| This thesis is a study in theoretical cosmology with an emphasis on the high-redshift universe and promising directions for future observations. In Chapters 2 and 3, we propose intensity mapping of spectral line emission from galaxies. This is a technique to observe the cumulative emission from many galaxies without resolving individual sources. We use analytic calculations and N-body simulations to predict the observational signal for different emission lines, including those from oxygen, carbon monoxide, and carbon.;In Chapter 4, we utilize the Fisher matrix formalism to determine how accurately futuristic 21cm observatories, dedicated to post-reionization redshifts, can constrain cosmological parameters. We find that these experiments may yield significantly better constraints than next generation Cosmic Microwave Background (CMB) experiments such as the Planck telescope.;In Chapter 5, we perform semi-numerical simulations of reionization including a significant contribution from X-rays. We find that the large mean-free-path of X-rays adds a uniform component to reionization. Using these simulations, we compute the impact on the kinetic Sunyaev-Zel'dovich effect and find that a contribution from X-rays could result in a lower value of the so-called ``patchy reionization'' component of the CMB power spectrum.;In Chapter 6, we present the first realistic simulations of the 3-dimensional distribution of the first galaxies including the newly discovered relative velocity between baryons and dark matter. We use these simulations to compute the 21cm signal at redshift 20 and find that the relative velocity effect enhances fluctuations on a scale of 100 Mpc, making observations more feasible than previously expected.;In the final chapter, we propose that the physics which sets the density profiles of dark matter halos can be understood in terms of dissipationless gravitational collapse in non-cosmological simulations. To explore this, we run N-body collapse simulations from cold and spherically symmetric initial conditions. We find that the final equilibrium states of these simulations have some common features with dark matter halos, including the form of the pseudo-phase-space density. |
