Thermodynamic reconstruction in observationally constrained dynamical dark energy models

Abstract

We perform an observational and thermodynamic analysis of four cosmological models $Λ$CDM, CPL, MPL, and the three-parameter MmAH parametrization. All dynamical models improve the fit relative to $Λ$CDM, with $Δχ^2 \sim 6$-$6.5$, with MPL providing the best fit, while MmAH remains viable. Although Bayesian evidence mildly favors $Λ$CDM due to its lower complexity, the dynamical dark energy models remain competitive alternatives given current observations. We then reconstruct thermodynamic quantities within observationally constrained cosmological models, extending previous theoretical studies. The bestfit heat capacity reconstruction indicates that the divergence associated with a second order thermodynamic phase transition coincides with the deceleration-acceleration transition only in $Λ$CDM, whereas for the dynamical dark energy models it occurs at distinct redshifts, suggesting that this coincidence is not universal. The generalized second law is satisfied for all models over $0 \le z \le 1$, while the Hessian analysis reveals a transient instability at the phase transition; however, the late time thermodynamic stability is model dependent, with CPL and MPL remaining stable and $Λ$CDM and MmAH failing to satisfy both stability conditions simultaneously. These results show that thermodynamic properties reconstructed within observationally constrained cosmological models provide a complementary probe of dark energy, with the thermodynamic phase transition emerging as an intrinsically model dependent phenomenon.