Remarks on Fundamental String Cosmology

In recent work, it was shown that velocity-dependent forces between moving strings or branes lead to an accelerating expanding universe without assuming the existence of a cosmological constant. Here we show that the repulsive velocity-dependent force arises in more general contexts and can lead to cosmic structure formation.Comment: 7 pages, harvma

Self-Gravitating Strings and String/Black Hole Correspondence

In a recent essay, we discussed the possibility of using polymer sizing to model the collapse of a single, long excited string to a black hole. In this letter, we apply this idea to bring further support to string/black hole correspondence. In particular, we reproduce Horowitz and Polchinki's results for self-gravitating fundamental strings and speculate on the nature of the quantum degrees of freedom of black holes in string theory.Comment: 8 pages, harvma

Geodesic Scattering of Solitonic Strings

We compute the metric on moduli space for the Dabholkar-Harvey string soliton in $D=4$ to lowest nontrivial order in the string tension. The metric is found to be flat, which implies trivial scattering of the solitons. This result is consistent with an earlier test-string calculation of the leading order dynamical force and a computation of the Veneziano amplitude for the scattering of macroscopic strings.Comment: 6 page

Spacetime Supersymmetry and Duality in String Theory

I discuss the role of spacetime supersymmetry in the interplay between strong/weak coupling duality and target space duality in string theory which arises in string/string duality. This can be seen via the construction of string soliton solutions which in $N=4$ compactifications of heterotic string theory break more than $1/2$ of the spacetime supersymmetries but whose analogs in $N=2$ and $N=1$ compactifications break precisely $1/2$ of the spacetime supersymmetries. As a result, these solutions may be interpreted as stable solitons in the latter two cases, and correspond to Bogomol'nyi-saturated states in their respective spectra.Comment: 6 pages, harvma

Black Hole Solutions in String Theory

We present two-parameter solutions of the low-energy four-dimensional heterotic string which in the extremal limit reduce to supersymmetric monopole, string and domain wall solutions. The effective scalar coupling to the Maxwell field, $e^{-\alpha \phi} F_{\mu\nu} F^{\mu\nu}$, gives rise to a new string black hole with $\alpha = \sqrt{3}$, in contrast to the pure dilaton black hole solution which has $\alpha=1$. Implications of string/fivebrane duality in $D=10$ to four-dimensional dualities are discussed.Comment: Based on talks given at MRST 94, McGill University and at Mini-workshop on Extended Objects, Newton Institute, March 1994, 4 pages, Te

Supersymmetry, Duality and Bound States

$P$-brane solutions of low-energy string actions have traditionally provided the first evidence for the existence of string dualities, in which fundamental and solitonic $p$-branes are identified with perturbative and non-perturbative BPS states. In this talk we discuss the composite nature of solutions, which allows for the interpretation of general solutions as bound states or intersections of maximally supersymmetric fundamental constituents. This feature lies at the heart of the recent success of string theory in reproducing the Beckenstein-Hawking black hole entropy formula.Comment: 8 pages, harvmac, talk given at Strings '9