Warm Ekpyrosis
  We propose a mechanism to generate a nearly scale-invariant spectrum of
adiabatic scalar perturbations about a stable, ekpyrotic background. The key
ingredient is a coupling between a single ekpyrotic field and a perfect fluid
of ultra-relativistic matter. This coupling introduces a friction term into the
equation of motion for the field, opposing the Hubble anti-friction, which can
be chosen such that an exactly scale-invariant (or nearly scale-invariant)
spectrum of adiabatic density perturbations is continuously produced throughout
the ekpyrotic phase. This mechanism eliminates the need for a second (entropic)
scalar field and hence any need for introducing a second phase for converting
entropic into curvature fluctuations. It also reduces the constraints on the
equation of state during the ekpyrotic phase and, thereby, the need for
parametric fine-tuning.
