Is expansion blind to the spatial curvature?

TL;DR

This paper explores a modified Einstein equation that makes cosmic expansion insensitive to spatial curvature, analyzing its cosmological implications, perturbations, and observational distinctions from standard models.

Contribution

It introduces a bi-connection modification of Einstein's equations where expansion is curvature-blind, and examines its perturbations and observational consequences.

Findings

Expansion is independent of spatial curvature in the new theory.

Two additional gauge-invariant perturbation modes are identified.

Differences from ΛCDM are negligible, but curvature effects could be distinguished with higher precision.

Abstract

In [arXiv:2204.13980], we proposed and motivated a modification of the Einstein equation as a function of the topology of the Universe in the form of a bi-connection theory. The new equation features an additional "topological term" related to a second non-dynamical reference connection and chosen as a function of the spacetime topology. In the present paper, we analyse the consequences for cosmology of this modification. First, we show that expansion becomes blind to the spatial curvature in this new theory, i.e. the expansion laws do not feature the spatial curvature parameter anymore (i.e. $\Omega_{\not= K} = 1, \ \forall \, \Omega_K$), while this curvature is still present in the evaluation of distances. Second, we derive the first order perturbations of this homogeneous solution. Two additional gauge invariant variables coming from the reference connection are present compared with general relativity: a scalar and a vector mode, both sourced by the shear of the cosmic fluid. Finally, we confront this model with observations. The differences with the $\Lambda$CDM model are negligible, in particular, the Hubble and curvature tensions are still present. Nevertheless, since the main difference between the two models is the influence of the background spatial curvature on the dynamics, an increased precision on the measure of that parameter might allow us to observationally distinguish them.