Browsing by Author "Lin, Weikang"
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Item Testing Gravity Theories Using Tensor Perturbations(American Physical Society, 2016-12-21) Lin, Weikang; Ishak-Boushaki, Mustapha; 0000 0001 2874 3832 (Ishak-Boushaki, M); Lin, Weikang; Ishak-Boushaki, MustaphaPrimordial gravitational waves constitute a promising probe of the very early Universe and the laws of gravity. We study in this work changes to tensor-mode perturbations that can arise in various proposed modified gravity theories. These include additional friction effects, nonstandard dispersion relations involving a massive graviton, a modified speed, and a small-scale modification. We introduce a physically motivated parametrization of these effects and use current available data to obtain exclusion regions in the parameter spaces. Taking into account the foreground subtraction, we then perform a forecast analysis focusing on the tensor-mode modified-gravity parameters as constrained by the future experiments COrE, Stage-IV and PIXIE. For a fiducial value of the tensor-to-scalar ratio r = 0.01, we find that an additional friction of 3.5-4.5% compared to GR will be detected at 3-σ by these experiments, while a decrease in friction will be more difficult to detect. The speed of gravitational waves needs to be by 5-15% different from the speed of light for detection. We find that the minimum detectable graviton mass is about 7.8 - 9.7 × 10⁻³³ eV, which is of the same order of magnitude as the graviton mass that allows massive gravity theories to produce late-time cosmic acceleration. Finally, we study the tensor-mode perturbations in modified gravity during inflation using our parametrization. We find that, in addition to being related to r, the tensor spectral index would be related to the friction parameter ν₀ by nT = -3ν₀ - r/8. Assuming that the friction parameter is unchanged throughout the history of the Universe, and that ν₀ is much larger than r, the future experiments considered here will be able to distinguish this modified-gravity consistency relation from the standard inflation consistency relation, and thus can be used as a further test of modified gravity. In summary, tensor-mode perturbations and cosmic-microwave-background B-mode polarization provide a complementary avenue to test gravity theories. © 2016 American Physical Society.Item Testing the Dark Sector Versus Modified Gravity Models in Cosmology(2018-08) Lin, Weikang; 0000-0003-2240-7031 (Lin, W); Ishak-Boushaki, MustaphaThis 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.