Unavoidable shear from quantum fluctuations in contracting cosmologies

Abstract Contracting cosmologies are known to be flawed with a shear instability, where the contribution from anisotropic stress overall energy density grows as $$a^{-6}$$ a - 6 , scale factor. Classically, whether or not this becomes important before bounce depends on its initial value, which can always sufficiently fine tuned make it irrelevant. However, vacuum quantum fluctuations inevitably provide non-vanishing source of stress. In work, we compute minimum amount that is obtained if one assumes vanishes initially, but lets build up. practice, consider massless test scalar field, and describe by means stochastic “inflation” (though here applied contracting phase) formalism. We find that, equation-of-state parameter contraction satisfies $$w>-1/9$$ w > 1 / 9 regardless when phase initiated, time at sizeable Hubble approaches Planck mass (which also expected take place). $$w<-1/9$$ < backreaction much earlier, point contraction.