This thesis aims to address some of the current mysteries in early Universe astrophysics. These include the high-z galaxies that have been discovered by the James Webb Space Telescope (JWST) and the population of early Universe black holes that exhibit higher mass and accretion rates than previously expected. We use the high resolution, high-z Renaissance simulations, built using the adaptive mesh-refinement code ENZO, to achieve this. First we apply post-processing methods to the simulations to investigate the formation and evolution of heavy seed black holes in rapidly assembling halos in the early Universe. We find that these black holes struggle to grow in high-z environments, while also investigating the potential for future Gravitational Wave detectors like LISA, to detect mergers of black holes similar to those simulated here. We also use the Renaissance simulations to investigate the validity of standard cosmology in light of the discovery of very high-z galaxies with JWST. Apparent tensions between observations and the limits to how stellar matter which forms from barynonic matter, whose abundance is dictated by ACDM, are starting to arise. This provides the perfect opportunity to use high-z simulations, like Renaissance, to test these theories directly. We find that Renaissance is largely able to explain the existence of these galaxies, while more recently discovered outliers could possibly be explained due to the lack of resolution on small areas of bursty star formation. The work in this thesis emphasises the importance of high-z simulations as we start to uncover the early Universe in incredible detail.