טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentRazamat Shlomo
SubjectTopics in String Theory: Closed String Tachyons and
Gauge/Gravity Duality
DepartmentDepartment of Physics
Supervisor Professor Oren Bergman
Full Thesis textFull thesis text - English Version


Abstract

This thesis centers on two aspects of string theory, the condensation of closed string tachyons and gauge/gravity duality.


In the first part we discuss gauge/gravity duality as it is manifested when considering D-branes localized in imaginary time. Such D-branes naturally arise when one considers the dynamical process of open string tachyon condensation, the so called S-brane. We argue that the effect of having D-branes in this set-up completely amounts to a change in the closed string background, realizing in the most basic sense the ideas behind gauge/gravity duality.


The second part of the thesis deals with a possible world-sheet construction of a string theory dual of free large N gauge theories. These field theories are extremely simple and thus completely tractable. However, till now finding a string theory dual for these proved to be a difficult task. In our research we discuss in great detail a concrete prescription for direct construction of such a string theory due to Gopakumar. We show that this prescription is sensible in many respects. However, we illustrate several puzzles rising from this prescription.


In the closed string tachyon part of the thesis we start with a discussion of a specific example of localized closed string tachyon condensation. These localized closed string tachyons, live on singularities in space-time. There exists a conjecture about the fate of these instabilities. This conjecture states that the condensation of the tachyons smoothes the singularity and eventually eliminates it. We compute the tachyon potential for a specific theory using closed string field theory. We show that our calculations support the above conjecture, and are consistent with previous calculations. We also pose some puzzles arising from our research.


In the last part of the thesis we discuss the role of bulk closed string tachyons. We address this issue using low energy effective field theory techniques. In particular, we argue in favor of the conjecture that the solitonic configurations of bulk closed string tachyons describe lower dimensional, non-critical string theories.