Scale-invariance in expanding and contracting universes from two-field models

Abstract

We study cosmological perturbations produced by the most general two-derivative actions involving two scalar fields, coupled to Einstein gravity, with an arbitrary field space metric, that admit scaling solutions. For contracting universes, we show that scale-invariant adiabatic perturbations can be produced continuously as modes leave the horizon for any equation of state parameter $w\ge 0$. The corresponding background solutions are unstable, which we argue is a universal feature of contracting models that yield scale-invariant spectra. For expanding universes, we find that nearly scale-invariant adiabatic perturbation spectra can only be produced for $w\approx -1$, and that the corresponding scaling solutions are attractors. The presence of a nontrivial metric on field space is a crucial ingredient in our results.

23 pages, oversight in perturbations calculation corrected, conclusions for expanding models modified

References in corpus (4)

• New Ekpyrotic Cosmology
• Generating Ekpyrotic Curvature Perturbations Before the Big Bang
• Solution of a Braneworld Big Crunch/Big Bang Cosmology
• Scaling solutions and geodesics in moduli space

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