Group velocity of gravitational waves in an expanding universe

The group velocity of gravitational waves in a flat Friedman–Robertson–Walker universe is investigated. For plane waves with wavelength well inside the horizon, and a universe filled with an ideal fluid with the pressure to density ratio less than 1/3, the group velocity is greater than the velocity of light. As a result, a planar pulse of gravitational waves propagating through the universe during the matter/dark energy dominated era arrives to the observer with the peak shifted towards the forefront. For gravitational waves emitted by inspiralling supermassive black holes at the edge of the observable universe, the typical shift that remains after the effects of nonplanarity are suppressed is of order of 10 ps.     

宇宙起源与阿里原初引力波探测

20世纪建立起的宇宙大爆炸模型取得了巨大的成功,但仍期待着新的革命性的突破。探知宇宙起源及其演化是新世纪对全世界科学家的新挑战,其重要性在近期公布的中国“十三五”规划纲要中得到了高度的肯定。现代宇宙学理论,暴胀以及非奇异宇宙模型如反弹等,预言了原初引力波的存在,但至今还没有被实验证实。不同于近期LIGO合作组探测到的黑洞引力波,原初引力波是宇宙诞生时期产生的,携带着丰富的宇宙学信息,是引力波探测的全新波段,是引力波探测的下一个突破方向。文章简述了中国的阿里原初引力波实验计划及相关的科学问题。     

Probing the origin of our universe through primordial gravitational waves by Ali CMB project

正Gravitational waves(GW),which were predicted by Einstein in 1916 based on the classical theory of General Relativity(GR),were recently detected by LIGO[1].This breakthrough is expected to initiate a novel probe of cosmology,the nature of gravity as well as fundamental physics.In general,signals of GWs can be classified into two categories,which     

Identifying the inflaton with primordial gravitational waves

We explore the ability of experimental physics to uncover the underlying structure of the gravitational Lagrangian describing inflation. While the observable degeneracy of the inflationary parameter space is large, future measurements of observables beyond the adiabatic and tensor two-point functions, such as non-gaussianity or isocurvature modes, might reduce this degeneracy. We show that, even in the absence of such observables, the range of possible inflaton potentials can be reduced with a precision measurement of the tensor spectral index, as might be possible with a direct detection of primordial gravitational waves.     

Tilt of primordial gravitational wave spectrum in a universe with sterile neutrinos

In this work,we constrain the spectral index ntof the primordial gravitational wave power spectrum in a universe with sterile neutrinos by using the Planck temperature data,the WMAP 9-year polarization data,the baryon acoustic oscillation data,and the BICEP2 data.We call this model theΛCDM+r+νs+ntmodel.The additional massive sterile neutrino species can significantly relieve the tension between the Planck and BICEP2 data,and thus can reduce the possible effects of this tension on the fit results of nt.To constrain the parameters of sterile neutrino,we also utilize the Hubble constant direct measurement data,the Planck Sunyaev-Zeldovich cluster counts data,the Planck CMB lensing data,and the cosmic shear data.We find that due to the fact that the BICEP2 data are most sensitive to the multipole～150 corresponding to k～0.01 Mpc-1,there exists a strong anticorrelation between ntand r0.002in the BICEP2 data,and this further results in a strongly blue-tilt spectrum.However,a slightly red-tilt tensor power spectrum is also allowed by the BICEP2 data in the region with larger value of r0.002.By using the full data sets,we obtain meffν,sterile=0.48+0.11-0.13eV,Neff=3.73+0.34-0.37,and nt=0.96+0.48-0.63for theΛCDM+r+νs+ntmodel.     

Polarized cosmological gravitational waves from primordial helical turbulence

We show that helical turbulence produced during a first-order phase transition generates circularly polarized cosmological gravitational waves (GWs). The characteristic frequency of these GWs for an extreme case of the phase transition model is around 10(-3)-10(-2) Hz with an energy density parameter as high as 10(-12)-10(-11). The possibility of detection is briefly discussed.     

Republication of: On the gravitational stability of the expanding universe

This is the republished English edition of a paper by E. Lifshitz, first published in 1946, in which the author investigates the gravitational stability of the non–stationary isotropic models of the universe. The paper has been selected by the Editors of General Relativity and Gravitation for re-publication in the Golden Oldies series of the journal. This republication is accompanied by an editorial note written by George F. R. Ellis, and by a brief biography of Evgenii Mikhailovich Lifshitz, written by Andrzej Krasiński.     

New cosmic microwave background constraint to primordial gravitational waves

Primordial gravitational waves (GWs) with frequencies > or approximately equal to 10(-15) Hz contribute to the radiation density of the Universe at the time of decoupling of the cosmic microwave background (CMB). This affects the CMB and matter power spectra in a manner identical to massless neutrinos, unless the initial density perturbation for the GWs is nonadiabatic, as may occur if such GWs are produced during inflation or some post-inflation phase transition. In either case, current observations provide a constraint to the GW amplitude that competes with that from big-bang nucleosynthesis (BBN), although it extends to much lower frequencies (approximately 10(-15) Hz rather than the approximately 10(-10) Hz from BBN): at 95% confidence level, omega(gw)h(2) 相似文献   

Propagation of transverse electromagnetic waves with gravitational perturbations in an isotropic universe

The combined behavior of gravitational and electromagnetic perturbations in the radiation-dominated plasma of an isotropic universe is considered. It is shown that transverse electromagnetic waves and vector and tensor gravitational perturbations are independent of one another. The propagation of transverse electromagnetic waves during the lepton and radiation-dominated phases is determined. It is shown that the gravitational perturbations help to excite longitudinal electromagnetic fields in the radiation-dominated plasma.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 49–54, December, 1985.     

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

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 ~ 1  MeV. GWS entering the horizon at that time will have today a frequency ν ~ 10⁻⁹ Hz, a region that is going to be probed by Pulsar Timing Arrays.     

Tachyons in an expanding universe

It has been suggested that causality problems associated with tachyons can be eliminated by the existence of a perferred frame of rest in which backward time travel is impossible. Furthermore, cosmology provides a de facto preferred rest frame. However, it is demonstrated here that the observed expansion of the universe raises further problems for the existence of tachyons.     

Low-energy behavior of a quantized scalar field in the linearly expanding universe

We study positive frequency mode solutions of a massive scalar field at an initial stage in the spatially flat, linearly expanding Robertson-Walker universe. The essential point is that the initial time is taken to be small enough, but nonzero (e.g., the Planck time), and the mode is determined by a kind of WKB condition. We calculate the created particle spectrum, and show that its low-energy behavior is considerably different from the previous results.     

Electrodynamics in an expanding universe

An algebraic form of the energy momentum tensor of the electromagnetic field is derived in terms of two scalars and two mutually orthogonal vector fields. Upon inserting this tensor into the field equations, solutions of the co-determined Einstein-Maxwell equations are obtained. The line element used is that corresponding to a conformal flat universe, whose form is then uniquely determined by the field equations. The case of a charged fluid is also considered and it is found that the particular form of the velocity field chosen limits the choice of the possible equation of state connecting the pressure and density distributions.