  We propose that the solution to the cosmological vacuum energy puzzle may
come from the infrared sector of the effective theory of gravity, where the
impact of the trace anomaly is of upmost relevance. We proceed by introducing
two auxiliary fields, which are capable of describing a diversity of quantum
states via specification of their macroscopic (IR) boundary conditions, in
contrast to ultraviolet quantum effects. Our investigation aims at finding a
realistic cosmological solution which interprets the observed cosmological
constant as a well defined deficit in the zero point energy density of the
Universe. The energy density arises from a phase transition, which alters the
properties of the quantum ground state. We explicitly formulate low energy
gravity as an effective field theory with a precise definition of the "point of
normalization" as the point at which the "renormalized cosmological constant"
is set to zero in the Minkowski vacuum, in which the Einstein equations are
automatically satisfied as the Ricci tensor identically vanishes. With this
definition the effective QFT of gravity has a predictive power. In particular,
it must predict the evolution of the system in any nontrivial geometry,
including the vacuum energy behaviour as a function of infrared, rather than
ultraviolet, input parameters.
