High Energy Physics

Quantum gravity in an AdS space-time is described by an SU(N) Yang-Mills theory on a sphere, a bounded many-body system. We argue that in the high temperature phase the theory is intrinsically non-perturbative in the large N limit. At any nonzero value of the \\\'t Hooft coupling $lambda$, an exponentially large (in N^2) number of free theory states of wide energy range (of order N) mix under the interaction. As a result the planar perturbation theory breaks down. We argue that an arrow of time emerges and the dual string configuration should be interpreted as a stringy black hole

We will discuss applications of the recently developed twistor-space methods in perturbative quantum field-theory. The two central topics will be 1) the unitarity bootstrap approach to hard scattering amplitudes in QCD and 2) the analysis of the UV structure of N=8 supergravity.

The exact boundary states for the rolling D-brane solution in two-dimensional black hole systems will be presented.I will study the physical significance of the solution in relation to the ``tachyon-radion correspondence\" and the ``black hole - string transition\". When the alpha\' corrections become larger, when at the same time the Hawking temperature coincide with the Hagedorn temperature, the phase transition occurs and the physics changes drastically. It also suggests the universal feature of the decaying D-brane and its failure in the strong quantum regime. The talk is based on my series of works hep-th/0605013, hep-th/0507040 in collaboration with Soo-jong Rey (SNU) and Yuji Sugawara (Tokyo).

I will discuss some ambiguities involved in using the AdS/CFT correspondence to calculate the ultra-relativistic jet quenching parameter for quarks moving in an N=4 super Yang-Mills thermal bath. Along the way, I will investigate the behavior of various string configurations on a five-dimensional AdS black hole background.

We discuss motivations, observational constraints and consequences of modifying the fundamental laws of gravity at large distances. Such modifications of gravity can be the reason for the observed late-time acceleration of the Universe, and can be differentiated from conventional dark energy via precision cosmology. The inevitable additional polarizations of graviton lead to observably large perihelion precession of the Lunar and Martian orbits. These theories also have potentially observable consequences at LHC .