Stable Palatini $f(\mathcal{R})$ braneworld

TL;DR

This paper investigates the stability of scalar perturbations in a Palatini $f( ext{R})$ braneworld model, developing new techniques to analyze coupled systems and identifying conditions for stable, nonlocalizable scalar modes that support viable four-dimensional gravity.

Contribution

It introduces a method to analyze scalar perturbations in Palatini $f( ext{R})$ braneworlds, demonstrating stability conditions and localization properties of scalar modes.

Findings

Scalar perturbations can oscillate stably over time.

Only some solutions support stable scalar perturbations.

Stable solutions feature nonlocalizable massless modes.

Abstract

We consider the static domain wall braneworld scenario constructed from the Palatini formalism $f(\mathcal{R})$ theory. We check the self-consistency under scalar perturbations. By using the scalar-tensor formalism we avoid dealing with the higher-order equations. We develop the techniques to deal with the coupled system. We show that under some conditions, the scalar perturbation simply oscillates with time, which guarantees the stability. We also discuss the localization condition of the scalar mode by analyzing the effective potential and the fifth dimensional profile of the scalar mode. We apply these results to an explicit example, and show that only some of the solutions allow for stable scalar perturbations. These stable solutions also give nonlocalizable massless mode. This is important for reproducing a viable four-dimensional gravity.