Samples from Planck TT+lowP chains in the Neff–H0 plane, colour-coded by σ8. The grey bands show the constraint H0 = (70.6 ± 3.3) km s-1Mpc-1 of Eq. (30). Notice that higher Neff brings H0 into better consistency with direct measurements, but increases σ8. Solid black contours show the constraints from Planck TT,TE,EE+lowP+BAO. Models with Neff< 3.046 (left of the solid vertical line) require photon heating after neutrino decoupling or incomplete thermalization. Dashed vertical lines correspond to specific fully-thermalized particle models, for example one additional massless boson that decoupled around the same time as the neutrinos (ΔNeff ≈ 0.57), or before muon annihilation (ΔNeff ≈ 0.39), or an additional sterile neutrino that decoupled around the same time as the active neutrinos (ΔNeff ≈ 1).