Dynamical systems of cosmological models for different possibilities of $G$ and $\rho_{\Lambda}$

Abstract

The present paper deals with the dynamics of spatially flat Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmological model with a time varying cosmological constant $\Lambda$ where $\Lambda$ evolves with the cosmic time (t) through the Hubble parameter (H). We consider that the model dynamics has a reflection symmetry $H \rightarrow -H $ with $\Lambda(H)$ expressed in the form of Taylor series with respect to H. Dynamical systems for three different cases based on the possibilities of gravitational constant G and the vacuum energy density $\rho_{\Lambda}$ have been analysed. In Case I, both G and $\rho_{\Lambda}$ are taken to be constant. We analyse stability of the system by using the notion of spectral radius, behavior of perturbation along each of the axis with respect to cosmic time and Poincare sphere. In Case II, we have dynamical system analysis for G=constant and $\rho_{\Lambda} \neq $ constant where we study stability by using the concept of spectral radius and perturbation function. In Case III, we take $G \neq$ constant and $\rho_{\Lambda} \neq$ constant where we introduce a new set of variables to set up the corresponding dynamical system. We find out the fixed points of the system and analyse the stability from different directions: by analysing behaviour of the perturbation along each of the axis, Center Manifold Theory and stability at infinity using Poincare sphere respectively. Phase plots and perturbation plots have been presented. We deeply study the cosmological scenario with respect to the fixed points obtained and analyse the late time behavior of the Universe. Our model agrees with the fact that the Universe is in the epoch of accelerated expansion. The EOS parameter $\omega_{eff}$, total energy density $\Omega_{tt}$ are also evaluated at the fixed points for each of the three cases and these values are in agreement with the observational values in [1].