Adiabatic and entropy perturbations propagation in a bouncing Universe

TL;DR

This paper demonstrates that in bouncing universe models with hydrodynamical fluids, primordial perturbations can pass smoothly through the bounce, highlighting the importance of entropy modes especially when the null energy condition is violated.

Contribution

It shows that entropy perturbations are essential in bouncing cosmologies and that adiabatic modes alone are insufficient and can be misleading during the bounce or NEC violation.

Findings

Primordial perturbations propagate smoothly through the bounce.

Entropy modes cannot be neglected in NEC violation scenarios.

Adiabatic modes exhibit fake divergences without entropy contributions.

Abstract

By studying some bouncing universe models dominated by a specific class of hydrodynamical fluids, we show that the primordial cosmological perturbations may propagate smoothly through a general relativistic bounce. We also find that the purely adiabatic modes, although almost always fruitfully investigated in all other contexts in cosmology, are meaningless in the bounce or null energy condition (NEC) violation cases since the entropy modes can never be neglected in these situations: the adiabatic modes exhibit a fake divergence that is compensated in the total Bardeen gravitational potential by inclusion of the entropy perturbations.