  One of the main challenges in cosmology is to quantify how small density
fluctuations at the recombination epoch z around 1000 evolved into the galaxies
and the large-scale structure we observe in the universe today. This thesis
discusses ways of probing the intermediate epoch, focusing on the thermal
history. The main emphasis is on the role played by non-linear feedback, where
a small fraction of matter forming luminous objects can inject enough energy
into the inter-galactic medium to radically alter subsequent events. Chapter 2,
"A Cosmology Primer", provides a 60-page introduction to cosmology that may be
useful for the reader with a general physics background.
