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Relying only on the standard model of elementary particles and gravity, westudy the details of a new source of gravitational waves whose origin is inquantum physics. Namely, it is well known that massless fields in curvedbackgrounds suffer from the so-called "trace anomaly". This anomaly can be castin terms of new scalar degrees of freedom which take account of macroscopiceffects of quantum matter in gravitational fields. The linearized effectiveaction for these fields describes scalar (as opposed to transverse)gravitational waves, which are absent in Einstein's theory. Since these newdegrees of freedom couple directly to the gauge field scalars in QCD, the epochof the QCD phase transition in early universe is a possible source ofprimordial cosmological gravitational radiation. While the anomaly is mostlikely fully unsuppressed at the QCD densities (temperature is much higher thanthe u and d quark masses), just to be careful we introduced the window functionwhich cuts-off very low frequencies where the anomaly effect might besuppressed. We then calculated the characteristic strain of the properlyadjusted gravitational waves signal today. The region of the parameter spacewith no window function gives a stronger signal, and both the strain and thefrequencies fall within the sensitivity of the near future gravitational waveexperiments (e.g. LISA and The Big Bang Observer). The possibility that one canstudy quantum physics with gravitational wave astronomy even in principle isexciting, and will be of value for future endeavors in this field.

Primordial scalar gravitational waves produced at the QCD phase transition due to the trace anomaly