Impact of anisotropic stress of free-streaming particles on gravitational waves induced by cosmological density perturbations
Impact of anisotropic stress of free-streaming particles on gravitational waves induced by cosmological density perturbations
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自由流动粒子各向异性应力对宇宙密度扰动引起的引力波的影响
DOI：
10.1103/physrevd.91.024030
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发表时间：
2015
影响因子：
5
通讯作者：
and N.Sugiyama
中科院分区：
文献类型：
--
作者：
S.Saga;K.Ichiki;and N.Sugiyama
引力波（GWs）不可避免地通过与一阶标量扰动的非线性耦合在二阶宇宙学扰动中产生，其存在已被最近的宇宙学观测所证实。到目前为止，二次诱导GWs的演化和谱都是只考虑一阶标量微扰的积源而导出的。本文研究了光子和中微子的纯二阶各向异性应力对GWs演化的影响，这在文献中被忽略了。基于宇宙摄动理论的形式论，我们对爱因斯坦-玻尔兹曼系统进行了全面的处理，以计算具有各向异性应力的GWs谱。我们发现，与没有各向异性应力的情况相比，光子各向异性应力在小尺度上放大了约150%的gw振幅，而中微子各向异性应力在小尺度上抑制了约30%的gw振幅。二阶各向异性应力对波数的变化没有影响。这一结果与线性顺序下的情况形成了明显的对比，在线性顺序下，各向异性应力的影响是在GWs振幅中阻尼的。
Gravitational waves (GWs) are inevitably induced at second order in cosmological perturbations through nonlinear couplings with first-order scalar perturbations, the existence of which is well established by recent cosmological observations. So far, the evolution and the spectrum of the secondary induced GWs have been derived by taking into account the sources of GWs only from the product of first-order scalar perturbations. Here we newly investigate the effects of purely second-order anisotropic stresses of photons and neutrinos on the evolution of GWs, which have been omitted in the literature. We present a full treatment of the Einstein–Boltzmann system to calculate the spectrum of GWs with anisotropic stress based on the formalism of the cosmological perturbation theory. We find that photon anisotropic stress amplifies the amplitude of GWs by about 150%, whereas neutrino anisotropic stress suppresses that of GWs by about 30% on small scalescompared to the case without anisotropic stress. The second-order anisotropic stress does not affect GWs with wave numbers. The result is in marked contrast with the case at linear order, where the effect of anisotropic stress is damping in the amplitude of GWs.