Revamped Braneworld Gravity

TL;DR

This paper analyzes a five-dimensional braneworld gravity model, addressing issues like ghosts and strong coupling, and proposes solutions involving ultralight gravitons and localized curvature to modify 4d gravity observables.

Contribution

It provides a comprehensive analysis of ghost-free regions, strong coupling scales, and mechanisms to avoid the vDVZ discontinuity in warped braneworld models with localized curvature.

Findings

Identified parameter regions avoiding ghosts and tachyons.

Derived the strong coupling scale formula for the model.

Demonstrated how ultralight gravitons can evade the vDVZ discontinuity.

Abstract

Gravity in five-dimensional braneworld backgrounds often exhibits problematic features, including kinetic ghosts, strong coupling, and the vDVZ discontinuity. These problems are an obstacle to producing and analyzing braneworld models with interesting and potentially observable modifications of 4d gravity. We examine these problems in a general AdS_5/AdS_4 setup with two branes and localized curvature from arbitrary brane kinetic terms. We use the interval approach and an explicit ``straight'' gauge-fixing. We compute the complete quadratic gauge-fixed effective 4d action, as well as the leading cubic order corrections. We compute the exact Green's function for gravity as seen on the brane. In the full parameter space, we exhibit the regions which avoid kinetic ghosts and tachyons. We give a general formula for the strong coupling scale, i.e. the energy scale at which the linearized treatment of gravity breaks down, for relevant regions of the parameter space. We show how the vDVZ discontinuity can be naturally but nontrivially avoided by ultralight graviton modes. We present a direct comparison of warping versus localized curvature in terms of their effects on graviton mode couplings. We exhibit the first example of DGP-like crossover behavior in a general warped setup.