Lensing Amplitude Bias (A_lens=1.18)

The Planck CMB temperature power spectrum shows more smoothing of its acoustic peaks than ΛCDM predicts from the measured cosmological parameters — a phenomenon attributed to gravitational lensing of CMB photons by intervening large-scale structure. When the amplitude of this lensing effect is allowed to float as a free parameter A_lens (where A_lens = 1 is the ΛCDM prediction), Planck's data prefers A_lens = 1.18 ± 0.065 — a 2.8σ excess. The universe appears to be lensing the CMB roughly 18% more strongly than the standard model predicts.

This anomaly is internally inconsistent with ΛCDM: either there is more matter along the line of sight than the model allows, or the lensing power spectrum deviates from GR predictions, or there is a systematic error in the Planck data pipeline. A_lens > 1 pulls the Planck best-fit parameters toward higher Ω_m and σ₈, exacerbating the S₈ tension with low-redshift surveys. If the excess lensing is physical, it suggests either a higher matter density, a modification to gravity on large scales, or a breakdown of the assumption that lensing is described by the standard GR Weyl potential. Planck's own team considers this one of the most puzzling internal anomalies in the dataset.