Planck 2015 results. XVII. Constraints on primordial non-Gaussianity (1502.01592v2)

Abstract: The Planck full mission cosmic microwave background(CMB) temperature and E-mode polarization maps are analysed to obtain constraints on primordial non-Gaussianity(NG). Using three classes of optimal bispectrum estimators - separable template-fitting (KSW), binned, and modal - we obtain consistent values for the local, equilateral, and orthogonal bispectrum amplitudes, quoting as our final result from temperature alone fNL^local=2.5+-5.7, fNL^equil=-16+-70 and fNL^ortho=-34+-33(68%CL). Combining temperature and polarization data we obtain fNL^local=0.8+-5.0, fNL^equil=-4+-43 and fNL^ortho=-26+-21 (68%CL). The results are based on cross-validation of these estimators on simulations, are stable across component separation techniques, pass an extensive suite of tests, and are consistent with Minkowski functionals based measurements. The effect of time-domain de-glitching systematics on the bispectrum is negligible. In spite of these test outcomes we conservatively label the results including polarization data as preliminary, owing to a known mismatch of the noise model in simulations and the data. Beyond fNL estimates, we present model-independent reconstructions of the CMB bispectrum and derive constraints on early universe scenarios that generate NG, including general single-field and axion inflation, initial state modifications, parity-violating tensor bispectra, and directionally dependent vector models. We also present a wide survey of scale-dependent oscillatory bispectra, and we look for isocurvature NG. Our constraint on the local primordial trispectrum amplitude is gNL^local=(-9.0+-7.7)x10^4 (68%CL), and we perform an analysis of additional trispectrum shapes. The global picture is one of consistency with the premises of the LambdaCDM cosmology, namely that the structure we observe today was sourced by adiabatic, passive, Gaussian, and primordial seed perturbations.[abridged]

• The paper establishes stringent limits on primordial non-Gaussianity, supporting the validity of simple inflationary models using bispectrum estimators.
• The analysis employs multiple methods including KSW, binned, and modal estimators on temperature and polarization data for robust cross-validation.
• The paper’s constraints disfavor certain multi-field inflation models and set a precise benchmark for future CMB polarization and non-Gaussianity studies.

Overview of the "Planck 2015 Results. XVII. Constraints on Primordial Non-Gaussianity"

This paper presents one of a series of results derived from the full mission data of the Planck satellite, focusing specifically on constraints on primordial non-Gaussianity (NG). Primordial NG serves as a critical test of inflationary cosmology models. These models, particularly those that involve multi-field or non-standard kinetic terms, can generate distinct non-Gaussian signatures in the cosmic microwave background (CMB).

Key Findings and Methodologies

The paper employs CMB temperature and polarization maps to extract information about primordial NG. Several classes of bispectrum estimators are utilized, including separable template-fitting (KSW), binned, and modal estimators, ensuring robust cross-validation across methods. This comprehensive approach allows the determination of NG amplitudes for various bispectrum shapes, such as local, equilateral, and orthogonal.

Primordial NG Constraints:

• Local Shape: The analysis reveals $f_{\rm NL}^{\rm local} = 2.5 \pm 5.7$ from temperature data and $0.8 \pm 5.0$ when combining temperature and polarization.
• Equilateral Shape: Constraints are found to be $f_{\rm NL}^{\rm equil} = -16 \pm 70$ for temperature and $-4 \pm 43$ for combined data.
• Orthogonal Shape: The paper reports $f_{\rm NL}^{\rm ortho} = -34 \pm 33$ from temperature and $-26 \pm 21$ for the combined case.

These constraints generally affirm the Gaussianity of the primordial perturbations and thus consistency with the simplest models of inflation.

Non-Primordial NG and Systematics

• Lensing-ISW Bispectrum: The paper highlights the significance of this secondary bispectrum, demonstrating its impact on NG analyses. The bias it introduces is systematically removed to refine primordial NG estimates.
• Point Sources and Glitch Residuals: The influence of these non-Gaussian sources is evaluated, confirming minimal impact on the main NG analysis.

Broader Implications

The Planck constraints on NG have profound implications for inflationary model parameters. They notably disfavor simple multi-field inflation models that predict higher levels of NG. The inclusion of both temperature and polarization data offers a more stringent test, improving overall constraints, especially for specific models such as equilateral and orthogonal.

Future Directions and Speculations

The analysis sets a stringent benchmark for future studies, encouraging advancements in CMB polarization measurements. As systematics in polarization data become better understood and managed, further tightening of NG constraints is expected. The exploration of other potential shapes and templates, informed by theoretical developments in inflationary physics, remains a fertile ground for future research. Moreover, complementary probes, such as higher-order polyspectra or cross-correlation studies with large-scale structure data, could provide additional insight into early universe physics.

In summary, the paper presents a meticulous and exhaustive analysis of Planck 2015 data, offering strong constraints on primordial NG. This contributes significantly to our understanding of inflationary physics and the initial conditions of our universe.