Recurrent Acceleration in Dilaton-Axion Cosmology

A class of Einstein-dilaton-axion models is found for which almost all flat expanding homogeneous and isotropic universes undergo recurrent periods of acceleration. We also extend recent results on eternally accelerating open universes.Comment: 8 pages, 7 figures. minor changes. Version 4 corrects a figure captio

p-Brane Democracy

The ten or eleven dimensional origin of central charges in the N=4 or N=8 supersymmetry algebra in four dimensions is reviewed: while some have a standard Kaluza-Klein interpretation as momenta in compact dimensions, most arise from $p$-form charges in the higher-dimensional supersymmetry algebra that are carried by $p$-brane `solitons'. Although $p=1$ is singled out by superstring perturbation theory, U-duality of N=8 superstring compactifications implies a complete `$p$-brane democracy' of the full non-perturbative theory. An `optimally democratic' perturbation theory is defined to be one in which the perturbative spectrum includes all particles with zero magnetic charge. Whereas the heterotic string is optimally democratic in this sense, the type II superstrings are not, although the 11-dimensional supermembrane might be.Comment: Some typos, and typographical problems, of previous revised version corrected. 15 pp. Latex. This paper will appear in the proceedings of the March 95 PASCOS/Johns Hopkins conferenc

Axion-Dilaton Domain Walls and Fake Supergravity

Dynamical systems methods are used to investigate domain-wall solutions of a two-parameter family of models in which gravity is coupled to an axion, and to a dilaton with an exponential potential of either sign. A complete global analysis is presented for (i) constant axion and (ii) flat walls, including a study of bifurcations and a new exact domain-wall solution with non-constant axion. We reconsider `fake supergravity' issues in light of these results. We show, by example, how domain walls determine multi-valued superpotentials that branch at stationary points that are not stationary points of the potential, and we apply this result to potentials with anti-de Sitter vacua. We also show by example that `adapted' truncation to a single-scalar model is sometimes inconsistent, and we propose a `generalized' fake supergravity formalism that applies in some such cases.Comment: 43pp, 19 figures; minor corrections and extensions; one additional figur

The Manifestly Sl(2;Z)-covariant Superstring

We present a manifestly Sl(2;Z)-covariant action for the type IIB superstring, and prove kappa-symmetry for on-shell IIB supergravity backgrounds.Comment: 13 pages, plain tex. Two minor corrections. Reference adde

Super D-branes

We present a manifestly Lorentz invariant, spacetime supersymmetric, and `$\kappa$-invariant' worldvolume action for all type II Dirichlet p-branes, $p\le9$, in a general type II supergravity background, including massive backgrounds in the IIA case. The $p=0,2$ cases are rederived from D=11. The $p=9$ case provides a supersymmetrization of the D=10 Born-Infeld action.Comment: 27pp. Typos corrected. To appear in Nucl. Phys.