Sharif Digital Repository

The purpose of this thesis is to study the behaviour of open strings between two parallel D-Branes in the presence of a background Kalb-Ramond field linearly varying perpendicular to D-Branes. In an attempt to solve the nonlinear equations of motion we found two sets of nontrivial solutions: String wrapped around a cylinder with constant velocity along its axis and a rotating string with oscillatory radius perpendicular to D-Branes, such that knots are shaped. Considering the boundary conditions, the constraints on these solutions, conserved currents and Mass are found. In particular calculated linear momentum for the first set of solutions suggests the possibility of using this setup as an... 

Since 1980’s, String Theory is considered as one of the main candidates for the unified theory of nature. One of the important features of this theory, is describing the Gravity as a renormalizable quantum theory. An interesting and important problem is the interaction of the string with classical fields in spacetime such as gravity. The conclusions which can be deduced may be very different from the standard results in case of absence of these classical fields.On the other side, for giving a solution to one of the unsolved problems in standard model of particle physics, the hierarchy problem, Lisa Randall and Raman Sundrum in1999, suggested a model which assumes that spacetime may has... 

In this Thesis we study the string theory discription of non-relativistic quantum ?eld theories.In other statement we study the duality between gravity and gauge theory with non-relativistic symmetry.We discuss that this duality works in non rel- ativistic sense.We study two kind of non-relativistic theories that one of them camefrom string theory and other model obtained by special transformation of original relativistic theory and we see that how duality can be work for these models.Also dual picture of semi-classical string in the background that found from string theorystudied.We also study the motion of particle in a hot plasma and we ?nd a sign of non-relativisic duality really works  

Not all effective theories in the low energy limit are compatible with quantum gravity. The goal of the swampland program is bringing the necessary criteria to test these effective theories and examine their compatibility with quantum gravity. This set of conjectures determines which theory is effective in the perspective of the self-consistent theory of quantum gravity which is actually string theory. A very important connection (through the swamp program) between high energy physics (UV) And the physics of the low energy limit (IR) can provide very important explanations for our questions. Among them are the cosmological constant problem in general relativity and the hierarchy problem in... 

Not all low-energy effective theories are compatible with quantum gravity. The purpose of swampland program is to provide the necessary characteristics to test these theories and measure their compatibility with quantum gravity. Accordingly, this group of conjectures determine which effective theory is in the landscape of self-consistent quantum gravity which is String Theory. An important connection is provided through the swampland program between the high-energy limit physics (UV) and the low-energy limit physics (IR) that can provide immensely important fundamental explanations for our profound and consistent questions such as the hierarchy problem in elementary particle physics and... 

Double Field Theory tries to incorporate T-duality, one of fundamental features of string theory, as a symmetry of a field Theory on a doubled space. It sets fields depend on winding mode conjugate coordinates as well as conventional space-time coordinates. In this review after presenting a brief introduction to string theory we investigate O(d,d,ℤ) duality group. Subsequently, we focus on Double Field Theory construction putting our attention on massless sector of string theory. We investigate on 2D nonlinear sigma model action in DFT and its gauge algebra. We follow this review by discussing constraints of DFT and Double Geometry. At the end we present some unsolved questions in this... 

We study a speci?c class of black holes which are called extremal. By de?ning extremality asdouble-horizon condition and imposing ?niteness assumption, we show that the dynamics of ?eldsat the horizon decouples from the rest of space. This decoupling occurs by vanishing all radialderivatives. We show this decoupling for the extremal black holes in the theories of gravity correctedby f(R)terms.We also show that these two above conditions leads to AdS × S1 near horizon geometry2of rotating black holes. This geometry which preserve SO(2, 1) × U(1) isometry group, is recentlyused in improving the entropy function method. Finally we show that decoupled equations of horizon can be derived from a... 

We extend the non-relativistic AdS/CFT correspondence to the fermionic fields. In particular we study the two point function of a fermionic perator in non-relativistic CFTs by making use of a massive fermion propagating in geometries with Schrodinger group isometry. Although the boundary of the geometries with Schrodinger group isometry differ from that in AdS geometries where the dictionary of AdS/CFT is established، using the general procedure of AdS/CFT correspondence، we see that the resultant two point function has the expected form for fermionic operators in non-relativistic CFTs، though a non-trivial regularization may be needed. Also we study Weyl symmetry for non-relativistic... 

Many physicists believed that all four forces were once unified at high energy levels but as the universe reduced into a lower-energy states the inherent symmetry between the forces began to break down.this broken symmetry caused the creation of four distinct forces of nature. The goal of the unified theory is an attempt to look back in time to when these forces where unified as one. In this thesis I tried to give an overview of the challenges which is againsted theorists in the way of unification. In the first chapter explores the theory that try to unify the electromagnetic and weak forces are investigated.it is done by considering guage principle and spontaneous symmetry breaking.in the... 

Multiverse scenario has been recently addressed by physicists and they have found it useful in many areas of physics. In this dissertation, I have treated some issues about multiverse such as the kinds of multiverses, the arguments for or against multiverse, areas in which multiverse is introduced, etc. This dissertation has six chapters and an appendix. In the chapter one, some classifications of multiverse is introduced, and also some definitions of the concept of a world is presented. Then, my classification and definition of a multiverse and a world is introduced. In the chapter two, the concept of possible worlds in philosophy and its relationship with multiverse hypothesis is examined....