Tilted Ekpyrosis
  We consider a simple model of cosmological collapse driven by canonical
fields with exponential potentials. We generalise the two-field ekpyrotic
collapse to consider non-orthogonal or tilted potentials and give the general
condition for isocurvature field fluctuations to have a scale-invariant
spectrum in this model. In particular we show that tilted potentials allow for
a slightly red spectrum of perturbations as required by current observations.
However a red spectrum of fluctuations implies that the two-field ekpyrotic
phase must have a finite duration and requires a preceding phase which sets the
initial conditions for what otherwise appears to be a fine-tuned trajectory in
the phase space.
