INVESTIGATING THE HYPERBOLIC AND HYBRID SCALAR FIELD COSMOLOGIES WITH VARYING COSMOLOGICAL CONSTANT IN f(R,T) GRAVITY

This paper investigated two scalar field cosmological models in f(R,T) gravity with cosmic transit and varying cosmological constant Λ(t). The cosmological constant tends to have a tiny positive value at the current epoch. The scalar field pressure pϕ shows a sign flipping for normal scalar field. For the phantom field, the scalar potential V(ϕ) is negative and the energy density ρϕ = Ek ± V takes negative values when the equation of state parameter ωϕ is less than -1. The WEC, ρ = ∑i ρi ≥ 0 and pi + ρi ≥ 0, is not violated but with an instability for the second model at late times. For a scalar field ϕ, the condition ρϕ + pϕ = ρϕ (1 + ωϕ) = 2Ek ≥ 0 allows for ρϕ < 0 if ωϕ < -1. The causality and energy conditions have been discussed for both models. The cosmology in both models was studied using a given function a(t) derived from the desired cosmic behavior, which is the opposite of the traditional view.