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Measurements of Microwave Background Radiation (MBR) on small angular scales will provide critical tests of models for structure formation, supply information about the evolution of the universe after recombination, and also help to constrain key cosmological parameters, including $\Omega_0$ and $H_0$. At small angles, reionization due to early star formation can erase primordial fluctuations and create new fluctuations. For my thesis, I focused on understanding secondary fluctuations arising from reionization, and will describe both analytical results and results of large-scale hydrodynamical simulations. I will present a formalism that we developed to compute fluctuations in both the microwave and X-ray skies.
If the universe was reionized at high redshift or never recombined, then electron scattering can erase microwave fluctuations at angles $<6^\circ$, but will create new anisotropies at the $1'$ level (Vishniac effect). The predicted level of anisotropies depends on $\Omega_0$, the ratio of the density in ionized baryons to the critical density ($\Omega_i$), and the shape of the power spectrum. Current observations do not yet constrain $\Omega_0$. A detection of Vishniac anisotropy at $\sim1'$ could be evidence for a low value of $\Omega_0$, provided the power spectrum at last scatter follows the observed distribution of galaxies down to about $10h\hbox{Mpc}^{-1}$.
Variations in the electron pressure can also distort the MBR spectrum (the Sunyaev-Zel'dovich effect). I computed the amplitude of these distortions for various cosmological scenarios using results from hydrodynamical simulations. In the standard CDM model at a beam throw of $0.1^\circ$, the amplitude is $1\times 10^{-5}$ if the experiment looks at a random part of the sky and $5\times 10^{-6}$ if it avoids bright clusters. Thus, SZ distortions could be the dominant source of anisotropy on small and intermediate angular scales. These fluctuations are likely to be moderately non-Gaussian and their complex energy dependence may account for some of the reported discrepancies between various experiments at moderate angular scales.
