Cosmological dynamics of holographic dark energy with non-minimally coupled scalar field

TL;DR

This paper investigates the cosmological evolution of a universe with holographic dark energy and a non-minimally coupled scalar field, analyzing fixed points, stability, and late-time behavior.

Contribution

It introduces a model combining holographic vacuum energy with a non-minimally coupled scalar field and analyzes its dynamical stability and cosmological evolution.

Findings

The universe transitions from stiff-fluid to dust and dark energy domination.

Stability requires negative coupling constant and small holographic parameter.

Late-time effective equation of state approaches -1.

Abstract

In this study, we consider FRW universe filled with matter, non-minimally coupling (NMC) scalar field under $V(\phi) = V_{0}\phi^{2}$ potential and holographic vacuum energy. Dark energy is contributed from both holographic vacuum energy and the NMC scalar field. NMC effective gravitational constant $G_\text{eff}(\phi)$, is naturally defined at the action level. Therefore, the gravitational constant in the holographic vacuum density is an effective one, i.e. $ \rho_{\Lambda} = {3c^{2}}/{8\pi G_{\text{eff}}L^{2}}\,. $ Apparent horizon is chosen as IR holographic cutoff scale as it is a trapped null surface. There are nine fixed points in this dynamical system with four independent dimensionless parameters. We consider flat case and find that viable cosmological evolution follows the sequence: an initial stiff-fluid-dominated phase, transitioning through a nearly dust-dominated era, and eventually reaching a stable dark energy-dominating state. Stability analysis requires that $\xi <0$ and $0 < c < 1$ for the theory to be physically valid. Since zero NMC coupling, $\xi=0$, is not allowed in the autonomous system, the model can not completely recover canonical scalar field case. That is to say, as $\xi \rightarrow 0^-$ and $c \rightarrow 0^+$, the model can only approach the canonical scalar case but can not completely recover it. To approach dust or stiff fluid dominations, both magnitudes of the NMC coupling and the holographic parameter must be small. Numerical integration shows that for any allowed values of $\xi$ and $c$, $w_\text{eff}$ approaches $-1$ at late times. Increasing of $c$ does not change shape of the $w_{\rm eff}$, but larger $c$ increases $w_\text{eff}$. As $\xi$ becomes stronger, dust era gradually disappears. Good behaviors of the dynamics require $-1 \ll \xi <0$ and $0 < c \ll 1$.