We describe methods to study the intergalactic medium (IGM) in both the local and high-redshift universe.; We propose two new techniques for studying galactic feedback at high redshifts. First, we study how observations of absorption produced by metals carried in the winds of early protogalaxies can constrain the process through which these winds enrich the IGM with heavy elements as well as the properties of early generations of protogalaxies. Second, we show that the next generation of radio telescopes will be able to observe 21 cm absorption by neutral hydrogen in “minihalos” (i.e., collapsed objects unable to form stars). We show that minihalo absorption measures the temperature of the IGM and the background radiation field.; Both of these methods of studying the high-redshift IGM require observations of absorption along the line of sight to background light sources. Gamma-ray bursts (GRBs) may constitute one such background source; however, the mysteries surrounding GRB progenitors compromise their cosmological utility. We show that positrons created in GRBs will continue to annihilate and produce an observable spectral signature long after the burst. Identifying a remnant in our Galaxy will clarify the usefulness of GRBs for studies of the high-redshift universe.; Many galaxy clusters contain jets produced by powerful radio galaxies, but the properties of the jets and their feedback on the host clusters are not well-understood. We show that, if mixing between the jet and cluster material is efficient, any positrons contained in the jet will eventually cool and annihilate with ambient electrons. Searches for such an annihilation feature will constrain the composition of jets and the history of jet feedback in clusters. We also show that the annihilation of positrons produced by cosmic ray interactions in galaxy clusters does not create a measurable signal.; Finally, the dynamics of galaxy cluster mergers offer a unique opportunity to study the self-interaction cross section of dark matter. We show that this cross section determines the final size of a merging dark matter subhalo, and we offer a suite of tests designed to measure the subhalo size and mass.
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