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 μHz with amplitude of h02Ωgw≈7.6×10−11, 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.
Highlights
The last scattering surface situated around zcrit ∼ 1100 represents a boundary beyond which the universe is opaque to the electromagnetic radiation
The Quantum Chromodynamics (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
The NANOGrav collaboration has claimed the detection of a possible stochastic gravitational wave background [6], the authors have emphasized the fact that no evidence for quadrupolar spatial correlations have been seen, a property expected to be present in order the signal be consistent with general relativity
Summary
The last scattering surface situated around zcrit ∼ 1100 represents a boundary beyond which the universe is opaque to the electromagnetic radiation. The NANOGrav collaboration has claimed the detection of a possible stochastic gravitational wave background [6], the authors have emphasized the fact that no evidence for quadrupolar spatial correlations have been seen, a property expected to be present in order the signal be consistent with general relativity. The physical parameters characterizing the transition depend on the adopted equation of state (EoS), which affects the properties of the gravitational waves generated during the process It was suggested by [9] that the NANOGrav signal could be explained in terms of a stochastic gravitational wave background generated during the QCD transition. The paper is organized as follows: in Section 2 the various equations of state and the corresponding parameters of the QCD transition will be examined; in Section 3 a selfconsistent method to estimate the duration of the transition will be presented; in Section 4 the gravitational wave spectrum resulting from each EoS will be discussed and in Section 5 our main conclusions will be summarized
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