Imprints of the QCD Phase Transition on the Spectrum of Gravitational Waves

TL;DR

This paper explores how the QCD phase transition affects the relic gravitational wave spectrum, revealing a step feature and the impact of inflationary scenarios on high-frequency modes.

Contribution

It compares different equations of state for the QCD transition, including a crossover and inflation effects, to understand their imprints on gravitational wave spectra.

Findings

QCD transition creates a step in the GW spectrum.

Entropy conservation determines step size in first two scenarios.

Inflation during transition suppresses high-frequency GW modes by ~10^(-12).

Abstract

We have investigated effects of the QCD phase transition on the relic GW spectrum applying several equations of state for the strongly interacting matter: Besides the bag model, which describes a first order transition, we use recent data from lattice calculations featuring a crossover. Finally, we include a short period of inflation during the transition which allows for a first order phase transition at finite baryon density. Our results show that the QCD transition imprints a step into the spectrum of GWs. Within the first two scenarios, entropy conservation leads to a step-size determined by the relativistic degrees of freedom before and after the transition. The inflation of the third scenario much stronger attenuates the high-frequency modes: An inflationary model being consistent with observation entails suppression of the spectral energy density by a factor of ~10^(-12).