<i>Planck</i>2018 results

<jats:p>We report on the implications for cosmic inflation of the 2018 release of the<jats:italic>Planck</jats:italic>cosmic microwave background (CMB) anisotropy measurements. The results are fully consistent with those reported using the data from the two previous<jats:italic>Planck</jats:italic>cosmological releases, but have smaller uncertainties thanks to improvements in the characterization of polarization at low and high multipoles.<jats:italic>Planck</jats:italic>temperature, polarization, and lensing data determine the spectral index of scalar perturbations to be<jats:italic>n</jats:italic><jats:sub>s</jats:sub> = 0.9649 ± 0.0042 at 68% CL. We find no evidence for a scale dependence of<jats:italic>n</jats:italic><jats:sub>s</jats:sub>, either as a running or as a running of the running. The Universe is found to be consistent with spatial flatness with a precision of 0.4% at 95% CL by combining<jats:italic>Planck</jats:italic>with a compilation of baryon acoustic oscillation data. The<jats:italic>Planck</jats:italic>95% CL upper limit on the tensor-to-scalar ratio,<jats:italic>r</jats:italic><jats:sub>0.002</jats:sub> <  0.10, is further tightened by combining with the BICEP2/Keck Array BK15 data to obtain<jats:italic>r</jats:italic><jats:sub>0.002</jats:sub> <  0.056. In the framework of standard single-field inflationary models with Einstein gravity, these results imply that: (a) the predictions of slow-roll models with a concave potential,<jats:italic>V</jats:italic>″(<jats:italic>ϕ</jats:italic>) < 0, are increasingly favoured by the data; and (b) based on two different methods for reconstructing the inflaton potential, we find no evidence for dynamics beyond slow roll. Three different methods for the non-parametric reconstruction of the primordial power spectrum consistently confirm a pure power law in the range of comoving scales 0.005 Mpc<jats:sup>−1</jats:sup> ≲ <jats:italic>k</jats:italic> ≲ 0.2 Mpc<jats:sup>−1</jats:sup>. A complementary analysis also finds no evidence for theoretically motivated parameterized features in the<jats:italic>Planck</jats:italic>power spectra. For the case of oscillatory features that are logarithmic or linear in<jats:italic>k</jats:italic>, this result is further strengthened by a new combined analysis including the<jats:italic>Planck</jats:italic>bispectrum data. The new<jats:italic>Planck</jats:italic>polarization data provide a stringent test of the adiabaticity of the initial conditions for the cosmological fluctuations. In correlated, mixed adiabatic and isocurvature models, the non-adiabatic contribution to the observed CMB temperature variance is constrained to 1.3%, 1.7%, and 1.7% at 95% CL for cold dark matter, neutrino density, and neutrino velocity, respectively.<jats:italic>Planck</jats:italic>power spectra plus lensing set constraints on the amplitude of compensated cold dark matter-baryon isocurvature perturbations that are consistent with current complementary measurements. The polarization data also provide improved constraints on inflationary models that predict a small statistically anisotropic quadupolar modulation of the primordial fluctuations. However, the polarization data do not support physical models for a scale-dependent dipolar modulation. All these findings support the key predictions of the standard single-field inflationary models, which will be further tested by future cosmological observations.</jats:p>