Collaborative Research: A Comprehensive Theoretical Study of Cosmic Magnetic Fields, their Origin, Evolution, and Signatures

人们用指南针来了解旅行的方向。这样的指南针之所以有效，是因为它的微小磁针与地球磁场对齐，并指示北方的方向。在比地球大得多的尺度上，我们的银河系也有自己的磁场。事实上，宇宙中所有的星系似乎都有磁场，而这些磁场会对星系一生的发展产生深远的影响。通过一系列复杂的计算机模拟，该项目旨在解开星系中磁场令人困惑的起源。这个项目完成后，可能会指出这些磁场的起源与宇宙本身的诞生相吻合。这样的结果可能会对我们目前对基础物理学的理解产生深远的影响，而对它的追求也符合美国在天体物理学领域发展科学领导地位的国家利益。该项目还将通过直接培训普通物理学和计算天体物理学的本科生和研究生来加强美国的科学劳动力。项目计划还包括一项强有力的教育和公众宣传计划。从技术上讲，观测表明，星系的磁场在星系的很大一部分上是相干的，磁场强度为微高斯数量级。这些磁场被认为是由未知性质的初始弱种子磁场放大而来。它们的起源有两种情景：(1)自下而上的天体物理学情景，所需的种子场是在较小的尺度上产生的；(2)自上而下的宇宙学情景，种子场是在早期宇宙中星系形成之前在现在的大尺度上产生的。根据目前的观察，本项目旨在区分这两种情况。为了实现这一目标，将在不同的天体物理环境（如星系团、星系和星际介质）中研究宇宙磁场的演化和观测特征。当初始条件对应于不同的磁发生情景时，数值模拟将模拟宇宙磁场在膨胀宇宙中随时间变化的耗散特性的演化。本项目分为以下相关部分：(i)磁发生机制的理论研究，包括宇宙学和天体物理情景；（ii）大尺度磁场演化的数值模拟，以分析天体物理机制是否可以导致与下限相当的河外磁场强度；（三）确定在大尺度上相干的宇宙磁场的潜在可观测特征——包括宇宙结构形成、微波背景波动或超高能量伽马射线传播。该项目将寻找原始宇宙磁场的特征，以区分宇宙种子和天体物理来源。

People use a compass to learn their direction of travel. Such a compass works because its tiny magnetic needle aligns with the magnetic field of Earth and indicates the direction of North. On scales much larger than Earth, our own Milky Way Galaxy also has its own magnetic field. In fact, all galaxies in the Universe appear to have magnetic fields, and these magnetic fields can have profound effects on a galaxy's development throughout its lifetime. Through a series of sophisticated computer simulations, this project aims to unlock the puzzling origin of the magnetic fields found in galaxies. Upon completion this project might point to an origin of these magnetic fields that coincides with the birth of the Universe itself. Such a result could have a profound influence on our current understanding of fundamental physics, and its pursuit serves the national interest of developing US scientific leadership in astrophysics. This project will also strengthen the US science workforce by directly training undergraduate and graduate students in general and computational astrophysics. Project plans also include a vigorous education and public outreach program.More technically, observations show that galaxies have magnetic fields with a component that is coherent over a large fraction of the galaxy with field strengths of order microgauss. These fields are assumed to be from the amplification of initial weak seed magnetic fields of unknown nature. The two scenarios of their origin are (1) a bottom-up astrophysical one, where the needed seed field is generated on smaller scales and (2) a top-down cosmological scenario where the seed field is generated prior to galaxy formation in the early Universe on scales that are large now. Based on current observations, this project aims to distinguish between these two scenarios. To achieve this goal, the evolution and observational signatures of cosmic magnetic fields will be studied in different astrophysical environments such as clusters, galaxies, and interstellar medium. Numerical simulations will model the evolution of cosmic magnetic fields in the expanding Universe with time varying dissipative properties when initial conditions correspond to the different scenarios of the magnetogenesis. This project is divided into the following related parts: (i) theoretical studies of magnetogenesis mechanisms including the cosmological and astrophysical scenarios; (ii) numerical simulations of large-scale magnetic fields evolution to analyze whether astrophysical mechanisms can lead to extragalactic magnetic field strengths comparable with the lower bounds; and (iii) determining potentially observable signatures of cosmic magnetic fields coherent at large scales---including cosmic structure formation, microwave background fluctuations, or ultra high energy gamma-ray propagation. The project will search for signatures of primordial cosmic magnetic fields that distinguish cosmological seeds from astrophysical sources.