Primordial (scalar curvature) power spectrum, reconstructed by using the Planck 2018 TTTEEE+lowE+lensing likelihood. This was done by sampling the parameters of an extended ΛCDM model, where the initial power spectrum was described with a varying number of movable spline nodes (from one to nine), rather than assumed to be a power law. The final reconstruction (bottom right plot) is obtained by marginalising, i.e., weighting each of the nine reconstructions by its own evidence. With two nodes (top left), the departure from scale invariance with ns − 1 ≃ −0.035 is nicely recovered. With three nodes the uncertainties at low ℓ (due to the small number of modes) and high-ℓ (due to noise) becomes visible. With a larger numbers of nodes, anomalies may be captured, and the most visible departure from a pure power law reflects the well–known power deficit at ℓ ≃ 20–30. However, the evidence-weighted plot (bottom right panel) shows that the evidence for such a spectral feature is actually not very significant. In Planck Collaboration XX (2016), this spectrum was reconstructed using three additional methods, with similar conclusions.
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