Non-singular big-bounces and evolution of linear fluctuations

Abstract

We consider evolutions of linear fluctuations as the background Friedmann world model goes from contracting to expanding phases through smooth and non-singular bouncing phases. As long as the gravity dominates over the pressure gradient in the perturbation equation the growing-mode in the expanding phase is characterized by a conserved amplitude, we call it a C-mode. In the spherical geometry with a pressureless medium, we show that there exists a special gauge-invariant combination Φwhich stays constant throughout the evolution from the big-bang to the big-crunch with the same value even after the bounce: it characterizes the coefficient of the C-mode. We show this result by using a bounce model where the pressure gradient term is negligible during the bounce; this requires additional presence of an exotic matter. In such a bounce, even in more general situations of the equation of states before and after the bounce, the C-mode in the expanding phase is affected only by the C-mode in the contracting phase, thus the growing mode in the contracting phase decays away as the world model enters expanding phase. In the case the background curvature has significant role during the bounce, the pressure gradient term becomes important and we cannot trace C-mode in the expanding phase to the one before the bounce. In such situations, perturbations in a fluid bounce model show exponential instability, whereas the ones in a scalar field bounce model show oscillatory behaviors.