Do active liquid crystals exist after all?

Quasi-long ranged order is the hallmark of two-dimensional liquid crystals. At equilibrium, this property implies that the correlation function of the local orientational order parameter decays with distance as a power law: i.e. C(r) ∼ |r|^–η, with η a temperature-dependent exponent. While in general non-universal, η = 1/4 universally at the Kosterlitz-Thouless transition, where orientational order is lost due to disclination unbinding. Does this definition of liquid crystal order in two dimensions also apply to active liquid crystals? And if not, are these more or less ordered than their passive counterpart? In this talk, I will share the outcome of a survey of experimental data and I will discuss it in the light of recent numerical simulations and analytical work.