Orateur
Description
ΛCDM fits CMB data remarkably well, and new physics is usually searched for by adding one parameter at a time: curvature, N_eff, dark-energy equation of state, varying constants, and others. Several have shown 2 to 3σ hints in Planck data. But when many extensions are tested on the same data, some such deviations are expected from noise alone: this is the look-elsewhere effect. Because the extensions are fitted to the same spectra, their significances are correlated, and the effect cannot be estimated by simply counting tests.
Based on a recent work, I will present an analytical framework that can be used to estimate the probability that a hint arises by chance among a set of extensions, accounting for correlations. For Planck, a 3σ local deviation corresponds to about 2.4σ when correcting for the look-elsewhere effect. Small-scale data, such as from the Simons Observatory LAT, break degeneracies and increase this penalty.