Non-perturbative gravity, the Hagedorn bounce and the cosmic microwave background

Abstract

In Biswas et al (2006 J. Cosmol. Astropart. Phys. JCAP03(2006)009 [hep-th/0508194]) it wasshown how non-perturbative corrections to gravity can resolve the big bang singularity,leading to a bouncing universe. Depending on the scale of the non-perturbative corrections,the temperature at the bounce may be close to or higher than the Hagedorn temperature.If matter is made up of strings, then massive string states will be excited near the bounce,and the bounce will occur inside (or at the onset of) the Hagedorn phase forstring matter. As we discuss in this paper, in this case cosmological fluctuationscan be generated via the string gas mechanism recently proposed in Nayeri et al (2005 Preprint hep-th/0511140). In fact, the model discussed here demonstratesexplicitly that it is possible to realize the assumptions made in Nayeri et al (2005Preprint hep-th/0511140) in the context of a concrete set of dynamical backgroundequations. We also calculate the spectral tilt of thermodynamic stringy fluctuationsgenerated in the Hagedorn regime in this bouncing universe scenario. Generally wefind a scale-invariant spectrum with a red tilt which is very small but does notvanish.