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The Effects of Tachyonic Instabilities and Non-Gaussian Perturbations During Cosmic Inflation and Reheating

Abolhasani, Ali Akbar | 2011

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 42740 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Firouzjahi, Hassan; Golshani, Mehdi; Sheikh Jabbari, Mohammad Mahdi
  7. Abstract:
  8. The cosmic inflation paradigmwas first introduced as a possible solution to the standardBig Bang cosmology problems. [1]. The inflationary models not only provide a solutionfor so the called flatness and horizon problems, but also they predict perturbations asprimordial seeds of large scale structures of Universe. Inflationary models can connecthigh-energy physics to the experiments. There is a mutual relation between inflationand high energy physics. It is necessary to find a fundamental description of inflationarymechanism and on the other hand the observational predictions of inflationary modelscan put bound on free parameters of associated fundamental theories. Thanks to vastdata obtained from recent cosmological observations such as WMAP7 [3] inflation hasemerged as the leading theory of the early universe and structure formation.In this thesis, we investigate the tachyonic effects and non-Gaussianities during cos-mological inflation and reheating. Tachyonic resonance preheating provides us with aefficient procedure for warming up the Universe at the end of inflation. Also tachy-onic instabilities during cosmic inflation and preheating may have important effectson cosmic microwave background radiation. Furthermore, non-Gaussian perturbationsemerging from tachyonic instabilities play crucial roles to put bounds on parameters ofthese models.This dissertation is organized as follows. In first chapter, we briefly reviewstandardcosmology, its problems and different inflationary models. In addition we review cos-mological perturbation theory and finally we compare predictions of these models withobservational data. In the second chapter, we study tachyonic effects during reheating.Specifically, in this chapter, tachyonic resonance preheating in expanding Universe isconsidered. In the remaining three chapters we investigate the general parameter spaceof the hybrid inflation model. First, we study the standard hybrid inflation as a two-fieldinflationary model. We show that perturbations of waterfall field does not contributeto the large scale curvature perturbations. We continue with studying double-inflationmodel in the next chapter. In contrast to the standard hybrid model, the waterfall fieldcan acquire non-zero classical values and also waterfall transition is mild. It is shownthat significant amount of large scale curvature perturbations are induced from the wa-terfall field perturbations. In the final chapter we investigate the parameter space ofhybrid inflation potential in more details. As one of our main result, we present a uni-versal classification of these models depending on the inflaton field value at the timeof phase transition and the sharpness of the phase transition. We find that for modelsin which the potential is vacuum dominated during inflation and the phase transition ismild the induced curvature perturbation from the waterfall field is too large to satisfythe COBE normalization
  9. Keywords:
  10. Inflationary Cosmology ; Reheating ; Cosmological Perturbations ; Tachyonic Instabilities

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