The Pre-Big Bang Scenario in String Cosmology

TL;DR

<3-5 sentences> The pre-big bang scenario in string cosmology reimagines the early universe as emerging from a weakly coupled, flat string vacuum into a dilaton-driven inflationary phase that approaches a high-curvature string regime, after which a non-perturbative exit leads to standard cosmology. The framework exploits string-duality and frame transformations (string vs Einstein frames) to generate a rich set of background solutions, while predicting distinctive spectra for relic gravitons, dilatons, and photons. Quantum fluctuations are amplified in a frame- and gauge-consistent manner, producing a growing gravitational-wave background with end-points tied to the string/Planck scales, and potentially detectable by future interferometers or space-based detectors. The paper also discusses initial-condition selection via asymptotic past triviality, the exit problem, and phenomenological constraints on dilatons and axions, highlighting both observational prospects and outstanding theoretical challenges in connecting string theory to cosmology.

Abstract

We review physical motivations, phenomenological consequences, and open problems of the so-called pre-big bang scenario in superstring cosmology.

Figures (31)

• Figure 1.1: Qualitative evolution of the curvature scale in the standard cosmological model, in conventional inflationary models and in string-cosmology models.
• Figure 1.2: Qualitative evolution of the Hubble horizon (dashed curve) and of the scale factor (solid curve) in the standard, post-big bang inflationary scenario.
• Figure 1.3: Qualitative evolution of the Hubble horizon (dashed curve) and of the scale factor (solid curve) in the pre-big bang inflationary scenario, in the S-frame, $a(t)$, and in the E-frame, $\widetilde{a}(t)$.
• Figure 2.1: According to the pre-big bang scenario, the dilaton starts in the asymptotic past of our Universe at very large negative values, and grows through a flat potential towards the strong-coupling regime. At present, it is either trapped at a minimum of the potential, or keeps growing monotonically towards $+\infty$ (see Section \ref{['Sec10']}).
• Figure 2.2: The four branches of the low-energy string cosmology backgrounds.