Backreaction and the Role of Spatial Curvature in the Cosmic Neighborhood

TL;DR

This study computes local universe averaged quantities using cosmic reconstructions, revealing significant spatial curvature effects and minimal backreaction, with implications for understanding cosmic structure and expansion.

Contribution

First direct computation of local spatially averaged dynamical quantities using a covariant formalism and cosmic reconstructions, highlighting curvature effects.

Findings

Spatial curvature contributes about 10% to the local energy budget.

Kinematical backreaction is negligible, around 1% at most.

No convergence to the global ΛCDM model within 300 Mpc/h scale.

Abstract

We present the first direct computation of spatially averaged dynamical quantities in the local Universe, employing the Cosmicflows-4++ reconstruction and a covariant scalar averaging formalism. We extract the domain-averaged density, expansion rate, spatial curvature, and kinematical backreaction over cosmologically relevant domains around our Galaxy, extending up to a comoving radius of $300~\mathrm{Mpc}/h$. The resulting domain-averaged present-day energy budget features nontrivial variations with scale that reflect a nested structure within the cosmic neighborhood, including a large-scale void shell encompassing the local cosmic web. Remarkably, we find significant contributions to this energy budget from the average spatial curvature at the $\mathcal{O}(10\%)$ level on all probed scales. By contrast, the kinematical backreaction remains much smaller throughout the surveyed volume, reaching at most a $\mathcal{O}(1\%)$ contribution on the smallest scales considered, i.e., $30~\mathrm{Mpc}/h$. Convergence to the global $\Lambda$CDM background is not observed within this range of scales.