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Structure Formation and Galactic Dynamics in Modified Gravity (MOG)

Vakili, Hajar | 2018

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 50712 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Rahvar, Sohrab; Mohammad Movahed, Sadegh
  7. Abstract:
  8. The standard model of Cosmology (SM), based on Einstein’s theory of general relativity (GR), is the best model in describing the cosmic-scale observations. In galactic scales, however, galactic dynamics show a discrepancy between the observed luminous mass and the predicted value from the theory. Two approaches are suggested to eliminate this inconsistency: assuming the existence of dark matter within the context of GR, or modifying the theory of gravity in galactic scales, like MOND or MOG. In this thesis, we study the spherical collapse and the formation of shell galaxies in MOG, in comparison with SM. We introduce the action and the field equations of MOG and the equation of motion of a test particle in the weak field limit of the theory. Therefore, we introduce the standard theory of gravitational instability in linear and non-linear regimes and calculate the collapse of a spherical overdense region with numerical methods under the MOG gravitational field. Our calculations exhibit, unlike the Newtonian case, a scale-dependency in MOG, which leads to significant results in automatic structure formation. The small-scale structures form over a longer time-scale compared to the large-scale structures in MOG, which gives us a clue to the solution for the missing satellite problem that exists in SM. In addition,we compare theoretically the formation of shells in Type I shell galaxies in MOG, MOND, and SM, to reveal the possible differences in shell structures and dynamics in different gravity theories. Formation of shell galaxies, via the collision of two galaxies, makes them suitable systems for testing the underlying gravitational potential. We study the motion of the incoming galaxy and its stars in the gravitational potential well of the primary galaxy, using numerical and analytical methods,and show that dynamical friction limits the range of the shell radii, in dark matters model, to low values. Comparing the models under the same initial conditions, we conclude that the first shell generation pattern shows a degeneracy with the age of the shells and in different theories, but the relative distance of the shells and the shell expansion velocity can break this degeneracy. Finally, we test the consistency of the models with the observational data of NGC 3923. The relative distance of the outermost opposite shells is more consistent in MOG than the other two models
  9. Keywords:
  10. Gravitation ; Dark Matter ; Cosmology ; Universe Large-Scale Structure ; Structure Formation ; Numerical Method ; Analytical Method ; Galaxies Formation

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