Abstract
Space-filling S-branes can mediate a transition between a contracting and an expanding universe in the Einstein frame. Following up on previous work that uncovered such bouncing solutions in the context of weakly coupled thermal configurations of a certain class of type II superstrings, we set up here the formalism in which we can study the evolution of metric fluctuations across such an S-brane. Our work shows that the specific nature of the S-brane, which is sourced by non-trivial massless thermal string states and appears when the universe reaches a maximal critical temperature, allows for a scale invariant spectrum of curvature fluctuations to manifest at late times via a stringy realization of the matter bounce scenario. The finite energy density at the transition from contraction to expansion provides calculational control over the propagation of the curvature perturbations through the bounce, furnishing a working proof of concept that such a stringy universe can result in viable late time cosmology.
Highlights
Since its inception, the inflationary universe scenario [1] has widely come to be accepted as one of the most promising phenomenological models of the early universe that successfully accounts for the initial conditions of the hot Big Bang.1 the paradigm is not without its conceptual [2, 3] as well as technical problems,2 and it is of more than just academic interest to study the possibility of alternative cosmological scenarios
We have studied the transfer of cosmological fluctuations through a stringy spacelike brane (S-brane), which mediates the transition between contracting and expanding cosmological phases related via string thermal duality symmetry
The S-brane itself is a space-filling defect that interpolates between the two dual geometrical phases of the underlying worldsheet CFT, and is sourced by non-trivial thermal string states that become massless at a critical maximal temperature
Summary
The inflationary universe scenario [1] has widely come to be accepted as one of the most promising phenomenological models of the early universe that successfully accounts for the initial conditions of the hot Big Bang. the paradigm is not without its conceptual [2, 3] as well as technical problems, and it is of more than just academic interest to study the possibility of alternative cosmological scenarios. As shown by Durrer and Vernizzi [49] (see [50]), in the case of an instantaneous transition between contraction and expansion, the generalization of the Israel junction conditions [54] to spacelike hypersurfaces [55, 56] implies that the coupling of the growing mode in the contracting phase to the constant mode in the expanding period is not suppressed unless the matching surface is a constant energy density surface This result had important implications for the transfer of fluctuations through examples such as an Ekpyrotic bounce. In this paper we study the cosmology induced in a concrete string theoretic construction [20,21,22,23] in which an S-brane mediates the transition between the contracting and expanding phases In these models, the S-brane is not located on a hypersurface of constant energy density. After contextualizing this calculation in the matter bounce scenario, we offer a summary and our concluding thoughts, taking care to highlight the open issues confronting our model
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