Sunggeun Lee

The Cosmology with the Dp-brane gas

We study the effect of the Dp-brane gas in string cosmology. When one kind of Dp-brane gas dominates, we find that the cosmology is equivalent to that of the Brans-Dicke theory with the perfect fluid type matter. We obtain

Chiral Rings and GSO Projection in Orbifolds

\gamma

Topology Change of Spacetime and Resolution of Spacetime Singularity in Emergent Gravity

, the equation of state parameter, in terms of p and the space-time dimension.

Localized tachyon condensation and G-parity conservation

The GSO projection in the twisted sector of orbifold background is sometimes subtle and incompatible descriptions are found in literatures. Here, from the equivalence of partition functions in NSR and GS formalisms, we give a simple rule of GSO projection for the chiral rings of string theory in C{sup r}/Z{sub n}, r = 1, 2, 3. Necessary constructions of chiral rings are given by explicit mode analysis.

Wilson loop and dimensional reduction in non-commutative gauge theories

Emergent gravity is based on the Darboux theorem or the Moser lemma in symplectic geometry stating that the electromagnetic force can always be eliminated by a local coordinate transformation as far as U(1) gauge theory is defined on a spacetime with symplectic structure. In this approach, the spacetime geometry is defined by U(1) gauge fields on noncommutative (NC) spacetime. Accordingly the topology of spacetime is determined by the topology of NC U(1) gauge fields. We show that the topology change of spacetime is ample in emergent gravity and the subsequent resolution of spacetime singularity is possible in NC spacetime. Therefore the emergent gravity approach provides a well-defined mechanism for the topology change of spacetime which does not suffer any spacetime singularity in sharp contrast to general relativity.

Closed string tachyon potential and tt* equation

We study the condensation of localized tachyon in non-supersymmetric orbifold 2/n. We first show that the G-parities of chiral primaries are preserved under the condensation of localized tachyon(CLT) given by the chiral primaries. Using this, we finalize the proof of the conjecture that the lowest-tachyon-mass-squared increases under CLT at the level of type-II string with full consideration of GSO projection. We also show the equivalence between the G-parity given by G = [jk1/n]+[jk2/n] coming from partition function and that given by G = {jk1/n}k2−{jk2/n}k1 coming from the monomial construction for the chiral primaires in the dual mirror picture.

Toda systems in closed string tachyon condensation

Using the AdS/CFT correspondence we study UV behavior of Wilson loops in various noncommutative gauge theories. We get an area law in most cases and try to identify its origin. In D3 case, we may identify the origin as the D1 dominance over the D3: as we go to the boundary of the AdS space, the effect of the flux of the D3 charge is highly suppressed, while the flux due to the D1 charge is enhanced. So near the boundary the theory is more like a theory on D1 brane than that on D3 brane. This phenomena is closely related to the dimensional reduction due to the strong magnetic field in the charged particle in the magnetic field. The linear potential is not due to the confinement by IR effect but is the analogue of Coulomb’s potential in 1+1 dimension.

Test of Emergent Gravity

Recently Dabholkar and Vafa proposed that closed string tachyon potential for non-supersymmetric orbifold C/Z3 in terms of the solution of a tt∗ equation. We extend this result to C2/Zn for n=3,4,5. Interestingly, the tachyon potentials for n=3 and 4 are still given in terms of the solutions of Painleve III type equation that appeared in the study of C1/Z3 with different boundary conditions. For C2/Z5 case, governing equations are of generalized Toda type. The potential is monotonically decreasing function of RG flow.

Chiral rings and Gliozzi-Scherk-Olive projection in orbifolds

We consider tt* equations appearing in the study of localized tachyon condensations. They are described by various Toda system when we consider the condensation by the lowest tachyon corresponding to the monomial xy. The tachyon potential is calculated as a solution to these equations. The Toda system appearing in the deformation of 2/n by xy is identical to that of Dn singularity deformed by x. For 3/n with xyz deformation, we find only generic non-simple form, similar to the case appearing in /5→/3 and we discuss the difficulties in these cases.