Cosmological Evolution of Viscous Dark Energy in f(Q,C) Gravity: Two-Fluid Approach

TL;DR

The paper investigates a viscous dark energy model within $f(Q,C)$ gravity using a two-fluid DM–DE framework and a DM–DE interaction to explain late-time cosmic acceleration without a cosmological constant. A tractable nonlinear form $f(Q,C)=a_1 Q^{\gamma}+a_2 C$ is combined with a DM-DE coupling and a phenomenological deceleration parameter $q(z)$ to derive an analytic $H(z)$, then confronted with joint data from cosmic chronometers, DESI Y1 BAO, Pantheon+ SN, and Planck priors. The analysis yields best-fit values $H_0=67.1\pm1.6$ km s$^{-1}$ Mpc$^{-1}$, $\alpha=1.51^{+0.18}_{-0.21}$, $\beta=1.367\pm0.041$, $r_d=146.9\pm3.4$ Mpc, and $M=-19.465\pm0.051$, with the data combination constraining different parameters in complementary ways. Physically, the model produces a positive energy density and negative pressure from viscosity, an evolving dark-energy EoS with present value $\omega_{\text{eff}}(0)\approx -0.60$ and a far-future approach to $-1$, while the total density parameter $\Omega$ remains consistent with a spatially flat universe ($\Omega=1$). Statefinder diagnostics show trajectories close to $\Lambda$CDM but with small quintessence-like deviations, and the inferred age $t_0\approx13.2$ Gyr agrees with Planck estimates. Overall, the work demonstrates that viscous $f(Q,C)$ cosmologies with DM–DE interaction offer a viable geometric route to describe late-time acceleration and invites further observational tests and model refinements.

Abstract

In this paper, we study the cosmological evolution of a viscous dark energy model within the framework of $f(Q, C)$ gravity, utilizing a two-fluid approach. The model incorporates non-metricity and boundary contributions to the total action, represented by the scalar quantities $Q$ and $C$. The viscosity in the dark energy fluid is modeled to understand the impact of bulk viscosity on cosmic expansion and the late-time acceleration of the universe. We derive the field equations using the modified FLRW metric and analyze the behavior of key cosmological parameters, including the energy density, pressure, and the equation of state (EoS) parameter. The effective EoS parameter is also studied in the context of cosmic evolution. We impose observational constraints on the Hubble parameter $H(z)$ using recent datasets from DESI-Y1, SDSS-IV, Pantheon+ (without SHOES calibration), and cosmic chronometer measurements. The analysis shows that the model effectively describes the universe's expansion history and predicts a transition from deceleration to acceleration, consistent with observational data. This model also provides an alternative explanation for cosmic acceleration without the need for a cosmological constant.

Figures (9)

• Figure 1: The likelihood contours for the model parameters, shown as $1\sigma$ and $2\sigma$ errors for the free parameters, are determined using the combined DESI-Y1, SDSS-IV, Pantheon$+$ (without SHOES) and Cosmic chronometer.
• Figure 2: The behavior of energy density of the fluid with the values of free parameters constraint by the combined data sets of DESI-Y1, SDSS-IV, Pantheon$+$ (without SHOES) and CC.
• Figure 3: The behavior of isotropic pressure of the fluid with the values of free parameters constraint by the combined data sets of DESI-Y1, SDSS-IV, Pantheon$+$ (without SHOES) and CC.
• Figure 4: The behavior of equation of state parameter of dark energy fluid with the values of free parameters constraint by the combined data sets of DESI-Y1, SDSS-IV, Pantheon$+$ (without SHOES) and CC.
• Figure 5: The behavior of equation of state parameter of viscous dark energy fluid with the values of free parameters constraint by the combined data sets of DESI-Y1, SDSS-IV, Pantheon$+$ (without SHOES) and CC.