Model-Independent Probes of Cosmology & Gravitation

Abstract

Cosmology & Gravitation are the fundamental studies in understanding the physics of the universe that we reside in. The approach in achieving the knowledge of the physical laws which govern our universe, is via the observations of the dynamics, geometries, and evolution of the astrophysical structures within it. Recent cosmological observations have tted well to a cosmological model known as CDM; where our universe's energy content is dominated a cosmological constant ( ) and Cold Dark Matter (CDM). The CDM model is based with gravity described by Einstein's theory of General Relativity (GR); where GR also provides the best description of gravity on all scales. However, the CDM plus GR model requires a cold, non-baryonic, non-visible CDM component, and DE to t the cosmological data. At present, there is no decisive detection of DM leaving an open window for Modi ed Gravity (MG) theories attempting to explain data without the inclusion of DM; and the CDM model contains loose constraints on the Epoch of Reionization (EoR), the epoch at which the rst galaxies and Super Massive Black Holes (SMBH) began to form. This thesis consists of model-independent probes of cosmology & gravitation. Part of this thesis involves searching for high redshift, z 6.5 quasars, with the VISTA Kilo-degree INfrared Galaxy (VIKING), where we expand on the search criteria used by Findlay et al. (2012) and Venemans et al. (2013), by applying various speci c cut methods, resulting in an extended search for these high redshift quasars within the VISTA Science Archive (VSA) database. These quasars can be used as cosmological probes of the EoR by constraining the redshift at which EoR begun, and the formation & evolution of the rst galaxies and SMBHs. Another part of this thesis is the prospects of testing gravity in very low acceleration regimes via Wide Binary (WB) stellar systems, with separations & 3 kAU. These WB systems can achieve low accelerations, on scales . 10 10 ms 2, which is comparable to the value where galaxy rotation curves attens due to DM or a MG. Thus, WBs can probe these low acceleration regimes without the presence of DM, hence making them `clean' and powerful probes of gravity. Our work consists of simulating a large sample of random orbits in various MG models, and predict the observed relative velocities and projected separations, comparing Newtonian prediction against other MG models. This work then follows into using the latest data release from GAIA to select a clean, unperturbed sample of WB systems, obtaining their projected velocities and separations. The selected WBs can then be followed-up with high-resolution ground-based spectroscopy, obtaining their radial velocities, allowing tests of gravity.