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arXiv 2005.06449astro-ph.COgr-qchep-th
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具有暴胀均匀磁场的原初磁螺旋度演化

Primordial magnetic helicity evolution with a homogeneous magnetic field from inflation

Axel Brandenburg, Ruth Durrer, Yiwen Huang, Tina Kahniashvili, Sayan Mandal, Shinji Mukohyama

摘要:

受暴胀过程中产生均匀磁场的磁成因的启发,我们研究了两种初始条件下原初等离子体运动的磁流体动力学演化:(i)具有无限关联长度的空间均匀场;(ii)零通量标度不变的统计均匀磁场.在这两种情况下,我们应用,为一个短的初始时间间隔,单色强迫在一定的波数,使相关长度是有限的,但远小于典型的长度尺度的湍流。特别是,我们调查的非螺旋和螺旋磁流体湍流的衰减。我们表明,在均匀磁场的存在下,螺旋和非螺旋小尺度场的衰减可以迅速发生。这是具有完全均匀磁场的系统的特殊性质,有时被认为是对缓慢变化的背景场的局部近似。它永远不会改变,并作为一个强加的磁场。这是在一个统计均匀的磁场的情况下,我们恢复熟悉的衰减特性:一个慢得多的衰减的磁能和相关长度的快速增长,特别是在磁幸福的情况下,形成鲜明对比。结果表明,均匀的磁场,如果在暴胀过程中产生的,应该持续小尺度场的影响下,可能是宇宙中的大尺度磁场的起源。

英文摘要:

Motivated by a scenario of magnetogenesis in which a homogeneous magnetic field is generated during inflation, we study the magnetohydrodynamic evolution of the primordial plasma motions for two kinds of initial conditions -- (i) a spatially homogeneous field with an unlimited correlation length, and (ii) a zero flux scale-invariant statistically homogeneous magnetic field. In both cases, we apply, for a short initial time interval, monochromatic forcing at a certain wave number so that the correlation length is finite, but much smaller than the typical length scale of turbulence. In particular, we investigate the decay of nonhelical and helical hydromagnetic turbulence. We show that, in the presence of a homogeneous magnetic field, the decay of helical and nonhelical small-scale fields can occur rapidly. This is a special property of a system with a perfectly homogeneous magnetic field, which is sometimes considered as a local approximation to a slowly varying background field. It can never change and acts as an imposed magnetic field. This is in a sharp contrast to the case of a statistically homogeneous magnetic field, where we recover familiar decay properties: a much slower decay of magnetic energy and a faster growth of the correlation length, especially in the case with magnetic felicity. The result suggests that a homogeneous magnetic field, if generated during inflation, should persist under the influence of small-scale fields and could be the origin of the large-scale magnetic field in the Universe.

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