Abstract:
We study the effect of the gravitational Chern-Simons term (GCST) in the (2+1)-dimensional anti-de Sitter (AdS$_{2+1}$) geometry. In the context of the gauge gravity, we obtain black hole solution and its boundary WZW theory. The BTZ black hole solution can still be retrieved but its gravitational mass and angular momentum become different from their inherent values. The deformation on these quantities due to the GCST can be summarized as $SO(1,1)$ times rescaling. The boundary WZW theory is found to be chiral, i.e., composed of the right moving part and the left moving part with different Kac-Moody levels. The statistical entropy is proportional to the area only for the large levels and vanishing GCST limit, but its coefficient is not the correct order in the Newton constant $G$. Some related physics are discussed.

Abstract:
We study the rotating tubular D2-brane as a time dependent supersymmetric solution of type-IIA string theory. We show that the Poynting angular momentum of the supertube can be replaced by the mechanical angular momentum without disturbing the 8 supersymmetries. Unlike the non-rotating supertube, whose cross section can take an arbitrary shape, the rotating supertube admits only the circular cross section. When there is no electric field on the world volume, the supersymmetry dictates the angular velocity of the tubular D2-brane to be inversely proportional to the magnetic field. This rotating supertube can be considered as the `blown-up' configuration of an array of spinning D0-particles and is T-dual to the spiraling D-helix whose pitch moves at the speed of light.

Abstract:
We study the effect of the Born-Infeld electric field on the supersymmetric configuration of various composite D-branes. We show that the generic values of the electric field do not affect the supersymmetry but, as it approaches $1/2\pi\alpha'$ keeping the magnetic field finite, various combinations of the magnetic fields allow up to 8 supersymmetries. We also explore the unbroken supersymmetries for two intersecting D-strings which are in uniform or relative motion. For a finite uniform Lorentz boost, 16 supersymmetries are guaranteed only when they are parallel. For an infinite one, 8 supersymmetries are preserved only when both the D-strings are oriented to the forward or backward direction of the boost. Under a finite relative boost, 8 supersymmetries are preserved only when the intersecting angle is less than $\pi/2$ and the intersecting point moves at the speed of light. As for an infinite relative boost, 8 supersymmetries are preserved regardless of the values of the intersecting angle.

Abstract:
We study `Myers effect' for a bunch of $D1$-branes with $IIB$ superstrings moving in one direction along the branes. We show that the `blown-up' configuration is the helical $D1$-brane, which is self-supported from collapse by the axial momentum flow. The tilting angle of the helix is determined by the number of $D1$-branes. The radius of the helix is stabilized to a certain value depending on the number of $D1$-branes and the momentum carried by $IIB$ superstrings. This is actually T-dual version of the supertube recently found as the `blown-up' configuration of a bunch of $IIA$ superstrings carrying $D0$-brane charge. It is found that the helical $D1$ configuration preserves one quarter of the supersymmetry of $IIB$ vacuum.

Abstract:
An exact solution, in which a D2-brane and an anti-D2-brane are connected by an elliptically tubular D2-brane, is obtained without any junction condition. The solution is shown to preserve one quarter of the supersymmetries of the type-IIA Minkowski vacuum. We show that the configuration cannot be obtained by "blowing-up" from some inhomogeneously D0-charged superstrings. The BPS bound tells us that it is rather composed of D0-charged D2-brane-anti-D2-brane pair and a strip of superstrings connecting them. We obtain the correction to the charges of the string end points in the constant magnetic background.

Abstract:
We study various bubble solutions in string/M theories obtained by double Wick rotations of (non-)extremal brane configurations. Typically, the geometry interpolates de Sitter space-time times non-compact extra-dimensional space in the near-bubble wall region and the asymptotic flat Minkowski space-time. These bubble solutions provide nice background geometries reconciling string/M theories with de Sitter space-time. For the application of these solutions to cosmology, we consider multi-bubble solutions and find landscapes of varying cosmological constant. Double Wick rotation in string/M theories, used in this paper, introduces imaginary higher-form fields. Rather than regard these fields as classical pathologies, we interpret them as semi-classical decay processes of de Sitter vacuum via the production of spherical branes. We speculate on the possibility of solving the cosmological constant problem making use of the condensation of the spherical membranes.

Abstract:
We consider the entropy problem of AdS_3 black holes using the conformal field theory at the horizon. We observe that the supersymmetry is enhanced at the horizon of massless AdS_3 black hole. This allows us to determine the vacuum of the modular invariant conformal field theory to be the NS-ground state (which corresponds to AdS_3 spacetime). This is smoothly related to the R-ground state (corresponding to massless black hole) by a spectral flow, which can be understood as a superconformal transformation.

Abstract:
The spin degrees of freedom for the relativistic particle are described by either Poincar\'{e} group variables (classically) or Grassmann variables (pseudo-classically). The relationship between those two descriptions are given. In doing that, appropriate constraints are constructed to put into the lagrangian. Especially a natural relation of Poincar\'{e} group variables and Grassmann variables is obtained. Hopf fibration relating the spin momentum to the group is just the right transformation of the spin momentum under Poincar\'{e} group. And with the relation just mentioned, pseudoclassical lagrangian is derived naturally from the classical one.

Abstract:
The `classical' model for a massive spinning particle, which was recently proposed, is derived from the isotropic rotator model. Through this derivation, we note that the spin can be understood as the relativistic extension of the isotropic rotator. Furthermore, the variables $t_\m$ corresponding to the $\p^*$ of the `pseudo-classical' model, are necessary for the covariant formulation. The dynamical term for these extra variables is naturally obtained and the meaning of the constraint term $p^\s\L_{\s\n}+mt_\n =0$, which was recently shown to give `quasi-supersymmetry', is clarified.

Abstract:
In the context of $ISO(2,1)$ gauge theory, we consider $(2+1)$-dimensional gravity with the gravitational Chern-Simons term (CST). This formulation allows the `exact' solution for the system coupled to a massive point particle (which is not the case in the conventional Chern-Simons gravity). The solution exhibits locally trivial structure even with the CST, although still shows globally nontrivialness such as the conical space and the helical time structure. Since the solution is exact, we can say the CST induces spin even for noncritical case of $\s+\al m\ne 0$.