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Research Interests

String theory provides a precise mathematical structure to address questions in low energy gravity, black hole physics, and realizes a fruitful and promising tool for construction of phenomenologically realistic models of elementary particle physics. It is a leading candidate for the consistent unification of gravitation and quantum mechanics. Much of the progress in string theory is attributed to understanding of static (time-independent) backgrounds, while deep cosmological foundations like the origin of time, the physics of space-like singularities, and the cosmological constant problem until recently appeared to be out of its grasp. Recently, important progress has been made toward formulating string theory in a cosmological setting. There is a hope that these new developments will help us understand the Universe we are living in.
With the formulation of the gauge/string duality by Maldacena, we learned that string theories and gauge theories are very closely related. Important progress in this field occurred in the last couple years when new computational tools and techniques were developed to study quantitatively gauge/string duality in the context of theories with reduced supersymmetry. These developments made Maldacena's duality a viable framework in which we can think concretely and analytically about the nonperturbative dynamics of gauge theories in 3+1 dimensions.

The focus of my research efforts is the development and understanding the synthesis of gravity and quantum field theory. Unfortunately, direct experimental probes of a quantum nature of gravity would require access to incredibly high energy, (i.e, the Planck mass, M_PL~ 10^19 GeV). Thus, the primary way at present for addressing candidate theories for such a synthesis is to establish a consistency in the theoretical framework describing this unification. As superstring theory currently is the only known candidate with finite corrections unifying quantum field theory and general relativity, my research concentrates in its development. I believe the most promising direction in string theory that would lead to its true test is understanding how strings describe cosmological backgrounds, motivated by current astrophysical measurements, and the nature of a black hole interior. Additionally, I would like to use insights from string theory and the gauge/string correspondence of Maldacena as a tool towards understanding nonperturbative phenomena in 3+1 dimensional gauge theories, which form a cornerstone of the Standard Model of Elementary Particles.
Analysis of the fluctuations of the Cosmic Microwave Background, when combined with the results of Type Ia supernovae observations, convincingly demonstrate the acceleration of our Universe.The simplest cosmological model to account for this result involves background space-times with a positive cosmological constant. However, corresponding de-Sitter space-time (and thus the presence of a genuine nonzero vacuum energy) present conceptual problems for the formulation of the string theory, when defined as an S-matrix theory. The problem is related to the existence of event horizon in de-Sitter space-time, which implies that it is not possible to construct a conventional S-matrix: a local observer inside his horizon is not able to isolate particles to be scattered. We proposed an alternative to a conventional S-matrix definition for string theory in accelerating universe. The basic idea is to exploit the correspondence between string theory and gauge theory, and rather when defining string theory as an S-matrix theory, realize it as a Quantum Field Theory in background de-Sitter space-time. The proposal provides a confirmation to the physical intuition that spontaneously broken symmetries of gauge theories are restored in accelerating universe, provided the Hubble scale is large enough. It can thus be used as an analytical tool to study these cosmological phase transitions. Much of the details of this "deformed" Maldacena correspondence remains to be uncovered.

One of the fundamental problems in cosmology is the origin of time and the orientability of the time arrow at the macroscopic level. Recently, Sen proposed to identify time in quantum cosmology with a scalar field (tachyon) representing a decay of an unstable D-brane (or brane anti-brane pair). Additionally, the non-equilibrium decay process naturally defines the direction of the entropy growth, and thus the macroscopic arrow of time. As the tachyon rolls down the potential, one is left with an interesting state of matter --- the "tachyon matter" which has finite energy density but the exponentially vanishing pressure. Tachyon matter has attracted considerable cosmological interest. We proposed and studied an effective field theoretical model describing a decaying tachyon on an unstable D-brane in string theory. Asymptotically, the tachyon field decouples from the supergravity modes, and the far future geometry approaches that of the space-like branes, S-branes. We proved that inclusion of the tachyon destabilizes the horizon of an S-brane to a genuine space-like curvature singularity. Such a singularity is expected on physical grounds in the supergravity approximation, where an S-brane is identified as a source of a space-like topological defect. An important open problem is the mechanism for the resolution of the space-like singularity of the unstable D-brane decay (or S-brane). I am currently working on this issue. It is likely that the observed singularity is the rudiment of the supergravity approximation. It is known that in the framework of the Boundary String Field Theory, which motivated our effective model, the rolling tachyon coupling to massive closed string modes becomes strong at string-scale times. Thus their production and the subsequent backreaction is important at very early stages of the tachyon decay. I'm studying whether the backreaction of the massive closed string modes resolves the space-like singularity in the supergravity description of the unstable D-brane decay.

Recently, Kachru et.al, made an important progress towards constructing four dimensional de-Sitter vacua in string theory with finite Newton's constant. Their construction requires extended objects of string theory (D-branes), and incorporation of nonperturbative (though under control) string theory corrections. I am further exploring these constructions, in particular attempting to construct specific string theory models that combine inflation and present day acceleration.

One of the most mysterious objects of our Universe are black holes. For one reason, they have thermodynamics unlike that of a conventional physical system described by quantum field theory. Specifically, the Bekenstein-Hawking entropy formular suggests that the fundamental degrees of freedom necessary to describe a black hole are characterized be a quantum field theory with one fewer space dimensions. String theory provided a satisfactory explanation for the thermodynamic properties of certain black holes. In addition to basic black holes arising in General Relativity, in string theory one encounters rather exotic black holes. We predicted and studied phase transitions in a class of stringy black holes as realizations of strongly coupled finite temperature phase transitions in gauge theories. While the gross features of the transitions were explained, some important characteristics (like the order of a transition, its dynamical onset, and the relation between confinement/deconfinement and chiral symmetry breaking/restoration) were left open. The dual string theory language is currently the only analytical framework to study these phenomena. Their clear explanation undoubtedly would improve our understanding of strong interactions in Nature.

A closely related problem, in which string theory can actually benefit from the holographic dual description in terms of gauge theory is the problem of a black hole interior. An interior of a black hole has a space-like singularity whose resolution in general relativity (or string theory) is currently not understood. Following gauge/string theory correspondence, certain black holes have a dual realization as finite temperature four-dimensional gauge theories in the deconfining phase. Consequently, studying the thermodynamics of these gauge theories should allow us to probe the physics behind the black hole horizon.

Last updated : 2010-11-02