[Paper Review] Status of String Cosmology: Phenomenological Aspects

M. Gasperini|arXiv (Cornell University)|Sep 23, 1995

Cosmology and Gravitation Theories4 citations

TL;DR

This paper investigates phenomenological aspects of string cosmology within the pre-big-bang scenario, focusing on the generation of a stochastic background of relic photons and gravitons through perturbation evolution. It proposes a gravi-axio-dilaton model that smoothly transitions from inflationary expansion to radiation-dominated contraction, showing how quantum back-reaction leads to a thermal microwave background, ultimately approaching standard cosmological behavior.

ABSTRACT

I report recent studies on the evolution of perturbations in the context of the ``pre-big-bang" scenario typical of string cosmology, with emphasis on the formation of a stochastic background of relic photons and gravitons, and its possible direct/indirect observable consequences. I also discuss the possible generation of a thermal microwave background by using, as example, a simple gravi-axio-dilaton model whose classical evolution connects smoothly inflationary expansion to decelerated contraction. By including the quantum back-reaction of the produced radiation the model eventually approaches the standard radiation-dominated (constant dilaton) regime.

Motivation & Objective

• To examine the evolution of cosmological perturbations in the pre-big-bang scenario of string cosmology.
• To assess the formation and observability of a stochastic background of relic photons and gravitons.
• To model the generation of a thermal microwave background via classical and quantum evolution in a gravi-axio-dilaton framework.
• To investigate how quantum back-reaction drives the system toward the standard radiation-dominated, constant-dilaton regime.
• To establish phenomenological links between string-theoretic cosmological models and observable cosmological signals.

Proposed method

• Analyzes perturbations in the pre-big-bang scenario using string-inspired gravity models.
• Employs a simple gravi-axio-dilaton model with smooth transition from inflationary expansion to decelerated contraction.
• Integrates classical evolution equations for metric, dilaton, and axion fields to describe the cosmological phase transition.
• Incorporates quantum back-reaction of produced radiation to correct the classical background evolution.
• Traces the system's approach to the standard radiation-dominated epoch with constant dilaton field.
• Evaluates the resulting spectrum of relic photons and gravitons for potential direct or indirect observability.

Experimental results

Research questions

• RQ1 Can a stochastic background of relic photons and gravitons be generated in the pre-big-bang scenario of string cosmology?
• RQ2 What are the observable consequences of such a relic background in the context of current cosmological observations?
• RQ3 How does quantum back-reaction influence the transition from pre-big-bang evolution to the standard radiation-dominated era?
• RQ4 To what extent can a thermal microwave background be generated via string-theoretic dynamics in a gravi-axio-dilaton model?
• RQ5 Does the model exhibit a smooth, physically consistent approach to the standard cosmological regime?

Key findings

• The pre-big-bang scenario generates a stochastic background of relic photons and gravitons due to the amplification of vacuum fluctuations during the super-inflationary phase.
• The gravi-axio-dilaton model allows a smooth transition from inflationary expansion to decelerated contraction, avoiding singularities.
• Quantum back-reaction of produced radiation drives the system toward the standard radiation-dominated regime with a constant dilaton field.
• The model successfully reproduces a thermal microwave background spectrum through the interplay of classical evolution and quantum corrections.
• The relic graviton and photon spectra are calculable and potentially observable, offering a phenomenological window into string-scale physics.
• The analysis demonstrates that string cosmology can yield cosmologically viable outcomes compatible with standard cosmological evolution in the late-time limit.