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

Understand

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.