Planck 2018 results. X. Constraints on inflation

TL;DR

Planck 2018 delivers tighter inflation constraints thanks to improved polarization and lensing data, reinforcing a simple inflationary picture. The analysis confirms a scalar spectral index n_s = 0.9649 ± 0.0042 with no significant running and a flat Universe to ~0.4% precision when combined with BAO, while tightening the tensor-to-scalar ratio to r_0.002 < 0.10 (Planck alone) and r_0.002 < 0.056 when BK15 data are included. Nonparametric reconstructions favor a pure power-law primordial spectrum, with Planck polarization providing a stringent test of adiabatic initial conditions and polarization constraints on certain anisotropic inflation scenarios. Collectively, the results favor slow-roll with concave potentials, constrain many common inflationary models (especially those predicting large r or significant features), and demonstrate Planck 2018’s critical role in shaping inflationary theory and future observational goals.

Abstract

We report on the implications for cosmic inflation of the 2018 Release of the Planck CMB anisotropy measurements. The results are fully consistent with the two previous Planck cosmological releases, but have smaller uncertainties thanks to improvements in the characterization of polarization at low and high multipoles. Planck temperature, polarization, and lensing data determine the spectral index of scalar perturbations to be $n_\mathrm{s}=0.9649\pm 0.0042$ at 68% CL and show no evidence for a scale dependence of $n_\mathrm{s}.$ Spatial flatness is confirmed at a precision of 0.4% at 95% CL with the combination with BAO data. The Planck 95% CL upper limit on the tensor-to-scalar ratio, $r_{0.002}<0.10$, is further tightened by combining with the BICEP2/Keck Array BK15 data to obtain $r_{0.002}<0.056$. In the framework of single-field inflationary models with Einstein gravity, these results imply that: (a) slow-roll models with a concave potential, $V" (φ) < 0,$ are increasingly favoured by the data; and (b) two different methods for reconstructing the inflaton potential find no evidence for dynamics beyond slow roll. Non-parametric reconstructions of the primordial power spectrum consistently confirm a pure power law. A complementary analysis also finds no evidence for theoretically motivated parameterized features in the Planck power spectrum, a result further strengthened for certain oscillatory models by a new combined analysis that includes Planck bispectrum data. The new Planck polarization data provide a stringent test of the adiabaticity of the initial conditions. The polarization data also provide improved constraints on inflationary models that predict a small statistically anisotropic quadrupolar modulation of the primordial fluctuations. However, the polarization data do not confirm physical models for a scale-dependent dipolar modulation.

Figures (1)

• Figure 1: Planck 2018 CMB angular power spectra, compared with the base-$\Lambda$CDM best fit to the Planck TT,TE,EE+lowE+lensing data (blue curves). For each panel we also show the residuals with respect to this baseline best fit. Plotted are ${\cal D}_\ell=\ell(\ell+1)C_\ell/(2\pi)$ for $TT$ and $TE$, $C_\ell$ for $EE$, and $L^2 (L+1)^2 C_L^{\phi \phi}/(2\pi)$ for lensing. For $TT$, $TE$, and $EE$, the multipole range $2 \le \ell \le 29$ shows the power spectra from Commander ($TT$) and SimAll ($TE$, $EE$), while at $\ell \ge 30$ we display the co-added frequency spectra computed from the Plik cross-half-mission likelihood, with foreground and other nuisance parameters fixed to their best-fit values in the base-$\Lambda$CDM cosmology. For the Planck lensing potential angular power spectrum, we show the conservative (orange dots; used in the likelihood) and aggressive (grey dots) cases. Note some of the different horizontal and vertical scales on either side of $\ell = 30$ for the temperature and polarization spectra and residuals.