String Gravity Research Articles

String theory and gravity

It is pointed out that string-loop effects may generate matter couplings for the dilaton allowing this scalar partner of the tensorial graviton to stay massless while contributing to macroscopic gravity in a way naturally compatible with existing experimental data. Under a certain assumption of universality of the dilaton coupling functions (possibly realized through a discrete symmetry such as S-duality), the cosmological evolution drives the dilaton towards values where it decouples from matter. At the present cosmological epoch, the coupling to matter of the dilaton should be very small, but non zero. This provides a new motivation for improving the experimental tests of Einstein's Equivalence Principle.

Physical properties of four-dimensional superstring gravity black hole solutions

We consider the physical properties of four-dimensional black hole solutions to the effective action describing the low energy dynamics of the gravitational sector of heterotic superstring theory. We compare the properties of the external field strengths in the perturbative solution to the full O(α′) string effective action equations, to those of exact solutions in a truncated action for charged black holes, and to the Kerr-Newman family of solutions of Einstein-Maxwell theory. We contrast the numerical results obtained in these approaches, and discuss limitations of the analyses. Finally we discuss how the new features of classical string gravity affect the standard tests of general relativity.

Classical hair for Kerr-Newman black holes in string gravity

We compute the external field strength hair present for black holes in string gravity. We retain all couplings in the effective lagrangian for string gravity through first order in α′, the string tension. We work in general black hole backgrounds, both rotating and charged, and compute both dilaton and axion hair to that order, thus expanding and systematizing previous treatments.

Anisotropic, time-dependent solutions in maximally Gauss-Bonnet extended gravity

In an arbitrary number of dimensions, we find the full exact anisotropic, time-dependent, diagonal-metric solutions to maximally Gauss-Bonnet extended gravity theory. This class of theories, for which the lagrangian is an arbitrary linear combination of dimensionally extended Euler forms, is the most general gravitational theory in which the field equations contain no more than second derivatives of the metric. We show that the space-time exponentially approaches an asymptotic state of constant, anisotropic curvature and prove three theorems concerning two generic types of singularities. The first theorem gives conditions for the existence of Kasner-like curvature singularities. For these the metric diverges as t Pi where ∑ p i = 2 k max- 1 and k max is the highest power of the curvature in the lagrangian. Other critical point singularities can arise from the polynomial nature of the theory. The remaining theorems demonstrate that the generic solution is extendible at all of these other critical points and that the generic critical points occur at moments of external volume density of space-time. We give an explicit coordinate transformation which produces a smooth extension through the critical point. The space-time may therefore alternately expand and contract for many cycles before expanding forever or contracting to a singularity. Many particular cases are treated in detail including several power series solutions, the generalized Kasner solution to general relativity with or without cosmological constant, the perturbative solution for quadratic string gravity, and five-dimensional extended gravity.

Gauged BRS symmetry for the Weyl x Lorentz x diffeomorphism group.

A local Becchi-Rouet-Stora operator corresponding to the Weyl\ifmmode\times\else\texttimes\fi{}Lorentz\ifmmode\times\else\texttimes\fi{}diffeomorphism symmetry is constructed in n-dimensional curved space-time. Actions invariant under subalgebras of the resulting local symmetry for the free bosonic string and Einstein gravity are derived.

Global aspects of fixing the gauge in the Polyakov string and Einstein gravity

It is shown that there exists a possible obstruction to a continuous global gauge choice in the Polyakov string theory and in four dimensional Einstein gravity. In many circumstances this obstruction results in no global gauge existing in these two theories.

Casimir effects in superstring theories

Compactification of the space in superstring theories is studied by calculating the one-loop effective potential provided that the supersymmetry is softly broken by some mechanism. The Casimir energy of closed string modes is shown to compactify the ten-dimensional Minkowski space M 10 into a stable M d ×T 10− d (1⩽ d<10) with a i= α′ R i =1 , where α′ and R i are the Regge slope and the ith radius of the torus T 10− d , respectively. In the SST-I model, although the closed string (gravity) sector decouples from the open string (Yang-Mills) sector in α′ → 0 limit, the former functions as an initiator as well as a stabilizer of the compactification.