Testing backreaction effects with type Ia supernova data and observational Hubble parameter data

TL;DR

This paper investigates the effects of backreaction in the universe using supernova and Hubble data, testing different metric models and finding that the choice of curvature parameter impacts data consistency.

Contribution

It introduces a backreaction model with CPL parameterization and compares its fit using two different metrics, highlighting issues with the evolving curvature assumption.

Findings

Parameter tensions increase with the template metric.

The standard FLRW metric shows better consistency with data.

Modifications are needed for the curvature prescription in backreaction models.

Abstract

The backreaction term ${\cal Q}_\CD$ and the averaged spatial Ricci scalar $\average{\CR}$ in the spatially averaged inhomogeneous Universe can be used to combine into effective perfect fluid energy density $\varrho_{\rm eff}^{\CD}$ and pressure $p_{\rm eff}^{\CD}$ that can be regarded as new effective sources for the backreaction effects. In order to model the realistic evolution of backreaction, we adopt the Chevallier-Polarski-Linder(CPL) parameterizations of the equation of state(EoS) of the effective perfect fluid. To deal with observations in the backreaction context, in this paper, we employ two metrics to describe the the late time Universe, one of them is the standard Friedmann-Lema\^{\i}tre-Robertson-Walker(FLRW) metric, and the other is a template metric with an evolving curvature parameter introduced by Larena et. al. in \cite{larena2009testing}. We also fit the CPL backreaction model using type Ia supernova(SN Ia) data and observational Hubble parameter data(OHD) with these two metrics, and find out that parameter tensions between two different data sets are larger when the backreaction model is equipped with the template metric, therefore we conclude that the prescription of the geometrical instantaneous spatially-constant curvature $\kappa_{\CD}$ needs to be modified.