A Study of Accelerating Cosmologies from Superstring/M theories

We study aspects of the accelerating cosmologies obtained from the compactification of vacuum solution and S2-branes of superstring/M theories. Parameter dependence of the resulting expansion of our universe and internal space is examined for all cases. We find that accelerating expansions are obtained also from spherical internal spaces, albeit the solution enters into contracting phase eventually. The relation between the models of SM2- and SD2-branes are also discussed, and a potential problem with SD2-brane model is noted.Comment: 20 pages, 17 figures, a typo corrected, to be published in Prog. Theor. Phy

Null-Brane Solutions in Supergravities

We find a new class of time-dependent brane solutions in supergravities in arbitrary dimensions $D$. These are general intersecting light-like branes (null-branes), and their superposition and intersection rules are obtained. This is achieved by directly solving bosonic field equations for supergravity coupled to a dilaton and antisymmetric tensor fields. We discuss their possible significance.Comment: 12 pages, v2: minor corrections, refs. added, v3: expressions of the solutions modifie

Introduction to branes and M-theory for relativists and cosmologists

We review the recent developments in superstrings. We start with a brief summary of various consistent superstring theories and discuss T-duality which necessarily leads to the presence of D-branes. The properties of D-branes are summarized and we discuss how these suggest the existence of 11-dimensional quantum theory, M-theory, which is believed to give rise to various superstrings as perturbative expansions around particular backgrounds in the theory. We also discuss the interpretation of brane solutions as black holes in string theories and statistical explanation of Bekenstein-Hawking entropy. The idea behind this interpretation is that there is a fundamental duality between closed (gravity) and open (gauge theory) string degrees of freedom, one of whose manifestation is what is kown as AdS/CFT correspondence. The idea is used to discuss the greybody factors for BTZ black holes. Finally the entropy of various balck holes are discussed in connection with Cardy-Verlinde formula.Comment: 28 pages, 7 figures, Latex. Lectures at the international workshop ``Brane world'' at YITP, 15--18 January 2002, to be published in Prog. Theor. Phys. Suppl. ptp style files included. v2: minor corrections, v3: minor corrections and refs. added, v4: eq.(7.14) correcte

Black Holes in the Dilatonic Einstein-Gauss-Bonnet Theory in Various Dimensions III -- Asymptotically AdS Black Holes with k=±1k=\pm 1 --

We study black hole solutions in the Einstein-Gauss-Bonnet gravity with the dilaton and a negative ``cosmological constant''. We derive the field equations for the static spherically symmetric ($k=1$) and hyperbolically symmetric ($k=-1$) spacetime in general $D$ dimensions. The system has some scaling symmetries which are used in our analysis of the solutions. We find exact solutions, i.e., regular AdS solution for $k=1$ and a massless black hole solution for $k=-1$. Nontrivial asymptotically AdS solutions are obtained numerically in D=4 -- 6 and 10 dimensional spacetimes. For spherically symmetric solutions, there is the minimum horizon radius below which no solution exists in D=4 -- 6. However in D=10, there is not such lower bound but the solution continues to exist to zero horizon size. For hyperbolically symmetric solution, there is the minimum horizon radius in all dimensions. Our solution can be used for investigations of the boundary theory through AdS/CFT correspondence.Comment: 25 pages, 16 figure

Interactions of Discrete States with Nonzero Ghost Number \\ in c=1c=1 2D2D Gravity

We study the interactions of the discrete states with nonzero ghost number in $c=1$ two-dimensional ($2D$) quantum gravity. By using the vertex operator representations, it is shown that their interactions are given by the structure constants of the group of the area preserving diffeomorphism similar to those of vanishing ghost number. The effective action for these states is also worked out. The result suggests the whole system has a BRST-like symmetry.Comment: 10 pages, OS-GE 25-9

On the No-ghost Theorem in String Theory

We give a simple proof of the no-ghost theorem in the critical bosonic string theory by using a similarity transformation.Comment: 5 pages, v2: a note added and a ref. added, v3: minor refinement of wording, published versio