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arXiv 1306.5634gr-qc
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具有负体粘滞系数的宇宙模型

Universe Models with Negative Bulk Viscosity

Iver Brevik, Øyvind Grøn

备注：
7 pages latex, no figures, published electronically in Astrophys. Space Sci
发表信息：
Astrophys. Space Sci. (2013) 347, 399
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摘要：

负温度的概念有时也被用于量子系统。最近的一个例子是在S. Braun等人[Science 339，52（2013）]，其中通过实验研究了超冷原子的吸引相互作用系综，并发现其对应于负温度系统，因为熵随着能谱高端能量的增加而减小。正如作者所建议的那样，研究标准宇宙学理论的适当推广是否有帮助，以阐明通常用正张应力（负压）解释的宇宙加速膨胀，这将是有趣的。在本说明中，我们采取这一基本思想，并研究了标准粘性宇宙学理论的推广，不是通过承认负温度，而是通过让体粘滞度取负值。显然，这种方法打破了标准热力学，但实际上可能被认为会导致与允许状态方程参数w小于-1的标准方法相同的奇怪结果。在负粘性主导的宇宙模型中，我们发现流体的熵随时间而减小，正如人们所期望的那样。此外，我们发现，从精华区到幻影区的流体过渡（从而通过幻影划分w=-1）实际上可以逆转。此外，在具有负体积粘度的流体的LCDM宇宙模型的推广中，我们发现粘度降低了宇宙的膨胀。

英文摘要：

The concept of negative temperatures has occasionally been used in connection with quantum systems. A recent example of this sort is reported in the paper of S. Braun et al. [Science 339,52 (2013)], where an attractively interacting ensemble of ultracold atoms is investigated experimentally and found to correspond to a negative-temperature system since the entropy decreases with increasing energy at the high end of the energy spectrum. As the authors suggest, it would be of interest to investigate whether a suitable generalization of standard cosmological theory could be helpful, in order to elucidate the observed accelerated expansion of the universe usually explained in terms of a positive tensile stress (negative pressure). In the present note we take up this basic idea and investigate a generalization of the standard viscous cosmological theory, not by admitting negative temperatures but instead by letting the bulk viscosity take negative values. Evidently, such an approach breaks standard thermodynamics, but may actually be regarded to lead to the same kind of bizarre consequences as the standard approach of admitting the equation-of-state parameter w to be less than -1. In universe models dominated by negative viscosity we find that the fluid's entropy decreases with time, as one would expect. Moreover, we find that the fluid transition from the quintessence region into the phantom region (thus passing the phantom divide w=-1) can actually be reversed. Also in generalizations of the LCDM-universe models with a fluid having negative bulk viscosity we find that the viscosity decreases the expansion of the universe.

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