Testing the Dark Sector Versus Modified Gravity Models in Cosmology

This dissertation studies cosmological and astrophysical tests of gravity theories and the dark sectors. We focus on three tests. All of them are promising for future observations.

The first test is about tensor-mode parameterization. While scalar-mode parameterization has been extensively studied, tensor-mode parameterization requires further investigations. This dissertation extends some previous work in the literature, and studies some physically motivated parameterization schemes, the current observational constraints, the future constraint forecast as well as the impacts on inflation consistency relation.

The second test is based on consistency tests. The idea is to compare constraints from different observations and check the consistency of a model. This dissertation introduces a novel measure, called the index of inconsistency (IOI). It is a simple and effective measure. We proposed a procedure based on IOI to test (in)consistencies of multiple observations. The difference between this consistency test and the first one is that we do not assume all data are correct. In fact, our procedure allows us to find observation outliers or the breakdown of the model.

The third test is about a phenomenological difference between modified gravity and dark matter. We studied properties of the faintest galaxy system in our local group, called the ultra-faint dwarf galaxies. We found a loss of correlation between their luminosity and stellar velocity dispersion. This does not favor the modified gravity hypothesis.