to alternative scenarios positing higher levels of primordial NG. Future studies may delve into even more complex models and extend these analyses with upcoming CMB observations.