Issues in cosmology: Observational tests and consistency with superstring physics

The CMB temperature anisotropy probes distances nearly to the edge of the observable universe and back to when the universe was a few 100,000 years old. Near-future satellite experiments will measure the CMB power spectrum with unprecedented accuracy. One use of this data will be to determine values for cosmological parameters. In this thesis parameter resolution is considered for two cases: a curved universe with a cosmological constant, and a flat universe with a negative pressure fluid of undetermined equation of state. Both of these cases are shown to have degeneracies with respect to some parameters. That is, if one parameter is changed, others can be adjusted to yield indistinguishable CMB spectra. Thus, some cosmological parameters, such as the matter density Ω m, Hubble constant h and equation of state of quintessence, can not be separately determined from CMB data alone, regardless of precision. However, other cosmological measurements (supernovae, age, baryon fraction, large scale structure, etc.) can be combined with the CMB to reduce the degeneracy. Depending on the range of parameters allowed by the combination of observations, substantial degeneracy may remain or it may be eliminated. A long-standing problem in superstring theories is the stabilization of moduli. Unstable moduli are disastrous for cosmology and high-energy physics. The coupling of moduli to ordinary matter fields in a cosmological setting, however, can result in a robust moduli stabilization mechanism. As an example, the problem of stabilizing the dilaton in a nonperturbative potential induced by gaugino condensates is reconsidered. A well-known difficulty is that the potential is so steep that the dilaton field tends to overrun the correct minimum and evolve to an observationally unacceptable vacuum unless the initial conditions are finely tuned. The generic coupling of the dilaton to thermal energy of matter fields (generic, for example, to some supergravity and string theories) results in a natural cosmological mechanism for gently relaxing the dilaton field into the correct minimum of the potential without fine-tuning of initial conditions. The effect has been neglected in previous treatments of the problem and may be relevant for stabilizing other moduli