  The trace anomaly of conformal matter implies the existence of massless
scalar poles in physical amplitudes involving the stress-energy tensor. These
poles may be described by a local effective action with massless scalar fields,
which couple to classical sources, contribute to gravitational scattering
processes, and can have long range gravitational effects at macroscopic scales.
In an effective field theory approach, the effective action of the anomaly is
an infrared relevant term that should be added to the Einstein-Hilbert action
of classical General Relativity to take account of macroscopic quantum effects.
The additional scalar degrees of freedom contained in this effective action may
be understood as responsible for both the Casimir effect in flat spacetime and
large quantum backreaction effects at the horizon scale of cosmological
spacetimes. These effects of the trace anomaly imply that the cosmological
vacuum energy is dynamical, and its value depends on macroscopic boundary
conditions at the cosmological horizon scale, rather than sensitivity to the
extreme ultraviolet Planck scale.
