Abstract
We propose a mechanism borrowed from string theory which yields a nonsingular transition from a phase of ekpyrotic contraction to the expanding phase of standard big bang cosmology. The same mechanism converts the initial vacuum spectrum of cosmological fluctuations before the bounce into a scale-invariant one, and also changes the spectrum of gravitational waves into an almost scale-invariant one. The scalar and tensor tilts are predicted to be the same, in contrast to the predictions from the ``string gas cosmology'' scenario. The amplitude of the gravitational wave spectrum depends on the ratio of the string scale to the Planck scale and may be in reach of upcoming experiments.
Highlights
The inflationary universe scenario [1] has become the standard paradigm of early universe cosmology
It is based on the assumption that there was a period of almost exponential expansion during a time period in the very early universe
Inflation provides a solution of the horizon and flatness problems of standard big bang cosmology, and provides a causal mechanism for producing cosmological perturbations and microwave background anisotropies based on the assumption that all fluctuation modes start our in their vacuum state inside the Hubble radius at early times [2]
Summary
The inflationary universe scenario [1] has become the standard paradigm of early universe cosmology. Inflation provides a solution of the horizon and flatness problems of standard big bang cosmology, and provides a causal mechanism for producing cosmological perturbations and microwave background anisotropies based on the assumption that all fluctuation modes start our in their vacuum state inside the Hubble radius at early times [2]. It can be shown that the adiabatic curvature fluctuations in a phase of ekpyrotic contraction retain a nearly vacuum spectrum [35] in spite of the fact that the spectrum of fluctuations of the scalar field φ obtains a scale-invariant spectrum [36]. The effect of the S-brane on the cosmological perturbations and gravitational waves converts initial vacuum fluctuations before the bounce to scale-invariant ones after the bounce. Since spatial curvature is not important in the early universe we will set it to zero
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