Stealth Acceleration and Modified Gravity

TL;DR

The paper develops ghost-free, asymmetric braneworld models in which ordinary matter can drive late-time acceleration on a Minkowski brane, avoiding de Sitter self-acceleration. A central challenge is the radion mode, whose presence and potential ghost-like behavior depend on the effective parameter χ; consistent cosmologies either decouple the radion or eliminate it via a symmetry, leading to radion-free or conformal cases. The cosmology is encoded in a modified Friedmann equation ρ = F(H^2), with acceleration arising in specific parameter regimes where the graviton is quasi-localised, yielding power-law expansion rather than exponential growth. Among the results, the conformal (χ=0) case yields the strongest late-time acceleration through nonlinear brane dynamics, and the work highlights the trade-offs between stability, observational viability, and theoretical naturalness in attempts to explain cosmic acceleration without a cosmological constant.

Abstract

We show how to construct consistent braneworld models which exhibit late time acceleration. Unlike self-acceleration, which has a de Sitter vacuum state, our models have the standard Minkowski vacuum and accelerate only in the presence of matter, which we dub ``stealth-acceleration''. We use an effective action for the brane which includes an induced gravity term, and allow for an asymmetric set-up. We study the linear stability of flat brane vacua and find the regions of parameter space where the set-up is stable. The 4-dimensional graviton is only quasi-localised in this set-up and as a result gravity is modified at late times. One of the two regions is strongly coupled and the scalar mode is eaten up by an extra symmetry that arises in this limit. Having filtered the well-defined theories we then focus on their cosmology. When the graviton is quasi-localised we find two main examples of acceleration. In each case, we provide an illustrative model and compare it to LambdaCDM.