Cosmological evolution of viscous dark energy in \(f(Q,C)\) gravity: two-fluid approach

Abstract

In this paper, we explore the cosmological evolution of a viscous dark energy model within the framework of \(f(Q, C)\) gravity, employing 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 introduced to investigate the impact of bulk viscosity on cosmic expansion and late-time acceleration. Field equations are derived in a modified FLRW background, and the dynamics of key cosmological quantities such as energy density, pressure, and the effective equation of state (EoS) parameter are analyzed. Observational constraints on \(H(z)\) are imposed using DESI BAO Measurements, Pantheon+ (without SHOES), and cosmic chronometer datasets. Results indicate that the model effectively captures the universe’s expansion history, including the deceleration–acceleration transition, consistent with observations. This framework provides an alternative explanation for late-time cosmic acceleration without invoking a cosmological constant.