RENORMALIZATION GROUP IMPROVING THE EFFECTIVE ACTION: A REVIEW

The existence of fluctuations together with interactions leads to scale-dependence, in the couplings of quantum field theories for the case of quantum fluctuations, and in the couplings of stochastic systems when the fluctuations are of a thermal or statistical nature. In both cases the effects of these fluctuations can be accounted for by solutions of the corresponding renormalization group equations. In this review, we show how the renormalization group equations are intimately connected with the effective action: given the effective action we can trivially extract the renormalization group equations; given the renormalization group equations the effects of these fluctuations can be included in the classical action by using what is known as improved perturbation theory (wherein the bare parameters appearing in tree-level expressions are replaced by their scale-dependent running forms). The improved action can then be used to reconstruct the effective action, up to finite renormalizations, and up to gradient terms.