Ekpyrotic collapse with multiple fields

Abstract

A scale-invariant spectrum of isocurvature perturbations is generated during collapse in thescaling solution in models where two or more fields have steep negative exponentialpotentials. The scale invariance of the spectrum is realized by a tachyonic instability in theisocurvature field. We show that this instability is due to the fact that the scaling solutionis a saddle point in the phase space. The late-time attractor is identified with asingle-field-dominated ekpyrotic collapse in which a steep blue spectrum for isocurvatureperturbations is found. Although quantum fluctuations do not necessarily to disrupt theclassical solution, an additional preceding stage is required to establish classicalhomogeneity.