07/11/2023
By Yousef Izadi

The Kennedy College of Science, Department of Physics & Applied Physics invites you to attend a Ph.D. dissertation defense by Yousef Izadi entitled "Modified Gravity Theories: Cosmological Tests and Black Hole Thermodynamics."

Degree: Doctoral
Date: Wednesday, July 19, 2023
Time: 1 p.m.
Location: Room: Olney 115, Zoom: Please email Yousef Izadi for the meeting link.

Committee Chair: Prof. Nishant Agarwal, Department of Physics & Applied Physics, University of Massachusetts Lowell

Committee Members:

  • Prof. Kunnat Sebastian, Department of Physics & Applied Physics, University of Massachusetts Lowell
  • Prof. Silas Laycock, Department of Physics & Applied Physics, University of Massachusetts Lowell
  • Prof. Amin Rezaei Akbarieh, Faculty of Physics, University of Tabriz

Abstract

General relativity (GR) is an extraordinarily successful theory of gravity and has displayed remarkable agreement with numerous experiments and observations. The theory has also made notable predictions, including the existence of black holes and gravitational waves. Moreover, its validity has been well-tested in regimes of weak gravitational fields such as precision tests within the domain of solar system gravity. However, despite these stunning accomplishments and mathematical elegance, both theoretical considerations and observational evidence strongly indicate that GR is incomplete. Fundamental theoretical issues are associated with GR in strong-gravity regimes, such as in singularities at the centers of black holes. Also, general relativity cannot provide a proper description of the observed late-time cosmic acceleration. These motivate us to search for alternative gravity theories and possible modifications of GR.

In this thesis, we test different theories of modified gravity using cosmological observations and black hole thermodynamics. The latter is motivated by the profound relationship between GR and thermodynamics, which should continue to hold for modifications of GR. In Chapter 1, we discuss the motivation behind modifying GR in more detail and introduce various modifications that are studied further in later chapters. In Chapter 2, using a special form of the quintessence model that is equivalent to the Chaplygin gas, we describe the spherical collapse of a matter overdensity. We study the cosmological properties of the quintessence field with a special potential and compare our results with the standard model of cosmology and the Einstein–de Sitter model. Chapter 3 introduces cubic Galilean massive gravity model as an extension of the de Rham-Gabadadze-Tolley (dRGT) theory of massive gravity. We present a detailed study of the cosmological aspects of this theory and show that cubic Galileon massive gravity theory is compatible with the observed accelerated expansion of the Universe. In Chapter 3, we present an extended dRGT massive gravity model that contains higher curvature contributions. We obtain the Einstein field equations and black hole solutions for this theory and study the thermodynamics of black holes within the framework of this model. In Chapter 4, we study the second law of black hole thermodynamics in higher curvature theories of gravity. We consider two cubic curvature terms and, by linearly perturbing their black hole solutions in the presence of infalling null matter, show that a modified version of Wald's entropy satisfies the second law. Chapter 5 is devoted to a conclusion and discussion of the main results of this thesis.