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Title:Topics in cosmology: structure formation, dark energy and recombination
Author(s):Alizadeh, Esfandiar
Director of Research:Wandelt, Benjamin D.
Doctoral Committee Chair(s):Fields, Brian D.
Doctoral Committee Member(s):Wandelt, Benjamin D.; Thaler, Jonathan J.; Weissman, Michael B.
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Dark Matter Halos
Dark Energy
Abstract:\indent The field of theoretical cosmology consists of numerous, inter-related branches, whose ambitious goal is to uncover the history of the universe from its beginning to its future. Achieving this, no doubt, requires a deep understanding of many areas of physics. In this thesis I touch upon a few of these areas in which I worked during my PhD studies. Chapter~(\ref{ch:accretion}) describes our work in finding the accretion and merger history of dark matter halos. Dark matter halos are the collapsed dark matter structures in the late time evolution of the universe, whose existence is vital for the formation of galaxies in the Universe as they act as the potential wells where normal matter (collectively called Baryons) can accumulate, cool, and form stars. It is then no surprise that the properties of galaxies depends on the properties of the dark matter halo in which it resides, including its merger history, i.e. the number of times it merged with other halos. Even though these merger rates can be calculated theoretically for infinitesimal time steps, in order to find the merger history over an extended period of time one had to use either Monte-Carlo simulations to build up the total rates of merging and accreting from the infinitesimal rates or use N-body simulations. In chapter~(\ref{ch:accretion}) we show how we used random walk formalism to write down an {\it analytical} (integral) equation for the merger history of halos. We have solved this equation numerically and find very good agreement with Monte-Carlo simulations. This work can be used in theories of galaxy formation and evolution. We then switch from the overdense regions of the Universe, halos, to the underdense ones, voids. These structures have not attracted as much attention from cosmologists as their overdense counterparts in probing the cosmological models. We show here that the shapes of voids as a probe can be of use for future surveys to pin down the equation of state of the dark energy, i.e. the ratio of its pressure to its energy density. As first approximation, voids can be considered to be ellipsoids whose axis ratio evolution depends on the cosmological parameters. This, together with the fact that the initial distribution of the axis ratios is known (because the intial density field is Gaussian) can be used to infer the equation of state of the dark energy statistically from the observation of voids at different redshifts and with different sizes. The standard method of Fisher matrices is then used to forecast how well a future survey can measure the equation of state. We find promising results with constraints coming from void ellipticity measurements comparable to those of other standard methods. Chapter~(\ref{ch:H2}) goes farther back in the history of the Universe. During the {\it recombination} era, when the Universe was around a thousandth of its present size, it became cool enough that free electrons got captured by free protons to make hydrogen atoms. Consequently, the Thompson scattering of photons off of free electrons dropped dramatically and the Universe became transparent to photon propagation. The Cosmic Microwave Background (CMB) is a remnant from this epoch, consisting of photons last scattered off of a free electron. A wealth of information is contained in the statistical properties of the CMB field. However, in order to take full advantage of this probe one needs to know the recombination history, i.e. the evolution of the number density of free electrons as a function of time, to sub-percent level accuracy during this era. There are a plethora of phenomena, from radiative transfer effects to atomic and molecular ones, that have the potential to change the recombination history to this level. Our work was to calculate the effect that the formation of hydrogen molecules will have on the recombination history. Even though the abundance of hydrogen molecules is very small, they still have the potential to change the recombination history by reshuffling photons from the blue side of the Ly-$\alpha$ line to its red side and vise-versa. To find the magnitude of the effect, we solve the appropriate rate equations for all of the bound-bound and bound free transitions for hundreds of ro-vibrational sub-levels of the ground state and the first three excited states of hydrogen molecule. We find that hydrogen molecules were not abundant enough to make a noticeable change in the recombination history. To put our work in context, in chapter~(\ref{ch:introduction}) I give an overview of theoretical cosmology, and I conclude in chapter~(\ref{ch:conclusion}). A number of appendices are added to clarify some of the more technical details.
Issue Date:2011-08-25
Rights Information:Copyright 2011 Esfandiar Alizadeh
Date Available in IDEALS:2011-08-25
Date Deposited:2011-08

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