An effective $\boldsymbol{\Lambda}$-Szekeres modelling of the local Universe with Cosmicflows-4

TL;DR

This paper models the local Universe's structure using a superposition of $oldsymbol{\Lambda}$-Szekeres patches calibrated with Cosmicflows-4 data to assess its impact on Hubble constant estimates.

Contribution

It introduces a $oldsymbol{\Lambda}$-Szekeres based framework calibrated with Cosmicflows-4 to quantify local structure effects on cosmological measurements.

Findings

Accounting for local structure increases the Hubble tension by about 0.5 km/s/Mpc.

The model predicts an inhomogeneous, anisotropic quasilocal expansion field.

Local structure bias can significantly affect low-redshift supernova-based $H_0$ estimates.

Abstract

We develop an effective description of the local cosmic environment, namely, for redshift $z \lesssim 0.1$, to quantify the bias induced by local structure on cosmological observables. Our approach models the metric of the nearby Universe as a superposition of multi-structured $\Lambda$-Szekeres patches, calibrated against the HAMLET peculiar velocity and density field reconstructions of Cosmicflows-4. From this framework we compute the fully inhomogeneous and anisotropic quasilocal expansion field predicted by our model, and use it to assess the impact of local structure on estimates of $H_0$. For this purpose we analyse low-redshift Type Ia supernovae from the Pantheon+ catalogue. We find that accounting for the local structure increases the Hubble tension, yielding a shift in the best-fit value of the Hubble constant of order $\Delta H_0 \approx 0.5\ \mathrm{km\,s^{-1}Mpc^{-1}}$.