Phantom Dark Energy Nature of String-Fluid Cosmological Models in \(\boldsymbol{f(Q)}\)-Gravity

The present investigation is focused on a curvatureless and torsionless modified nonmetricity theory of gravity in the context of string fluid. For that, we use an anisotropic, locally rotationally symmetric (LRS), Bianchi Type I space-time universe with the arbitrary function \(f(Q)=Q+\Lambda\), where \(Q\) is the nonmetricity scalar and \(\Lambda\) is the cosmological constant. We solve the field equations using a time-dependent deceleration parameter and obtain various cosmological parameters. To obtain the best-fit values of the model parameters, we use the observational constraints on Hubble function \(H(z)\) and the apparent magnitude \(m(z)\) using the observational datasets \(H(z)\) and SNe Ia. Using these values of model parameters, we investigate cosmological scenarios of the model. We observe the behavior of the variable dark energy equation of state (EoS) parameter \(\omega\) in the context of a string fluid universe and find the present value of the effective EoS parameter \(\omega^{\textrm{eff}}<-1\) that corresponds to phantom dark energy. Also, we analyze the statefinder parameters and estimate the present age of the universe.