Translating nano-Hertz gravitational wave background into primordial perturbations taking account of the cosmological QCD phase transition

Understand

The evidence of the nano-Hertz stochastic gravitational wave (GW) background is reported by multiple pulsar timing array collaborations.

• While a prominent candidate of the origin is astrophysical from supermassive black hole binaries, alternative models involving GWs induced by primordial curvature perturbations can explain the inferred GW spectrum.
• Serendipitously, the nano-Hertz range coincides with the Hubble scale during the cosmological quantum chromodynamics (QCD) phase transition.
• The influence of the QCD phase transition can modify the spectrum of induced GWs within the nano-Hertz frequency range, necessitating careful analysis.