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An old question surrounding bouncing models concerns their stability undervector perturbations. Considering perfect fluids or scalar fields, vectorperturbations evolve kinematically as $a^{-2}$, where $a$ is the scale factor.Consequently, a definite answer concerning the bounce stability depends on anarbitrary constant, therefore, there is no definitive answer. In this paper, weconsider a more general situation where the primeval material medium is anon-ideal fluid, and its shear viscosity is capable of producing torqueoscillations, which can create and dynamically sustain vector perturbationsalong cosmic evolution. In this framework, one can set that vectorperturbations have a quantum mechanical origin, coming from quantum vacuumfluctuations in the far past of the bouncing model, as it is done with scalarand tensor perturbations. Under this prescription, one can calculate theirevolution during the whole history of the bouncing model, and precisely inferthe conditions under which they remain linear before the expanding phase. It isshown that such linearity conditions impose constraints on the free parametersof bouncing models, which are mild, although not trivial, allowing a largeclass of possibilities. Such conditions impose that vector perturbations arealso not observationally relevant in the expanding phase. The conclusion isthat bouncing models are generally stable under vector perturbations. As theyare also stable under scalar and tensor perturbations, we conclude thatbouncing models are generally stable under perturbations originated fromquantum vacuum perturbations in the far past of their contracting phase.

Vector perturbations in bouncing cosmology