Oscillating scalar fields in extended quintessence

TL;DR

This paper investigates the effects of a rapidly oscillating scalar field nonminimally coupled to gravity on the universe's expansion, focusing on the equation of state and constraints from gravitational constant variations.

Contribution

It provides a detailed analysis of how nonminimal coupling influences oscillating scalar fields and their role as dark energy candidates, including constraints from gravitational constant variation.

Findings

Change in equation of state parameter depends on the potential exponent n.

Constraints on G variation limit effects at late times but not in the early universe.

Additional effective energy density from oscillations remains subdominant.

Abstract

We study a rapidly-oscillating scalar field with potential $V(\phi) = k|\phi|^n$ nonminally coupled to the Ricci scalar $R$ via a term of the form $(1- 8 \pi G_0 \xi \phi^2) R$ in the action. In the weak coupling limit, we calculate the effect of the nonminimal coupling on the time-averaged equation of state parameter $\gamma = (p + \rho)/\rho$. The change in $\langle \gamma \rangle$ is always negative for $n \ge 2$ and always positive for $n < 0.71$ (which includes the case where the oscillating scalar field could serve as dark energy), while it can be either positive or negative for intermediate values of $n$. Constraints on the time-variation of $G$ force this change to be infinitesimally small at the present time whenever the scalar field dominates the expansion, but constraints in the early universe are not as stringent. The rapid oscillation induced in $G$ also produces an additional contribution to the Friedman equation that behaves like an effective energy density with a stiff equation of state, but we show that, under reasonable assumptions, this effective energy density is always smaller than the density of the scalar field itself.