Testing the models of cosmological structure formation

Abstract

The introduction describes promising theories which extend the Standard Model - inflationary and topological defect models. Although inflation solves some important problems the model is poorly motivated in terms of currently understood particle physics. Furthermore, conclusive tests of inflation are elusive. Topological defects, while less of a panacea for the problems of the field, are well motivated by theories of Grand Unification at T ~ 10¹⁶ GeV and make plausible candidates for the source of primordial inhomogeneities. Crucially, cosmic strings, the best investigated class of topological defect models, have testable consequences for the microwave background and the lensing of galaxies. Chapters 2 - 4 adapt the second of these effects into a search method for strings. Chapter 2 draws heavily on simulations of string networks to set limits on the shape and distribution of horizon panning strings. Chapter 3 investigates the appearance of galaxies lensed by such strings. In chapter 4 this knowledge is then built into an algorithm which searches for strings on Schmidt plates. Ultimately only a weak density limit of ≲ 90 long strings per horizon volume can be set with this survey medium . Finally, an extension of the work to deeper surveys is considered - running the search algorithm on the large area Sloan survey should test the cosmic string model conclusively.The introduction in chapter 1 also exposes the most significant grey areas in the Standard Hot Big Bang model: (a) determining the form and density of the energy content of the Universe, and reconciling this to (b) the age of the Universe, and (c) the observed clustering of galaxies. This thesis makes two contributions to this area. Chapter 5 discusses a variant of the Cold Dark Matter model in which a dark matter component decays radiatively at early times. The model has the virtue that it can accommodate the low apparent value Ωh inferred from observed large-scale galaxy clustering and the high measured values of Ω and h. Limits on the small-scale clustering predicted by such models constrains the mass and lifetime of the decaying component to 0.5 < m < 30 keV, 0.2 < ᵀ< 500 years. Chapter 6 contributes to the observational tests of large-scale galaxy clustering by constraining the clustering signal of a sample of high-redshift radio galaxies. The clustering measured here is consistent with that measured with other radio surveys. The high redshift data are important as they give clues to the evolution of the density field with time. Forms of bias evolution which arise from continuity constraints on the equations of motion for galaxies and the mass (i.e. b[z] = b[0] - a + a [ l + z]) are shown to be entirely consistent with the data, as are unbiased b[z] = b[0] models. Schemes in which bias in galaxy numbers arises from the differential formation and motion of dark matter haloes (predicting b[z] = b[0] - a + a[ l + z]²) are shown to be only marginally consistent with the clustering measured at the highest redsliifts, (z ≃ 1.5). Improvements to the dataset which would allow more conclusive limits are discussed.