The Cosmic Stories: Beginning, Evolution and Present Days of the Universe

Citation

Tseliakhovich, Dmitriy (2012) The Cosmic Stories: Beginning, Evolution and Present Days of the Universe. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/T279-PV49. https://resolver.caltech.edu/CaltechTHESIS:01042012-160039824

Abstract

This work presents three studies of independent astrophysical phenomena which cover a full timeline of the universe from the epoch of inflation to the present day. Along with our results we provide concise overviews of the considered phenomena and outline major open questions.

The first part of this work is focused on the epoch of inflation. We analyze the evolution of early density fluctuations which originate during inflation and connect physical fields driving inflation with observable parameters. We study several inflationary scenarios, specifically one field inflation, in which the only field present during that epoch is the inflaton field and two field inflation, in which along with the inflaton field the epoch of inflation is effected by the second scalar field - curvaton field. Single field inflationary models predict nearly Gaussian initial conditions and hence a detection of non-Gaussianity would be a signature of more complex inflationary scenarios. In this work we study the effect of primordial non-Gaussianity on the cosmic microwave background (CMB) and on large-scale structure in a two-field inflationary model in which both the inflaton and curvaton fields contribute to the primordial density fluctuations. We show that in addition to the previously described enhancement of the galaxy bias on large scales, this setup results in large-scale stochasticity. We provide joint constraints on the local non-Gaussianity parameter f _nl and the ratio of the amplitude of primordial perturbations due to the inflaton and curvaton using WMAP and Sloan Digital Sky Survey (SDSS) data.

The second and largest part of this study is focused on the formation of the first cosmic structures and the effect of relative velocity between dark matter and baryonic fluids. In that part we discuss a very important and previously unnoticed effect which significantly changes the process of structure formation in the early universe. At the time of recombination, baryons and photons decoupled and the sound speed in the baryonic fluid dropped from relativistic, to the thermal velocity of the hydrogen atoms. This is less than the relative velocity of baryons and dark matter computed via linear perturbation theory, so we infer that there are supersonic coherent flows of the baryons relative to the underlying potential wells created by the dark matter. As a result, the advection of small-scale perturbations (near the baryonic Jeans scale) by large-scale velocity flows is important for the formation of the first structures. This effect involves a quadratic term in the cosmological perturbation theory equations and hence has not been included in studies based on linear perturbation theory. We show that the relative motion suppresses the abundance of the first bound objects, even if one only investigates dark matter halos, and leads to qualitative changes in their spatial distribution, such as introducing scale-dependent bias and stochasticity. We further discuss possible observable implications of this effect for high-redshift galaxy clustering and reionization. Specifically we discuss in detail the effect of the relative velocity on the gas content in the early galaxies, minihaloes and the first stars. This part of the thesis also includes a concise overview of the recent studies that investigated various aspects of the relative velocity effect and showed its importance for topics ranging from star formation to precision cosmology.

The third and final part of the thesis covers interaction between expanding shocks of the supernovae explosions with the interstellar medium. The shocks of supernovae remnants represent a unique cosmic environment which allows detailed studies of plasma physics and high-energy astrophysics phenomena in conditions unreachable in the Earth-based laboratories. Specifically, shocks of supernovae remnants are associated with production of cosmic rays - the most energetic particles that we can observe. In our study we are specifically focused on the science of Balmer-dominated shocks (BDS) - a subset of collisionless, fast shocks dominated by hydrogen line emission with both broad and narrow components. The unique feature of BDS is that they are directly observable and their observations provide an opportunity for direct testing of the phase space structure and ion velocity distribution inside of shocks.

Understanding of physical phenomena occurring inside of astrophysical shocks requires precise knowledge of cross sections for high-nl proton-hydrogen collisions. Until now scientists have been using approximations for these cross sections which fall short of the precision needed for robust analysis of the observed data and can no longer satisfy needs of the astrophysical community. Guided by the demand in high-precision calculations of the cross sections we developed and implemented a robust method for direct solution of the Schroedinger partial differential equation on a grid. In this work we provide a detailed description of our computational algorithm for calculating cross sections in high-nl proton-hydrogen collisions and show results for n ≤ 4. We describe the code we developed, show the results of consistency tests and describe possible extensions. Finally, we show how our results are applied to the studies of Balmer-dominated shocks and specifically how the precise cross sections for n ≤ 4 can be used in computing Balmer decrement - the ratio of H_alpha and H_beta line intensities.

Thesis Files