Gravitational Waves from the Cosmological Quark-Hadron Phase Transition Revisited

TL;DR

This study calculates the gravitational wave spectrum from the QCD phase transition using lattice-based equations of state, finding it unlikely to explain NANOGrav signals but potentially detectable by future space interferometers.

Contribution

It provides a self-consistent calculation of the gravitational wave spectrum from the QCD transition with lattice-based equations of state, differing from previous studies.

Findings

The gravitational wave peak is around 0.28 μHz, insufficient to explain NANOGrav.

The QCD gravitational wave background could be detected by the Big Bang Observer above 1 mHz.

The transition duration was estimated self-consistently with cosmological dynamics.

Abstract

The recent claim by the NANOGrav collaboration of a possible detection of an isotropic gravitational wave background stimulated a series of investigations searching for the origin of such a signal. The QCD phase transition appears as a natural candidate and in this paper the gravitational spectrum generated during the conversion of quarks into hadrons is calculated. Here, contrary to recent studies, equations of state for the quark-gluon plasma issued from the lattice approach were adopted. The duration of the transition, an important parameter affecting the amplitude of the gravitational wave spectrum, was estimated self-consistently with the dynamics of the universe controlled by the Einstein equations. The gravitational signal generated during the transition peaks around 0,28 \mu Hz, being unable to explain the claimed NANOGrav signal. However, the expected QCD gravitational wave background could be detected by the planned spatial interferometer Big Bang Observer in its advanced version for frequencies above 1.0 mHz. This possible detection assumes that algorithms recently proposed will be able to disentangle the cosmological signal from that expected for the astrophysical background generated by black hole binaries.