  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.
