Cosmic acceleration in f(Q) gravity: a nonlinear equation of state approach

Abstract We investigate late-time cosmic acceleration within the framework of f (Q) gravity supplemented by a phenomenological nonlinear equation of state of the form p = A - B. p = A ρ - B ρ. Adopting a power-law ansatz f (Q) =, (Q/Q₀) ^n, f (Q) = γ, (Q / Q 0) n, we derive analytic expressions for the energy density and Hubble function and confront the model with a combined dataset consisting of CC, SNIa, BAO, quasar, and GRB observations covering 0. 106 0. 106 z 2. 33. A Bayesian MCMC analysis constrains the parameter set H₀, A, B, c, n H 0, A, B, c, n and yields H₀ 71. 6^+1. 0-₁. ₉~ km~ s^-1~ Mpc^-1 H 0 ≃ 71. 6 - 1. 9 + 1. 0 km s - 1 Mpc - 1 for the joint sample, together with a transition redshift zₓₑ 0. 68. z tr ≈ 0. 68. Cosmographic diagnostics indicate a present-day deceleration parameter q₀ -0. 59, q 0 ≈ - 0. 59, with mild deviations of the jerk and snap parameters from their Λ CDM values at low redshift. Thermodynamic and causal consistency is ensured by the condition 0 cₛ² (z) 1, 0 ≤ c s 2