Scale Invariance via a Phase of Slow Expansion
  We consider a cosmological scenario in which a scale-invariant spectrum of
curvature perturbations is generated by a rapidly-evolving equation of state on
a slowly expanding background. This scenario generalizes the "adiabatic
ekpyrotic" mechanism proposed recently in arXiv:0910.2230. Whereas the original
proposal assumed a slowly contracting background, the present work shows that
the mechanism works equally well on an expanding background. This greatly
expands the realm of broader cosmological scenarios in which this mechanism can
be embedded. We present a phase space analysis and show that both the expanding
and contracting versions of the scenario are dynamical attractors, with the
expanding branch having a broader basin of attraction. In both cases, a finite
range of scale invariant modes can be generated within the regime of validity
of perturbation theory.
