Rational Conformal Field Theories With G_2 Holonomy

We study conformal field theories for strings propagating on compact, seven-dimensional manifolds with G_2 holonomy. In particular, we describe the construction of rational examples of such models. We argue that analogues of Gepner models are to be constructed based not on N=1 minimal models, but on Z_2 orbifolds of N=2 models. In Z_2 orbifolds of Gepner models times a circle, it turns out that unless all levels are even, there are no new Ramond ground states from twisted sectors. In examples such as the quintic Calabi-Yau, this reflects the fact that the classical geometric orbifold singularity can not be resolved without violating G_2 holonomy. We also comment on supersymmetric boundary states in such theories, which correspond to D-branes wrapping supersymmetric cycles in the geometry.Comment: 20 pages, harvmac(b); v2: ref. adde

The Tachyon does Matter

We review the concept of S-branes introduced by Gutperle and Strominger hep-th/0202210. Using the effective spacetime description of the rolling tachyon worldsheets discussed by Sen, we analyze the possibility that the gravitational backreaction of tachyon matter is important in the time-dependent process. We show that this is indeed the case in the example of the S0-brane in 4-dimensional Einstein-Maxwell theory. This talk is based on hep-th/0207235.Comment: 8 pages, Talk given at 35th Symposium Ahrenshoop, Contribution to the Proceeding

Conformal field theory, boundary conditions and applications to string theory

This is an introduction to two-dimensional conformal field theory and its applications in string theory. Modern concepts of conformal field theory are explained, and it is outlined how they are used in recent studies of D-branes in the strong curvature regime by means of CFT on surfaces with boundary.Comment: 45 pages, LaTeX2

A new kind of McKay correspondence from non-Abelian gauge theories

The boundary chiral ring of a 2d gauged linear sigma model on a K\"ahler manifold $X$ classifies the topological D-brane sectors and the massless open strings between them. While it is determined at small volume by simple group theory, its continuation to generic volume provides highly non-trivial information about the $D$-branes on $X$, related to the derived category $D^\flat(X)$. We use this correspondence to elaborate on an extended notion of McKay correspondence that captures more general than orbifold singularities. As an illustration, we work out this new notion of McKay correspondence for a class of non-compact Calabi-Yau singularities related to Grassmannians.Comment: 29 pages, harvmac(b), 2 fig

Dark Matter and The Anthropic Principle

We evaluate the problem of galaxy formation in the landscape approach to phenomenology of the axion sector. With other parameters of standard LambdaCDM cosmology held fixed, the density of cold dark matter is bounded below relative to the density of baryonic matter by the requirement that structure should form before the era of cosmological constant domination of the universe. Galaxies comparable to the Milky Way can only form if the ratio also satisfies an upper bound. The resulting constraint on the density of dark matter is too loose to select a low axion decay constant or small initial displacement angle on anthropic grounds.Comment: 17 pages, 1 figur

About the morphology of dwarf spheroidal galaxies and their dark matter content

The morphological properties of the Carina, Sculptor and Fornax dwarfs are investigated using new wide field data with a total area of 29 square degrees. The stellar density maps are derived, hinting that Sculptor possesses tidal tails indicating interaction with the Milky Way. Contrary to previous studies we cannot find any sign of breaks in the density profiles for the Carina and Fornax dwarfs. The possible existence of tidal tails in Sculptor and of King limiting radii in Fornax and Carina are used to derive global M/L ratios, without using kinematic data. By matching those M/L ratios to kinematically derived values we are able to constrain the orbital parameters of the three dwarfs. Fornax cannot have M/L smaller than 3 and must be close to its perigalacticon now. The other extreme is Sculptor that needs to be on an orbit with an eccentricity bigger than 0.5 to be able to form tidal tails despite its kinematic M/L.Comment: 9 pages, 7 figures, accepted by A&

Polynomial Structure of the (Open) Topological String Partition Function

In this paper we show that the polynomial structure of the topological string partition function found by Yamaguchi and Yau for the quintic holds for an arbitrary Calabi-Yau manifold with any number of moduli. Furthermore, we generalize these results to the open topological string partition function as discussed recently by Walcher and reproduce his results for the real quintic.Comment: 15 page