New method to constrain the relativistic free-streaming gas in the Universe

Zhao, Wen; Zhang, Yang; Xia, Tianyang

doi:10.1016/j.physletb.2009.05.046

Cited by 17 publications

(9 citation statements)

References 46 publications

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“…For easy reference, we list these projects in Table III For the known damping factor, if r and n t are given, the spectrum (8) is determined. Then, we use (18) to find the characteristic strain h c (f ) and obtain P g (∆ ′ ) through (30) for known PTA observations. Finally, we can calculate the SNR for these observations through (29).…”

Section: A Potential Pta Projects In the Futurementioning

confidence: 99%

“…In all these previous works, a simple evolution model is used to calculate the RGW power spectrum, where the effects of free-streaming fluids and various cosmic phase transitions are neglected. However, it was known that, the relativistic free-streaming gas (such as the neutrinos in the Universe) gives rise to an anisotropic term in the evolution equation of gravitational waves, which can significantly damp the RGW spectra at f > 10 −16 Hz [29,30]. Another effect is caused by the successive changes in the relativistic degrees of freedom during the radiation-dominant stage, i.e.…”

Section: Introductionmentioning

confidence: 99%

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Detecting relic gravitational waves by pulsar timing arrays: Effects of cosmic phase transitions and relativistic free-streaming gases

et al. 2016

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Relic gravitational waves (RGWs) generated in the early Universe form a stochastic GW background, which can be directly probed by measuring the timing residuals of millisecond pulsars. In this paper, we investigate the constraints on the RGWs and on the inflationary parameters by the observations of current and potential future pulsar timing arrays. In particular, we focus on effects of various cosmic phase transitions (e.g. e + e − annihilation, QCD transition and SUSY breaking) and relativistic free-streaming gases (neutrinos and dark fluids) in the general scenario of the early Universe, which have been neglected in the previous works. We find that the phase transitions can significantly damp the RGWs in the sensitive frequency range of pulsar timing arrays, and the upper limits of tensor-to-scalar ratio r increase by a factor ∼ 2 for both current and future observations. However, the effects of free-steaming neutrinos and dark fluids are all too small to be detected. Meanwhile, we find that, if the effective equation of state w in the early Universe is larger than 1/3, i.e. deviating from the standard hot big bang scenario, the detection of RGWs by pulsar timing arrays becomes much more promising.

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Section: A Potential Pta Projects In the Futurementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Detecting relic gravitational waves by pulsar timing arrays: Effects of cosmic phase transitions and relativistic free-streaming gases

et al. 2016

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“…However, GWs are sourced by the anisotropic stress part of the energy-momentum tensor of matter, so that the vacuum approximation is well-motivated only when this can be neglected. It is already known that the anisotropic stress of free streaming neutrinos acts as an effective viscosity, absorbing gravitational waves in the low frequency region, thus resulting in a damping of the B-modes of CMB [34][35][36][37][38].…”

Section: Introductionmentioning

confidence: 99%

Signatures of the neutrino thermal history in the spectrum of primordial gravitational waves

2010

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In this paper we study the effect of the anisotropic stress generated by neutrinos on the propagation of primordial cosmological gravitational waves. The presence of anisotropic stress, like the one generated by free-streaming neutrinos, partially absorbs the gravitational waves (GWs) propagating across the Universe. We find that in the standard case of three neutrino families, 22% of the intensity of the wave is absorbed, in fair agreement with previous studies. We have also calculated the maximum possible amount of damping, corresponding to the case of a flat Universe completely dominated by ultrarelativistic collisionless particles. In this case 43% of the intensity of the wave is absorbed. Finally, we have taken into account the effect of collisions, using a simple form for the collision term parameterized by the mean time between interactions, that allows to go smoothly from the case of a tightly coupled fluid to that of a collisionless gas. The dependence of the absorption on the neutrino energy density and on the effectiveness of the interactions opens the interesting possibility of observing spectral features related to particular events in the thermal history of the Universe, like neutrino decoupling and electron-positron annihilation, both occurring at T similar to 1 MeV. GWS entering the horizon at that time will have today a frequency nu similar to 10(-9) Hz, a region that is going to be probed by Pulsar Timing Arrays

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“…However, GWs are sourced by the anisotropic stress part of the energy-momentum tensor of matter, so that the above assumption is well-motivated only when anisotropic stress can be neglected. It is already known that the anisotropic stress of free streaming neutrinos acts as an effective viscosity, absorbing gravitational waves in the low frequency region, thus resulting in a damping of the B-modes of CMB [12,13,14,15,16]. ‡…”

Section: Introductionmentioning

confidence: 99%

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

Lattanzi¹,

Benini²,

Montani³

2010

Class. Quantum Grav.

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. 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.

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New method to constrain the relativistic free-streaming gas in the Universe

Cited by 17 publications

References 46 publications

Detecting relic gravitational waves by pulsar timing arrays: Effects of cosmic phase transitions and relativistic free-streaming gases

Detecting relic gravitational waves by pulsar timing arrays: Effects of cosmic phase transitions and relativistic free-streaming gases

Signatures of the neutrino thermal history in the spectrum of primordial gravitational waves

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

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