Abstract

The importance of determining the most general, self-consistent scalar-tensor theory of gravity in connection with either inflation or dark energy has been recently stressed in a series of papers by Koyama and collaborators and Langlois and collaborators. A large class of theories is represented by theso-called Horndeski/beyond Horndesky Lagrangian, which guarantees the absence of ghost instabilities. The aim of this master thesis is to investigate the large-scale cosmological implications of a recently proposed Òquadratic Degenerate High OrderÓ class of scalar-tensor theories of gravity, analyzing the behavour ofÊ scalar and tensor field equationsÊandÊstudying scalarÊperturbations in the vicinity of a static spherically-symmetric cosmic structure.