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The Observational Tests of Dark Energy and Modified Gravity Theories

Baghram, Shant | 2011

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 42054 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Rahvar, Sohrab
  7. Abstract:
  8. The cosmological observations, like the luminosity distance of type Ia Supernovas, Cosmic Microwave Background radiation (CMB), Baryon Acoustic Oscillations (BAO) and Large Scale Structure (LSS) observations indicate that the Universe is nearly flat and also it is in a phase of acceleration. In the context of standard cosmological models, it means that the Universe is made of an unknown component (Dark Energy), which approximately has 70% of total energy-momentum of the Universe. The Cosmological Constant(CC) in the paradigm of ΛCDM model is the easiest model to describe the acceleration of the Universe. On the other hand the fine tuning (CC problem) and coincidence problems open new horizons for alternative theories such as Dark Energy(DE) and Modified Gravity (MG) theories. In this thesis, first by using the SNIa data, CMB, BAO, gas mass fraction in cluster of galaxies, and growth index in LSS observation we constrain a generic model of f(R) gravity model in Palatini formalism and we show that ΛCDM model as a special case of this generic action, matches with observational data. This result is also in agreement with local gravity tests. In the next part, we introduce the reconstruction method, where by using the SNIa data, we reconstruct the dynamics of the Universe and by Inverse problem method we find the MG action. We also show that by having more than 1500 SNIa data with the Union Sample catalogue quality, we will be able to distinguish the ΛCDM model from the alternative theories. In the next step, as any MG model is equivalent with a smooth dark energy model in the background dynamics of Universe, we study the structure formation in MG models and show that we can distinguish MG from DE, by using LSS observations like power spectrum, growth index and Integrated Sachs Wolfe effect. Finally we show the importance of nonlinear structure formation in the studying of accelerating universe models. We find the matter power spectrum in nonlinear regime, using the stable clustering hypothesis in phase space. Then we introduce a new method, where we probe the gravitational effect of Dark Matter(DM) substructure through the Shapiro time delay and Doppler effect, using the power spectrum of a pulsar frequency change. We compare our predictions with future experiments designed for gravitational wave detection by pulsar timing. Although our signal is quite low in comparison with SKA experiment, but this method is a promising tool in nonlinear structure formation studies and DM astronomy.
  9. Keywords:
  10. Cosmology ; Generalized Gravity ; Dark Energy ; Structure Formation ; Accelerating Universe ; Observational Tests

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