  As a low energy effective field theory, classical General Relativity receives
an infrared relevant modification from the conformal trace anomaly of the
energy-momentum tensor of massless, or nearly massless, quantum fields. The
local form of the effective action associated with the trace anomaly is
expressed in terms of a dynamical scalar field that couples to the conformal
factor of the spacetime metric, allowing it to propagate over macroscopic
distances. Linearized around flat spacetime, this semi-classical EFT admits
scalar gravitational wave solutions in addition to the transversely polarized
tensor waves of the classical Einstein theory. The amplitude of the scalar wave
modes, as well as their energy and energy flux which are positive and contain a
monopole moment, are computed. Astrophysical sources for scalar gravitational
waves are considered, with the excited gluonic condensates in the interiors of
neutron stars in merger events with other compact objects likely to provide the
strongest burst signals.
