A possible signature of cosmic neutrino decoupling in the nHz region of the spectrum of primordial gravitational waves

Abstract

. A possible signature of cosmic neutrino decoupling in the nHz region of the spectrum of primordial gravitational waves. Classical and Quantum Gravity, IOP Publishing, 2010, 27 (19) Abstract. In this paper we study the effect of cosmic neutrino decoupling on the spectrum of cosmological gravitational waves (GWs). At temperatures T 1 MeV, neutrinos constitute a perfect fluid and do not hinder GW propagation, while for T 1 MeV they free-stream and have an effective viscosity that damps cosmological GWs by a constant amount. In the intermediate regime, corresponding to neutrino decoupling, the damping is frequency-dependent. GWs entering the horizon during neutrino decoupling have a frequency f ∼ 1 nHz, corresponding to a frequency region that will be probed by Pulsar Timing Arrays (PTAs). In particular, we show how neutrino decoupling induces a spectral feature in the spectrum of cosmological GWs just below 1 nHz. We briefly discuss the conditions for a detection of this feature and conclude that it is unlikely to be observed by PTAs.

The scalar, vector, and tensor modes in gravitational wave turbulence simulations

“…In addition there might be parity violating (or CP violating) processes in the early universe that lead to the generation of helical magnetic fields, or helical turbulent motions, and thus produce circularly polarized GWs [32]. In particular, the detection of circularly polarized GWs will shed light on phenomena of fundamental symmetry § We discard the damping due to neutrino free streaming [2][3][4][5][6][7][8] or from anisotropic stresses [9] The commonly discussed sources of GWs at the nHz regime are mergers of supermassive black holes [19,27,28]. breaking in the early universe, such as parity violation, and potentially can serve as an explanation of the lepto-and baryogenesis problem (see Refs.…”

Reproducing neutrino effects on the matter power spectrum through a degenerate Fermi gas approach

“…In this case, the same arguments that one could use for disregarding the contribution from σ to the evolution of the perturbations for ultra-relativistic and non-relativistic are analogously maintained. In case of cosmic neutrinos, one certainly should not discard the contribution due to the anisotropic stress (multipole l = 2) in case of analyzing CMB and CνB anisotropies due to scalar perturbations and the CνB coupling to gravitational waves due to tensorial perturbations [22,23]. Evidently, that was not the case of the analysis that we have performed.…”

Analytic description of the damping of gravitational waves by free streaming neutrinos

“…[10], [11], [12], [13]). The present approach represents a trade off between the use of reliable sophisticated numerical techniques to calculate χ(s, Q) and the necessity of evaluating the exact series solution for χ(s, Q) to sufficient accuracy.…”