${\mathbf{\Omega }}_1{\mathbf{\Omega }}_2$-$\mathbf{\Lambda }$CDM: A promising phenomenological extension of the standard model of cosmology

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 $H_0=69.74\pm 0.77\mathrm{km}{\mathrm{s}}^{-1}\mathrm{Mp}{\mathrm{c}}^{-1}$ (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.

arXiv comment12 pages, 2 figures; Major updates; Matches the version published in Physics of The Dark Universe

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