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Particles in the early Universe: Interface of Cosmology and Particles Massive Neutrinos and Cosmology

Rezaie, Mehdi | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 47214 (04)
  4. University: Sharif University of Technology
  5. Department: Physic
  6. Advisor(s): Golshani, Mehdi; Farzan, Yasaman
  7. Abstract:
  8. Neutrino flavor oscillations are clear evidence of massive neutrinos. It has been for more than a decade that neutrino physics became one of active topics in the field of particle physics and various experiments started studying the flavor oscillation of neutrinos. Unfortunately, experiments studying the flavor oscillations are only sensitive to the mass square difference, and they are incapable of providing more information about the absolute mass scale of neutrinos. Additionally, sterile neutrinos having non-standard interactions are under vast investigations and studies. Determination of neutrino masses and their effective number is among the most challenging problems by which physicists are baffled. Presence of neutrinos, affect the evolution of the universe, namely the expansion history and the evolution of other perturbations. Assuming Big Bang as the time origin of the universe, neutrinos postpone the equality time between matter and radiation and their free-streamings reduce the growth rate of matter perturbations. The delay in equality time along with the suppression of matter perturbations growth rate, affect the cosmological observables to a measureable extent. That is to say, cosmological observations are a robust tool to put more tight constraints upon the neutrino parameters. Cosmic Microwave Background (CMB) and Large Scale Structures (LSS) are two notable examples of these observations. In recent years, precise measurements of CMB by Planck and WMAP, along with vast studies of LSS by SDSS and 6dF, have broadened our knowledge in the field of precision cosmology. Therefore, observational cosmology can be considered as a robust approach to shed more light on the effective number of neutrinos and their masses. In this thesis, impacts of neutrinos on the cosmological observables are studied and after that, recent bounds on the effective number of neutrinos and their masses are reviewed from cosmological point of view. Based on current Planck 2015 data, the upper bound on the sum of neutrino masses is 0.23 eV, and the effective number of neutrinos is 3.2±0.5 (within 95% C.L.). This means that there is no evidence for extra radiation (ΔN_eff≥1), although models of sterile neutrinos, not thermalized in the cosmic plasma with non-standard interactions (ΔN_eff≤0.5), can not be ruled out
  9. Keywords:
  10. Cosmic Microwave Background (CMB) ; Universe Large-Scale Structure ; Cosmic Rays ; Massive Neutrinos ; Cosmological Observables

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