[Paper Review] Planck 2018 results. IX. Constraints on primordial non-Gaussianity

Planck Collaboration, Y. Akrami+158 more|arXiv (Cornell University)|May 14, 2019

Cosmology and Gravitation TheoriesPhysics and Astronomy208 references153 citations

TL;DR

Planck analyzes full-mission CMB temperature and E-mode polarization to constrain primordial non-Gaussianity, finding tight limits on local, equilateral, and orthogonal f_NL and exploring a wide range of NG shapes.

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).

Motivation & Objective

• Motivate and quantify primordial non-Gaussianity as a probe of inflationary models.
• Combine Planck temperature and low-multipole polarization data to constrain NG shapes.
• Test a wide range of NG templates including local, equilateral, orthogonal, and scale-dependent models.
• Assess robustness through extensive validation against foregrounds and instrumental systematics.

Proposed method

• Use separable template-fitting, binned, and optimal modal bispectrum estimators to measure the CMB bispectrum.
• Incorporate low-multipole polarization (4 ≤ ℓ < 40) data to improve constraints.
• Evaluate non-primordial contributions to the bispectrum, such as lensing and extragalactic point sources.
• Provide model-independent reconstructions of the CMB bispectrum.
• Extend analysis to the trispectrum to constrain g_NL_local and other trispectrum shapes.

Experimental results

Research questions

• RQ1 What are the amplitudes of primordial NG for local, equilateral, and orthogonal shapes using Planck data?
• RQ2 Do polarization data improve NG constraints and robustness of results?
• RQ3 What is the status of NG for a broad set of models including running, isocurvature, resonance, and parity-violating scenarios?
• RQ4 How do non-primordial contributions (lensing, point sources) affect NG estimates?
• RQ5 What do Planck NG constraints imply for early-Universe inflationary models?

Key findings

• Combined temperature and polarization analysis yields f_NL^local = -0.9 ± 5.1 (68% CL).
• f_NL^equil = -26 ± 47 (68% CL).
• f_NL^ortho = -38 ± 24 (68% CL).
• g_NL^local = (-5.8 ± 6.5) × 10^4 (68% CL).
• Polarization-only bispectra provide robust constraints with sensitivity comparable to temperature-based results and agree with previous measurements.
• Lensing bispectrum detected at 3.5σ significance, rejecting the null hypothesis; no convincing signal for other NG shapes beyond the standard ones.