Induced gravitational waves as a cosmological probe of the sound speed during the QCD phase transition

Understand

The standard model of particle physics is known to be intriguingly successful.

• However their rich phenomena represented by the phase transitions (PTs) have not been completely understood yet, including the possibility of the existence of unknown dark sectors.
• In this Letter, we investigate the measurement of the equation of state parameter $w$ and the sound speed $c_{\rm s}$ of the PT plasma with use of the gravitational waves (GWs) of the universe.
• Though the propagation of GW is insensitive to $c_{\rm s}$ in itself, the sound speed value affects the dynamics of primordial density (or scalar curvature) perturbations and the induced GW by their horizon reentry can then be an indirect probe both $w$ and $c_{\rm s}$.