Vector perturbations in bouncing cosmology

Understand

An old question surrounding bouncing models concerns their stability under vector perturbations.

• Considering perfect fluids or scalar fields, vector perturbations evolve kinematically as $a^{-2}$, where $a$ is the scale factor.
• Consequently, a definite answer concerning the bounce stability depends on an arbitrary constant, therefore, there is no definitive answer.
• In this paper, we consider a more general situation where the primeval material medium is a non-ideal fluid, and its shear viscosity is capable of producing torque oscillations, which can create and dynamically sustain vector perturbations along cosmic evolution.