The matter with(in) CPL

TL;DR

This paper proposes a two-parameter extension of the $ ext{Lambda}$CDM model with a modified dust equation of state below a certain redshift, fitting current data and offering a natural interpretation in cosmological backreaction.

Contribution

It introduces a novel phenomenological model with a variable dust equation of state and compares it with standard models using recent observational data.

Findings

Model fits data as well as CPL parameterization.

Model is preferred over $ ext{Lambda}$CDM with >3 sigma significance.

Predicts a specific evolution of the growth factor affecting low redshift measurements.

Abstract

We introduce a two-parameter phenomenological extension of the $\Lambda$CDM model in which the equation of state parameter of the ``dust'' fluid becomes different from zero for redshifts below a transition value $z_t$. Using data from DESI DR2 BAO, DESY5 Sn~Ia and CMB distance priors ($R,l_A,\omega_b$) data, we compare our model with the standard CPL parameterization $w_0-w_a$ for dynamical dark energy. Using the Deviance Information Criteria (DIC), we find that the two models are essentially indistinguishable ($\Delta$DIC $<$ 2) and preferred over $\Lambda$CDM with a significance $\geq 3 \sigma$. We discuss how this parameterization finds a natural interpretation in the context of cosmological backreaction and derive a prediction for the evolution of the growth factor, discussing its impact on low redshift $f\sigma_8$ measurements.