Brane-World Multigravity

TL;DR

This thesis explores multigravity in brane-world models with extra dimensions, demonstrating how ultralight gravitons and modified gravity at cosmological scales can emerge from these higher-dimensional setups.

Contribution

It introduces a viable six-dimensional multigravity model with positive tension branes, addressing previous issues like the van Dam-Veltman-Zakharov discontinuity.

Findings

Ultralight graviton Kaluza-Klein states arise from negative tension branes.

Radions associated with negative tension branes have wrong sign kinetic terms.

A stable six-dimensional multigravity model with positive tension branes is proposed.

Abstract

In this thesis, we discuss the idea of multigravity, namely the possibility that a significant component of gravity that we feel at intermediate distances (1 mm < r < 10^26 cm) is due to massive but ultralight gravitons. We demonstrate how this phenomenon can be realized in brane-world models in a spacetime with more than four dimensions and discuss how modifications of gravity at cosmological scales emerge as a consequence. Firstly, we consider five dimensional multigravity models with flat branes. We see how the existence of freely moving negative tension branes gives rise to ultralight graviton Kaluza-Klein states. Secondly, we study the moduli corresponding to the position of the branes and the size of the extra dimension, the radions and the dilaton respectively. We show that the radions corresponding to negative tension branes have wrong sign kinetic term. We also derive a stabilization condition for the dilaton in a brane model with general bulk scalar field dynamics. Thirdly, we show how we can reproduce the five dimensional multigravity models with Anti-de Sitter branes of positive tension only. Moreover, we demonstrate how the van Dam-Veltman-Zakharov discontinuity can be circumvented at tree level in Anti-de Sitter space. Finally, we discuss how the above multigravity models can be replicated in six dimensions with flat positive tension branes. This last possibility is the first theoretically and phenomenologically viable realization of the multigravity scenario.