An effective Λ-Szekeres modelling of the local Universe with Cosmicflows-4

Abstract We develop an effective description of the local cosmic environment — namely, for redshift z ≲ 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 Λ-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 Δ H 0 ≈ 0.5 km s -1 Mpc -1 .