# Observational evidence for a local underdensity in the Universe and its effect on the measurement of the Hubble Constant

## TL;DR

This study identifies a significant local underdensity in galaxy cluster distribution, which affects local measurements of the Hubble Constant and may partly explain discrepancies with cosmic microwave background estimates.

## Contribution

It provides observational evidence of a local matter underdensity using galaxy cluster data and assesses its impact on Hubble Constant measurements within a LambdaCDM framework.

## Key findings

- Detected a 30-60% underdensity extending up to 170 Mpc.
- The underdensity increases the local Hubble parameter by about 5.5%.
- Probability of such an underdensity in LambdaCDM is around 10%.

## Abstract

For precision cosmological studies it is important to know the local properties of the reference point from which we observe the Universe. Particularly for the determination of the Hubble constant with low-redshift distance indicators, the values observed depend on the average matter density within the distance range covered. Here we used the spatial distribution of galaxy clusters to map the matter density distribution. The study is based on our CLASSIX galaxy cluster survey, which is highly complete and well characterised with galaxy clusters detected in X-rays. We find a local underdensity in the cluster distribution of about 30 - 60% which extends ~85 Mpc to the north and ~170 Mpc to the south. For three regions for which the galaxy density distribution has previously been studied, we find good agreement between the density distribution of clusters and galaxies. Correcting for the bias in the cluster distribution we infer an underdensity in the matter distribution of about -0.3 +- 0.15 (-0.2 +- 0.1) in a region with a radius of about 100 (~140) Mpc. Calculating the probability of finding such an underdensity theoretically in a LambdaCDM universe with concordance cosmological parameters, we find a probability characterised by sigma-values of 1.3-3.7. This indicates low probabilities, but with values of around 10% at the lower uncertainty limit, the existence of an underdensity cannot be ruled out. Inside this underdensity, the observed Hubble parameter will be larger by about 5.5 +2.1-2.8%, which explains part of the discrepancy between the locally measured value of H_0 compared to the value of H_0 inferred from the Planck observations of cosmic microwave background anisotropies. If distance indicators outside the local underdensity are included, as in many modern analyses, this effect is diluted.