Planck and the local Universe: quantifying the tension

TL;DR

This paper assesses the tension between Planck cosmological constraints and local measurements of the Hubble constant and universe age, proposing a new statistical method to quantify this tension and exploring potential explanations within and beyond the standard cosmological model.

Contribution

It introduces a novel evidence ratio statistic to quantify tension between datasets and applies it to Planck and local measurements, analyzing implications for cosmology.

Findings

Planck data shows strong tension with local measurements (odds ~ 1:50).

Small neutrino masses reduce tension; masses above 0.15 eV increase tension.

Degenerate neutrino hierarchy is disfavored by data.

Abstract

We use the latest Planck constraints, and in particular constraints on the derived parameters (Hubble constant and age of the Universe) for the local universe and compare them with local measurements of the same quantities. We propose a way to quantify whether cosmological parameters constraints from two different experiments are in tension or not. Our statistic, T, is an evidence ratio and therefore can be interpreted with the widely used Jeffrey's scale. We find that in the framework of the LCDM model, the Planck inferred two dimensional, joint, posterior distribution for the Hubble constant and age of the Universe is in "strong" tension with the local measurements; the odds being ~ 1:50. We explore several possibilities for explaining this tension and examine the consequences both in terms of unknown errors and deviations from the LCDM model. In some one-parameter LCDM model extensions, tension is reduced whereas in other extensions, tension is instead increased. In particular, small total neutrino masses are favored and a total neutrino mass above 0.15 eV makes the tension "highly significant" (odds ~ 1:150). A consequence of accepting this interpretation of the tension is that the degenerate neutrino hierarchy is highly disfavoured by cosmological data and the direct hierarchy is slightly favored over the inverse.