Planck 2018 results. IX. Constraints on primordial non-Gaussianity

Abstract: We analyse the Planck full-mission cosmic microwave background (CMB) temperature and E-mode polarization maps to obtain constraints on primordial non-Gaussianity (NG). We compare estimates obtained from separable template-fitting, binned, and modal bispectrum estimators, finding consistent values for the local, equilateral, and orthogonal bispectrum amplitudes. Our combined temperature and polarization analysis produces the following results: f_NL^local = -0.9 +- 5.1; f_NL^equil = -26 +- 47; and f_NL^ortho = - 38 +- 24 (68%CL, statistical). These results include the low-multipole (4 <= l < 40) polarization data, not included in our previous analysis, pass an extensive battery of tests, and are stable with respect to our 2015 measurements. Polarization bispectra display a significant improvement in robustness; they can now be used independently to set NG constraints. We consider a large number of additional cases, e.g. scale-dependent feature and resonance bispectra, isocurvature primordial NG, and parity-breaking models, where we also place tight constraints but do not detect any signal. The non-primordial lensing bispectrum is detected with an improved significance compared to 2015, excluding the null hypothesis at 3.5 sigma. We present model-independent reconstructions and analyses of the CMB bispectrum. Our final constraint on the local trispectrum shape is g_NLl^local = (-5.8 +-6.5) x 10^4 (68%CL, statistical), while constraints for other trispectra are also determined. We constrain the parameter space of different early-Universe scenarios, including general single-field models of inflation, multi-field and axion field parity-breaking models. Our results provide a high-precision test for structure-formation scenarios, in complete agreement with the basic picture of the LambdaCDM cosmology regarding the statistics of the initial conditions (abridged).

• The paper refines constraints on primordial non-Gaussianity using full-mission Planck temperature and polarization data.
• It employs multiple bispectrum estimators and four component separation methods to minimize systematic biases in CMB analysis.
• The results support Gaussian primordial fluctuations, thereby limiting the parameter space for various inflationary models.

Overview of "Planck 2018 results. IX. Constraints on primordial non-Gaussianity"

The paper "Planck 2018 results. IX. Constraints on primordial non-Gaussianity" provides a comprehensive analysis of the non-Gaussian features in the cosmic microwave background (CMB) data obtained from the full-mission observations of the Planck satellite. This study marks a significant step in constraining various models of the early Universe, particularly those predicting deviations from Gaussianity in the primordial perturbations.

Key Methodologies

1. Data and Estimators: The authors utilize the full-mission Planck CMB temperature and E-mode polarization maps, employing four component-separation techniques: SMICA, SEVEM, NILC, and Commander. They apply multiple bispectrum estimators, including the KSW, modal, and binned methods, to ensure robustness against potential systematic biases and foreground contamination.
2. Non-Gaussianity Templates: The study focuses on three primary shapes of primordial non-Gaussianity: local, equilateral, and orthogonal. Each shape is linked to specific theoretical models of inflation, and their amplitudes are encapsulated in the parameters $f_{\rm NL}^{\rm local}$, $f_{\rm NL}^{\rm equil}$, and $f_{\rm NL}^{\rm ortho}$.
3. Lensing and Point Source Contaminants: A meticulous analysis is conducted to quantify non-primordial sources of non-Gaussianity, namely lensing-induced bispectrum and contributions from extragalactic point sources. The study finds significant detection of the lensing bispectrum, excluding the null hypothesis with high confidence.

Main Results

• The combined temperature and polarization analysis yields $f_{\rm NL}^{\rm local} = -0.9 \pm 5.1$, $f_{\rm NL}^{\rm equil} = -26 \pm 47$, and $f_{\rm NL}^{\rm ortho} = -38 \pm 24$. These results are consistent with a Gaussian primordial density perturbation and align with the $\Lambda$CDM paradigm.
• Non-primary sources, particularly the lensing bispectrum, are detected with improved significance over prior analyses, crucially influencing the local $f_{\rm NL}$.

Implications for Early Universe Models

The constraints provided by this analysis place stringent limits on several classes of inflationary models:

• Single-Field Models: The observed bounds on equilateral and orthogonal non-Gaussianity limit the parameter space for single-field models with non-standard kinetic terms, such as Dirac-Born-Infeld (DBI) inflation.
• Multi-Field Models: The lack of detected local non-Gaussianity constrains the parameter space for multi-field scenarios, such as curvaton models, requiring a reassessment of their contributions to primordial perturbations.
• Alternative Scenarios: Models proposing substantial running or scale-dependent non-Gaussianity, as well as those suggesting residual isocurvature perturbations, are closely examined, though no significant deviations from Gaussianity are found.

Future Prospects

The analysis demonstrates a progressing sensitivity in the evaluation of primordial non-Gaussianity, marking an advancement towards more precise testing of the initial conditions of the Universe. Achieving $f_{\rm NL}$ sensitivities at the level of unity in future surveys is highlighted as a critical goal to distinguish between competing inflationary models, potentially unveiling new physics beyond the standard inflationary paradigm.

Overall, this paper contributes a critical step in cosmology by placing unprecedented constraints on the primordial non-Gaussianity and thereby enhancing our understanding of the Universe's inflationary phase.