Can cosmic voids ease the Hubble tension? Local expansion in $w_0w_a$CDM

Abstract

We investigate the effect of local cosmic voids on low-redshift measurements of the Hubble rate in flat $w_0w_a$CDM cosmologies. Using a hydrodynamical model for isolated spherical inverse top-hat underdensities, we compute the void-induced Hubble shift as a function of redshift, enclosed density contrast $\delta_{\rm E}$ and cosmological parameters. We find that the effect is mainly controlled by the matter sector, through $\delta_{\rm E}$ and $\Omega_{\rm m,0}$, while dynamical dark energy gives only subdominant late-time corrections. For a Planck-calibrated $\Lambda$CDM background, matching the SH0ES value requires a present-day void with $\delta_{\rm E}(z=0)\simeq -0.44$, substantially deeper than the KBC-like local underdensity. A KBC-like void lowers the SH0ES-Planck discrepancy to about $2\sigma$, but is not deep enough to fully reconcile the two measurements. Allowing for dynamical dark energy, including regions motivated by recent DES and DESI analyses, changes this result only at the percent level. We also show that the reported redshift dependence of locally inferred $H_0$ values can be represented phenomenologically by an effective enclosed matter profile. This reconstruction should be interpreted as a consistency test of local-structure effects rather than as explanation for an evolving background value of $H_0$. Overall, local underdensities can affect low-redshift Hubble-rate inferences, but they do not resolve the Hubble tension within the spherical void setup considered here.