  This is a pedagogical guide to works on this subject which began in the 80s
but has seen vibrant activities in the last decade. It aims to help orient
readers, especially students, who wish to enter into research but bewildered by
the vast and diverse literature on this subject. We describe the three main
veins of activities: the Euclidean zero-mode dominance, the Lorentzian
interacting quantum field theory and the classical stochastic field theory
approaches in some detail, explaining the underlying physics and the
technicalities of each. We show how these approaches are interconnected, and
highlight recent papers which contain germs of worthy directions for future
developments.
