Abstract
The theory of Ratio Gravity (RG) proposes that the curvature of 3+1 spacetime originates from a deformation of a Cross Ratio, where similar mathematical structure to general relativity emerges. This paper (in the first half of the thesis) studies RG using the framework of the Newman-Penrose spin formalism. After proposing the general formalism, we move on to study a homogeneous and isotropic universe with RG. It is noted that the theory contains a component of dynamical dark energy with novel equation of state. In the second half of the thesis, based on the postulate above - the deformation of the cross ratio to associate with the physical model of gravity, we further develop a new mechanism to generate dynamical dark energy - a quintessence field coupled with gravity. Such model causes the dark energy behaving differently in early and late time universe. In the radiation-dominated-era and matter-dominated-era, the related analytical solutions of the quintessence field have an interesting property - starting as a constant field, then oscillating as the universe expands. By Markov Chain Monte Carlo search of the parameter space with the local measurement (Type Ia supernovae) in the Bayesian framework, the probed range of H0 (within 1) overlaps the H0 value inferred from Planck CMB dataset by CDM model.