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

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.