Can we be tricked into thinking that w is less than -1?

TL;DR

Brans-Dicke scalar-tensor gravity is examined as a possible way to produce an apparent $w_{ m eff}<-1$ without violating the dominant energy condition. The authors perform a perturbative Einstein-frame analysis, considering both a dark-energy component and BD-driven acceleration, and they impose stringent constraints from the time-variation of Newton's constant. They find that achieving $w_{ m eff}<-1$ requires highly contrived BD dynamics (e.g., the scalar near a local maximum today) and large potential slopes, while solar-system bounds force tiny variations in $G$; consequently, phantom-like behavior is not generically realized in this framework. Overall, the work argues that BD realizations yielding $w_{ m eff}<-1$ demand fine-tuning or alternative physics, making such an outcome unlikely in natural BD cosmologies.

Abstract

Dark energy candidates for which the equation-of-state parameter w is less than -1 violate the dominant energy condition, and are typically unstable. In scalar-tensor theories of gravity, however, the expansion of the universe can mimic the behavior of general relativity with w<-1 dark energy, without violating any energy conditions. We examine whether this possibility is phenomenologically viable by studying Brans-Dicke models and characterizing both the naturalness of the models themselves, and additional observational constraints from limits on the time-dependence of Newton's constant. We find that only highly contrived models would lead observers to measure w<-1.

Figures (4)

• Figure 1: $w_{\rm eff}$ as a function of $y\equiv \frac{1}{H_0}\, \frac{\dot G_{\rm eff}}{G_{\rm eff}}$ and $\lambda$.
• Figure 2: $w_{\rm eff}$ as a function of $y\equiv \frac{1}{H_0}\, \frac{\dot G_{\rm eff}}{G_{\rm eff}}$ with $\omega=1$.
• Figure 3: $w_{\rm eff}$ as a function of $y\equiv \frac{1}{H_0}\, \frac{\dot G_{\rm eff}}{G_{\rm eff}}$ with $\omega=40000$ and $\lambda\gg\alpha$.
• Figure 4: $w_{\rm eff}$ as a function of both $y\equiv \frac{1}{H_0}$ and $\lambda$ with $\omega=40000$.