I will investigate the primordial squeezed limit bispectrum as produced by inflation in single-field models. Previous results have argued that generically, single-field inflation produces a negligible bispectrum. However, more careful evaluation yields a more ambiguous result. I will discuss an alternate method for calculating the squeezed limit bispectrum for a general single-field inflation model. I will also explore slow-roll inflation with a non-standard initial state, where we find an enhanced squeezed-limit. I will discuss the detectability of such models in various cosmological observables such as the Cosmic Microwave Background (CMB), Large Scale Structure, and mu-distortion of the CMB. / text

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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. / Physics

published_or_final_version / Physics / Master / Master of Philosophy

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In this thesis we study aspects of the cosmological electroweak phase transition which are relevant to the possibility of baryogenesis at this epoch. We focus on two issues: first, requiring that the observed baryon number be of electroweak origin places strong constraints on electroweak physics, and second, baryogenesis at the electroweak scale may be driven by an asymmetry generated at the GUT scale. We use the effective potential at finite temperature as a means of analyzing phase transitions associated with spontaneous symmetry breaking. We develop the theory with two basic examples: the scalar and Abelian Higgs models. Infrared divergences near the phase transition make the one-loop description unreliable, and indeed invalidate conventional perturbation theory. Borrowing a method from studies of QCD at high temperatures, we demonstrate that the summation of ring diagrams cures the leading infrared divergences and achieves a more reliable perturbative expansion. We then apply this formalism to the minimal Standard Model, following previous work, and confirm weak first-order behavior at the phase transition. We show that requiring the baryon number not be erased by sphaleron processes after the phase transition places a stringent bound on the Higgs mass, which is incompatible with experiment. This cosmological bound, however, may be relaxed by extending the scalar sector of the Standard Model. We consider the two simplest such extensions, the addition of a gauge singlet and of a second doublet. We demonstrate that ring-improvement in the singlet extension alters previous arguments at the one-loop level and yields a more restrictive bound on the Higgs mass. While ring-improvement in the two-doublet model, in principle, also reduces the Higgs mass bound found earlier at one loop, the multitude of new couplings in this model does not permit a definitive statement. We then investigate a mechanism for generating the observed baryon asymmetry (n_B/S~ 10-¹⁰) at the electroweak phase transition from a pre-existing leptonic asymmetry (L_T/s~ 10-⁵) produced at the GUT scale. This mechanism works by charge transport in a strongly first-order phase transition and avoids the need for large CP-violation at the electroweak scale.

Since the discovery that the large-scale dynamics of galaxy clusters are dominated by dark matter, cosmologists have aspired to measure the spatial distribution of dark matter and identify its nature. Gravitational lensing, especially employing the Hubble Space Telescope (HST) has emerged as the tool-of-choice for mapping dark matter. Standing on the shoulders of the pioneering 1990's, this thesis is the first homogeneous lensing study of clusters with HST. We measure the mass and structure of an objectively-selected sample of X-ray luminous clusters at a single epoch (z ~ 0.2). We present observations often clusters (L(_x)≥8x10(^44)[0.1=2.4 keV] ergs(^-1)) and use the numerous gravitationally-lensed features in these data to constrain a detailed model of the central regions (r ~ 500 kpc) of each cluster. Our models provide an unprecedented view of cluster morphology, revealing that 60% of the sample contain significant substructure. Chandra X-ray observations confirm this is a signature of dynamical immaturity, and show that the mean temperature of the intra-cluster medium of the morphologically complex clusters is ~ 25% higher than then regular siblings. This offset results in a critical, and previously unexplored, systematic uncertainty in the use of clusters to normalise the mass power spectrum. We also use the detailed morphology of the clusters to constrain the nature of dark matter. We then exploit the clusters as gravitational telescopes, using ground-based near-infrared imaging to construct a sample of 60 gravitationally magnified Extremely Red Objects (EROs), a population that is believed to harbour important clues on the formation epoch and mechanism of massive galaxies. This unique sample overcomes the faintness of EROs (R ≥ 23, K ≥ 18) to uncover a wealth of morphological, photometric and spectroscopic evidence of diversity in both passively evolving and dusty active EROs. Coupled with our deep number counts (to K ~ 22), these observations provide important new constraints on competing theoretical models of galaxy formation.