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Adiabatic CMB perturbations in pre-big bang string cosmology

Kari Enqvist, Martin S. Sloth

TL;DR

The paper shows that in pre-big-bang string cosmology, a massive axion acquiring a periodic potential and decaying to photons can convert initial fluctuations into adiabatic CMB perturbations. Depending on the axion’s spectral tilt $\gamma = 3/2 - r$ and the damping from the potential, the resulting CMB spectrum can be nearly scale-invariant with a small positive or negative tilt; positive tilt gives a modest blue tilt $Δn \approx 0.26$, while negative tilt can yield $0<Δn\lesssim 1$ under suitable conditions. The authors also analyze the axion's decay timing, reheating temperature $T_{RH}$, and entropy production, showing that late-time entropy release can dilute relics and keep BBN consistent, with a broad range of $T_{RH}$ depending on model parameters. Overall, the scenario suggests a viable route to adiabatic dominance in the CMB within the PBB framework, potentially coexisting with residual isocurvature components and leaving observable imprints in the gravitational-wave spectrum.

Abstract

We consider the pre-big bang scenario with a massive axion field which starts to dominate energy density when oscillating in an instanton-induced potential and subsequently reheats the universe as it decays into photons, thus creating adiabatic CMB perturbations. We find that the fluctuations in the axion field can give rise to a nearly flat spectrum of adiabatic perturbations with a spectral tilt $Δn$ in the range $-0.1 \lesssim Δn \lesssim 1$.

Adiabatic CMB perturbations in pre-big bang string cosmology

TL;DR

The paper shows that in pre-big-bang string cosmology, a massive axion acquiring a periodic potential and decaying to photons can convert initial fluctuations into adiabatic CMB perturbations. Depending on the axion’s spectral tilt and the damping from the potential, the resulting CMB spectrum can be nearly scale-invariant with a small positive or negative tilt; positive tilt gives a modest blue tilt , while negative tilt can yield under suitable conditions. The authors also analyze the axion's decay timing, reheating temperature , and entropy production, showing that late-time entropy release can dilute relics and keep BBN consistent, with a broad range of depending on model parameters. Overall, the scenario suggests a viable route to adiabatic dominance in the CMB within the PBB framework, potentially coexisting with residual isocurvature components and leaving observable imprints in the gravitational-wave spectrum.

Abstract

We consider the pre-big bang scenario with a massive axion field which starts to dominate energy density when oscillating in an instanton-induced potential and subsequently reheats the universe as it decays into photons, thus creating adiabatic CMB perturbations. We find that the fluctuations in the axion field can give rise to a nearly flat spectrum of adiabatic perturbations with a spectral tilt in the range .

Paper Structure

This paper contains 8 sections, 57 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Initial axion field fluctuations
  3. Decaying axion as the origin of initial density perturbations
  4. Negative tilt fluctuations
  5. Positive tilt fluctuations
  6. Spectral tilt of the CMB anisotropies
  7. Reheat temperature and entropy production
  8. Conclusion

Figures (1)

  • Figure 1: Levels of constant $(\delta\rho_a)_k/\rho_a$ as a function of $M_s/\psi_0$ on the horizontal axis and $\gamma$ on the vertical axis. The three lines correspond to $(\delta\rho_a)_k/\rho_a=10^{-3}$, $10^{-4}$, and $10^{-5}$.