$\mathbf{\Omega_1\Omega_2}$-$\mathbf{\Lambda}$CDM: A promising phenomenological extension of the standard model of cosmology

TL;DR

This paper introduces an extended cosmological model, $oldsymbol{\Omega_1oldsymbol{\Omega_2}}$-$oldsymbol{\Lambda}$CDM, adding terms to dark energy density, which fits observational data well and offers a flexible late-time universe evolution.

Contribution

It proposes a novel phenomenological extension to $oldsymbol{\Lambda}$CDM$, incorporating additional terms in the energy density expansion, and demonstrates its consistency with observational data.

Findings

The model fits Planck and DESI data effectively.

It predicts a Hubble constant consistent with TRGB measurements.

Dark energy equation of state shows a smooth transition across the phantom divide.

Abstract

We investigate a phenomenological extension of the standard $\Lambda$CDM framework, the $\Omega_1\Omega_2$-$\Lambda$CDM model, in which the total energy density of the universe is expanded in powers of $1+z$. This parameterization recovers $\Lambda$CDM and introduces two additional terms, $\Omega_1(1+z)$ and $\Omega_2(1+z)^2$, in the dark energy sector alongside the cosmological constant, leading to a physically interpretable and observationally testable effective dark energy dynamics. Using Planck cosmic microwave background (CMB) data, we find that the model allows additional freedom in the late-time expansion history while preserving standard early-universe physics. When DESI baryon acoustic oscillation (BAO) data are included, the inferred Hubble constant is $H_0 = 69.74 \pm 0.77~\mathrm{km\;s^{-1}\;Mpc^{-1}}$ (68% CL), consistent with Tip of the Red Giant Branch (TRGB) measurements from the Carnegie--Chicago Hubble Program (CCHP). The reconstructed dark energy equation of state exhibits a smooth transition across the phantom divide followed by asymptotic de Sitter behavior, modifying late-time dynamics while maintaining standard early-time cosmology. Overall, controlled late-time deviations from $\Lambda$CDM improve cosmological concordance.