[Paper Review] Particle Creation in Pre-Big-Bang Cosmology: theory and observational consequences

Ruth Durrer, Kerstin E. Kunze+1 more|arXiv (Cornell University)|Oct 20, 2000

Cosmology and Gravitation Theories4 citations

TL;DR

This paper investigates particle creation in pre-Big-Bang cosmology, a string-theory-inspired model where quantum fluctuations generate primordial perturbations. It finds that tensor modes yield a blue spectrum with $n_T = 3$, incompatible with observations unless modified by a dilaton potential, while axionic and electromagnetic seeds may seed large-scale structure and primordial magnetic fields, though only axions yield a viable spectrum under certain conditions.

ABSTRACT

We present some phenomenological aspects of the pre-big-bang cosmological model inspired by the duality properties of string theory. In particular, assuming the spatial sections of the homogeneous background geometry to be isotropic, we discuss the quantum production of perturbations of the background fields (gravitons, dilatons, moduli fields), as well as the production of particles which do not contribute to the background, which we call ``seeds''. As such we consider the cases of electromagnetic and axionic seeds. We also discuss their possible observational consequences, for example, we study whether they can provide the origin of primordial galactic magnetic fields, and whether they can generate the initial fluctuations leading to the formation of large-scale structure and the measured cosmic microwave background anisotropies. We finally analyze axion and photon production in four dimensional anisotropic pre-big-bang cosmological models.

Motivation & Objective

• To explore the phenomenological consequences of particle production in the pre-Big-Bang (PBB) cosmological model, inspired by string theory duality.
• To assess whether quantum fluctuations of background fields (gravitons, dilatons, moduli) and non-background fields ('seeds') can generate observable primordial structures.
• To determine if electromagnetic or axionic seeds can account for the origin of primordial galactic magnetic fields and large-scale structure.
• To evaluate whether the PBB model can reproduce observed cosmic microwave background (CMB) anisotropies, particularly through scale-invariant perturbations.
• To analyze the impact of anisotropy in four-dimensional PBB models on particle production and spectral outcomes.

Proposed method

• Uses the low-energy effective action of string theory in $D$ dimensions, including the dilaton, antisymmetric tensor field $B_{\mu\nu}$, and matter action $S_M$, to derive cosmological equations.
• Applies scale-factor duality and time-reversal symmetry to map standard post-big-bang solutions into pre-big-bang solutions with accelerated expansion.
• Analyzes parametric amplification of vacuum fluctuations during the transition from pre- to post-big-bang era, focusing on scalar, tensor, and dilaton/moduli perturbations.
• Computes second-order energy-momentum tensor perturbations from 'seed' fields (electromagnetic and Kalb-Ramond axions) to assess their geometric impact.
• Performs multipole expansion of temperature anisotropies to quantify contributions from seed fields to CMB fluctuations.
• Considers both isotropic and anisotropic four-dimensional cosmological models, computing energy spectra for massless axions and photons under varying internal dimension contraction rates.

Experimental results

Research questions

• RQ1 Can the pre-Big-Bang model generate a scale-invariant spectrum of adiabatic perturbations compatible with CMB observations?
• RQ2 Do electromagnetic seed fluctuations produce observable primordial magnetic fields or contribute significantly to CMB anisotropies?
• RQ3 Can Kalb-Ramond axions with masses up to 100 MeV produce a flat or slightly blue spectrum consistent with current data?
• RQ4 How does the inclusion of a dilaton potential affect the spectral index of dilaton and moduli field perturbations?
• RQ5 Do anisotropic pre-Big-Bang models yield different particle production spectra compared to isotropic models?

Key findings

• Tensor metric perturbations in the PBB model yield a blue-tilted power spectrum with $n_T = 3$, which is incompatible with CMB observations unless modified by a dilaton potential.
• Scalar perturbations generically have a red spectrum with $n = 0$, leading to excessive large-scale inhomogeneities that conflict with observations unless corrected by a dilaton potential.
• Dilaton and moduli field perturbations can achieve a scale-invariant Harrison-Zel’dovich spectrum ($n = 1$) if an exponential dilaton potential is included, making them consistent with CMB data.
• Electromagnetic seed fluctuations produce a blue spectrum that decays rapidly on large scales, rendering them ineffective for generating observable primordial magnetic fields, though they contribute negligibly to CMB anisotropy.
• Kalb-Ramond axions can produce a flat or slightly blue spectrum depending on the contraction rate of internal dimensions, with a spectral index compatible with current data under reasonable assumptions.
• In four-dimensional anisotropic PBB models, axion spectra remain infra-red divergent and axion spectra are unphysically red, while photon spectra remain blue, indicating the same fundamental issues as in the isotropic case.