Abstract
For precision cosmological studies it is important to know the local
properties of our reference point from which we observe the Universe. In
particular for the determination of the Hubble constant with low redshift
distance indicators, the values observed will depend on the local average
matter density. In this work we used the spatial distribution of galaxy
clusters to map the matter density distribution in the local Universe. The
study is based on our CLASSIX galaxy cluster survey, where galaxy clusters are
detected by their X-ray emission. In total 1653 galaxy clusters are involved in
this study. We find a local underdensity in the cluster distribution of about
30 - 60% which extends about 85 Mpc to the north and about 170 Mpc to the
South. For three regions for which the galaxy density distribution was studied
previously, 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 -30 +- 15% (-20 +- 10%) in a
region with a radius of about 100 (140) Mpc. Calculating the probability of
finding such an underdensity through structure formation theory 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 around 10% at the lower uncertainty limit, this finding is not
so unlikely. 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 the Hubble
parameter inferred from the Planck observations of cosmic microwave background
anisotropies.
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