论文标题

关于不同时期宇宙学散装粘度的评论

Remarks on cosmological bulk viscosity in different epochs

论文作者

Brevik, Iver, Normann, Ben David

论文摘要

本文的意图主要是两个方面。 \ textit {first},我们指出了由Husdal(2016)计算的Lepton ERA中的散装粘度与我们对当今的体积粘度计算时的数值一致性,当时功能形式为$ζ\,\ sim \ sim \ sim \ sim \sqrtρ$。因此,从现象学的角度来看,我们似乎拥有一个粘度的安萨兹,它可以将宇宙的婴儿($ \ sim 1 $ s)与现在桥接起来。这也可以看作是早期宇宙学与当今宇宙学之间的一种对称性:非常值得注意的是,早期宇宙中基于动力学理论的散装粘度和当前宇宙中基于实验的散装粘度可以由相同的简单分析公式覆盖。 \ textit {second},我们将Kasner宇宙视为Bianchi-Type I的典型各向异性模型,研究了该几何模型是否与流体中恒定的粘度系数兼容。也许令人惊讶的是,剪切粘度的存在与Kasner模型不相容。相比之下,在模型的{\ is istropic}版本中,散装粘度是无问题的。在Zel'Dovich(刚性)流体的特殊情况下,Kasner度量中的三个相等指数甚至由单独的大量粘度决定,而与流体能量密度的值无关。我们还与其他一些粘性宇宙学方法进行了简短的比较。

The intention of this paper is mainly two-fold. \textit{First}, we point out a striking numerical agreement between the bulk viscosity in the lepton era calculated by Husdal (2016) and our own calculations of the present-day bulk viscosity when the functional form is $ζ\,\sim\sqrtρ$. From a phenomenological point of view, we thus seem to have an ansatz for the viscosity which bridges the infancy of the Universe ($\sim 1$ s) with the present. This can also be looked upon as a kind of symmetry between the early-time cosmology and the present-day cosmology: it is quite remarkable that the kinetic theory-based bulk viscosity in the early universe and the experimentally-based bulk viscosity in the present universe can be covered by the same simple analytical formula. \textit{Second}, we consider the Kasner universe as a typical anisotropic model of Bianchi-type I, investigating whether this geometrical model is compatible with constant viscosity coefficients in the fluid. Perhaps surprisingly, the existence of a shear viscosity turns out to be incompatible with the Kasner model. By contrast, a bulk viscosity is non-problematic in the {\it isotropic} version of the model. In the special case of a Zel'dovich (stiff) fluid, the three equal exponents in the Kasner metric are even determined by the bulk viscosity alone, independent of the value of the fluid energy density. We also give a brief comparison with some other recent approaches to viscous cosmology.

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