Investigating the hyperbolic and hybrid scalar field cosmologies with varying cosmological constant in $f(R,T)$ gravity

Abstract

This paper investigated two scalar field cosmological models in $f(R,T)$ gravity with cosmic transit and varying cosmological constant $\Lambda(t)$.The cosmological constant tends to have a tiny positive value in the current epoch.The scalar field pressure $p_{\phi}$ shows a sign reversal for a normal scalar field. For the phantom field, the scalar potential $V(\phi)$ is negative and the energy density $\rho_{\phi}=E_k+V$ takes negative values when the equation of state parameter $\omega_{\phi}$ is less than $-1$. While the weak energy condition WEC implies that the total energy density $\rho=\sum_i\rho_i\geq 0$, we still can have a negative $\rho$ term as long as the total $\rho$ is positive. In the current work we argue that the WEC, $\rho=\sum_i \rho_i \geq 0$ and $p_i+\rho_i \geq 0$, is not violated but with an instability for the second model at late-times. For a scalar field $\phi$, The condition $\rho_{\phi}+p_{\phi}=\rho_{\phi} (1+\omega_{\phi})=2E_k\geq 0$ allows for $\rho_{\phi}<0$ if $\omega_{\phi}<-1$. 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.