"For almost a century the theory of general relativity (GR) has been known to describe the force of gravity with impeccable agreement with observations. ... Far from a purely academic exercise, the existence of consistent alternatives to describe the theory of gravitation is actually essential to test the theory of GR. Furthermore the open questions that remain behind the puzzles at the interface between gravity/cosmology and particle physics such as the hierarchy problem, the old and the origin of the late-time acceleration of the Universe have pushed the search for alternatives to GR."