翻转场揭示的宇宙网络特征
The features of the Cosmic Web unveiled by the flip-flop field
摘要：
只有在暗物质的理论框架中才能理解宇宙中观察到的结构。N体模拟对于分析暗物质网的形成和演化是必不可少的。在大多数研究中，使用了两个基本场-密度场和速度场。然而，暗物质提供了两个额外的领域，这是唯一的无碰撞介质。它们是欧拉空间中的多流场和拉格朗日空间中的触发场。触发器字段表示每个无碰撞流体元素的基本体积的符号反转的数量。这个场可以通过在模拟的每个时间步长对每个粒子处的雅可比矩阵的符号反转进行计数来估计。雅可比矩阵通过拉格朗日子流形的数值微分来计算，即，由三个拉格朗日坐标和三个欧拉坐标构成的六维空间中的三维暗物质层。我们提出了从z = 50到现在的时间z = 0的翻转场的演变的统计研究的结果。一些统计特征表明，从z = 30到现在，触发场的模式保持非常稳定。因此，在z = 0时计算的翻转场存储了大量关于暗物质网的动力学历史的信息。特别是，触发器最有趣的特性之一是保存暗物质晕合并历史信息的独特能力。
英文摘要：
Understanding of the observed structure in the universe can be reached only in the theoretical framework of dark matter. N-body simulations are indispensable for the analysis of the formation and evolution of the dark matter web. Two primary fields - density and velocity fields - are used in most of studies. However dark matter provides two additional fields which are unique for collisionless media only. These are the multi- stream field in Eulerian space and flip-flop field in Lagrangian space. The flip-flop field represents the number of sign reversals of an elementary volume of each collisionless fluid element. This field can be estimated by counting the sign reversals of the Jacobian at each particle at every time step of the simulation. The Jacobian is evaluated by numerical differentiation of the Lagrangian submanifold, i.e., the three-dimensional dark matter sheet in the six-dimensional space formed by three Lagrangian and three Eulerian coordinates. We present the results of the statistical study of the evolution of the flip-flop field from z = 50 to the present time z = 0. A number of statistical characteristics show that the pattern of the flip-flop field remains remarkably stable from z = 30 to the present time. As a result the flip-flop field evaluated at z = 0 stores a wealth of information about the dynamical history of the dark matter web. In particular one of the most intriguing properties of the flip-flop is a unique capability to preserve the information about the merging history of dark matter haloes.
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