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 $$p = A\rho - B\sqrt{\rho }.$$ p = A ρ - B ρ . Adopting a power-law ansatz $$f(Q)=\gamma ,(Q/Q_0)^{n},$$ 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 $$0.106 0.106 < z < 2.33 . A Bayesian MCMC analysis constrains the parameter set $${H_0, A, B, c, n}$$ H 0 , A , B , c , n and yields $$H_0 \simeq 71.6^{+1.0}_{-1.9}~\text {km}~\text {s}^{-1}~\text {Mpc}^{-1}$$ H 0 ≃ 71 . 6 - 1.9 + 1.0 km s - 1 Mpc - 1 for the joint sample, together with a transition redshift $$z_{\textrm{tr}}\approx 0.68.$$ z tr ≈ 0.68 . Cosmographic diagnostics indicate a present-day deceleration parameter $$q_0\approx -0.59,$$ q 0 ≈ - 0.59 , with mild deviations of the jerk and snap parameters from their $$\Lambda $$ Λ CDM values at low redshift. Thermodynamic and causal consistency is ensured by the condition $$0\le c_s^2(z)\le 1,$$ 0 ≤ c s 2