  Relying only on the standard model of elementary particles and gravity, we
study the details of a new source of gravitational waves whose origin is in
quantum physics. Namely, it is well known that massless fields in curved
backgrounds suffer from the so-called "trace anomaly". This anomaly can be cast
in terms of new scalar degrees of freedom which take account of macroscopic
effects of quantum matter in gravitational fields. The linearized effective
action for these fields describes scalar (as opposed to transverse)
gravitational waves, which are absent in Einstein's theory. Since these new
degrees of freedom couple directly to the gauge field scalars in QCD, the epoch
of the QCD phase transition in early universe is a possible source of
primordial cosmological gravitational radiation. While the anomaly is most
likely fully unsuppressed at the QCD densities (temperature is much higher than
the u and d quark masses), just to be careful we introduced the window function
which cuts-off very low frequencies where the anomaly effect might be
suppressed. We then calculated the characteristic strain of the properly
adjusted gravitational waves signal today. The region of the parameter space
with no window function gives a stronger signal, and both the strain and the
frequencies fall within the sensitivity of the near future gravitational wave
experiments (e.g. LISA and The Big Bang Observer). The possibility that one can
study quantum physics with gravitational wave astronomy even in principle is
exciting, and will be of value for future endeavors in this field.
