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arXiv 1012.4794astro-ph.CO
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原始磁场大尺度尾的演化

The Evolution of the Large-Scale Tail of Primordial Magnetic Fields

Karsten Jedamzik, Guenter Sigl

摘要:

宇宙磁场可能在早期宇宙相变(例如QCD或电弱相变)期间产生。在当前时期,这些场的剩余部分的大小主要取决于它们(最初是超哈勃的)大尺度尾部的指数$n$,即B(lambda)~ lambda^(-n)。有人声称因果关系要求n=5/2,与更早的n=3/2的主张相反。在这里,我们详细分析这个问题。首先,我们注意到,与目前的信念相反,大规模的磁场尾部不是建立在相变本身,而是continuoulsy演变到目前的时代。忽略湍流,我们发现n=7/2,即非常强的抑制大尺度场。然而,在不可避免的存在的湍流,我们发现,大规模的磁场尾部有足够的时间来发展的流体湍流。对于白噪声流体湍流,这产生了n=3/2,直到某个尺度,并且对于磁场频谱,n=5/2。当考虑到原始粘度和流体流动耗散时,这幅图也没有改变。由此看来,源自宇宙相变的可感知的原始磁场似乎是可能的。

英文摘要:

Cosmic magnetic fields may be generated during early cosmic phase transition, such as the QCD- or electroweak- transitions. The magnitude of the remainder of such fields at the present epoch crucially depends on the exponent $n$ of their (initially super-Hubble) large-scale tail, i.e. B(lambda) ~ lambda^(-n). It has been claimed that causality requires n=5/2, contrary to much earlier claims of n=3/2. Here we analyze this question in detail. First, we note that contrary to current belief, the large-scale magnetic field tail is not established at the phase transition itself, but rather continuoulsy evolves up to the present epoch. Neglecting turbulent flows we find n=7/2, i.e. very strongly suppressed large-scale fields. However, in the inevitable presence of turbulent flows we find that the large-scale magnetic field tail has sufficient time to evolve to that of the fluid turbulence. For white noise fluid turbulence this yields n=3/2 up to a certain scale and n=5/2 beyond for the magnetic field spectrum. This picture is also not changed when primordial viscosity and fluid flow dissipation is taken into account. Appreciable primordial magnetic fields originating from cosmic phase transitions seem thus possible.

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