How a brane cosmological constant can trick us into thinking that w < -1

TL;DR

The paper demonstrates that in a DGP braneworld with a brane cosmological constant, gravitational leakage into the extra dimension can screen the cosmological constant and produce an effective $w_{\rm eff}<-1$ without phantom energy, ghosts, or a big rip. The expansion history is derived and shown to closely resemble constant-$w$ models in luminosity-distance data, yet it carries distinct signatures in growth of structure and local gravity tests. Key results include the explicit $H(z)/H_0$ relation, solar-system precession constraints $\Delta\dot{\phi} = -\dfrac{3}{8 r_0}$, and a time-varying $G_{\rm eff}$ that enhances linear growth by measurable amounts. These findings position modified gravity as a viable alternative to phantom dark energy and outline concrete observational tests to differentiate the scenarios.

Abstract

Observations exploring the contemporary cosmic acceleration have sparked interest in dark energy models possessing equations of state with w < -1. We review how the cosmic expansion history of a Dvali-Gabadadze-Porrati (DGP) braneworld model with a standard brane cosmological constant can mimic that of ordinary 4-dimensional gravity with w < -1 "phantom" dark energy for observationally relevant redshifts. We reinterpret the effective phantom nature of the dark energy as arising from dynamical-screening of the brane cosmological constant in DGP. This unusual variety of expansion history is thus possible without violating the null-energy condition, without ghosts and without any big rip, in a model which seems no more contrived than most evolving dark energy models. We indicate ways by which one may observationally test this effective w < -1 possibility, and differentiate it from ``ordinary'' phantom dark-energy.

Figures (1)

• Figure 1: $d^{\rm w}_L(z)/d^{\Lambda CDM}_L(z)$ and $d^{\lambda DGP}_L(z)/d^{\Lambda CDM}_L(z)$ for a variety of models. The reference model is the best-fit flat $\Lambda$CDM, with $\Omega^0_M =0.27$. The dotted curves are for the constant-${\rm w}$ models with $(\Omega^{\rm w}_M,{\rm w}) = (0.28,-1.02)$,$(0.294,-1.05)$, and $(0.316,-1.1)$ from top to bottom. The quantity $\Omega^{\rm w}_M$ is chosen to match the $\Lambda$CDM model at low--$z$ for a given ${\rm w}$. The solid curves are for the screened-cosmological-constant DGP models with $(\Omega^{\lambda DGP}_M,r_0H_0) = (0.292,5.9)$,$(0.34,1.83)$, and $(0.42,0.86)$ from top to bottom where the parameters were chosen to best mimic the corresponding ${\rm w}$--models.