Pressure Parametrization of Dark Energy: First and Second-Order Constraints with Latest Cosmological Data

Abstract

We explore an extension of the $\hat{I}›$CDM model in which the pressure $p$ of the dark energy (DE) fluid evolves with the expansion of the Universe, expressed as a function of the scale factor $a$. The corresponding energy density $\ddot{I}$ is derived from the continuity equation, resulting in a dynamical equation-of-state parameter $w\equiv p/\ddot{I}$ during the late-time expansion of the Universe. The pressure is modeled using a Taylor expansion around the present epoch ($a=1$), introducing deviations from a cosmological constant within the dynamical dark energy (DDE) framework. At first order, a single new parameter $\hat{I}{\text{c◯}}_1$ captures linear deviations, while a second-order parameter, $\hat{I}{\text{c◯}}_2$, accounts for quadratic evolution in the pressure. We constrain the first- and second-order DDE models using multiple observational datasets and compare their performance against $\hat{I}›$CDM and the CPL parameterization. A joint analysis of Planck CMB, DESI, and DESY5 data yields the strongest evidence for DDE, with a $2.7\ddot{I}ƒ$ deviation in the first-order model and over $4\ddot{I}ƒ$ in the second-order model, providing strong statistical support for a departure from a cosmological constant. The reconstructed DE evolution in the second-order case reveals a distinctive non-monotonic behavior in both energy density and $w_{\mathrm{DE}}(a)$, including clear phantom-crossing phenomena. Notably, the late-time evolution of $w_{\mathrm{DE}}(a)$ remains consistent across datasets and shows strong agreement with the CPL parameterization, underscoring the robustness of the pressure-based approach.

29 pages including references, 4 tables, and 11 figures