Decoupling Solution Moduli of Bigravity

A complete classification of exact solutions of ghost-free, massive bigravity is derived which enables the dynamical decoupling of the background, and the foreground metrics. The general decoupling solution space of the two metrics is constructed. Within this branch of the solution space the foreground metric theory becomes general relativity (GR) with an additional effective cosmological constant, and the background metric dynamics is governed by plain GR.