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arXiv 1508.06910gr-qcastro-ph.COhep-th
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宇宙尺度上的电动力学

Electrodynamics on Cosmological Scales

Li-Xin Li

摘要:

麦克斯韦方程组不能描述一个均匀的、各向同性的、具有均匀分布的净电荷的宇宙,因为在这样的宇宙中,电磁场张量必须处处为零。对于具有非零净电荷的封闭宇宙,无论时空对称性和电荷分布如何,麦克斯韦方程组总是不成立。这两个悖论表明,麦克斯韦方程组需要修改,以适用于整个宇宙。我们考虑了两类修正的麦克斯韦方程组,它们都能解决这个悖论。一个是包含光子质量项的Proca型方程。这种电磁场方程可以自然地从自发对称性破缺和量子场论中的希格斯机制中产生,其中光子通过吃掉无质量的戈德斯通玻色子获得质量。然而,当对称性恢复时,光子失去了它们的质量,悖论再次出现。另一类修正的麦克斯韦方程组,在我们看来更有吸引力,它包含一个电磁势矢量与时空曲率张量耦合的项。这类电磁场方程没有引入新的维度参数,并返回到平坦或里奇平坦时空中的麦克斯韦方程。我们表明,曲率耦合项可以自然地产生的模糊性,在扩展麦克斯韦方程组从一个平坦的时空,弯曲的时空通过最小替换规则。研究了修正的麦克斯韦方程组的一些结果。结果表明,在合理的参数下,修正不影响已有的实验和观测。然而,具有曲率耦合项的场方程可以在质量密度高或电磁辐射的重力在动力学中起主导作用的天体物理环境中检验,例如,中子星和早期宇宙的内部。

英文摘要:

Maxwell's equations cannot describe a homogeneous and isotropic universe with a uniformly distributed net charge, because the electromagnetic field tensor in such a universe must be vanishing everywhere. For a closed universe with a nonzero net charge, Maxwell's equations always fail regardless of the spacetime symmetry and the charge distribution. The two paradoxes indicate that Maxwell's equations need be modified to be applicable to the universe as a whole. We consider two types of modified Maxwell equations, both can address the paradoxes. One is the Proca-type equation which contains a photon mass term. This type of electromagnetic field equations can naturally arise from spontaneous symmetry breaking and the Higgs mechanism in quantum field theory, where photons acquire a mass by eating massless Goldstone bosons. However, photons loose their mass when symmetry is restored, and the paradoxes reappear. The other type of modified Maxwell equations, which are more attractive in our opinions, contain a term with the electromagnetic potential vector coupled to the spacetime curvature tensor. This type of electromagnetic field equations do not introduce a new dimensional parameter and return to Maxwell's equations in a flat or Ricci-flat spacetime. We show that the curvature-coupled term can naturally arise from the ambiguity in extending Maxwell's equations from a flat spacetime to a curved spacetime through the minimal substitution rule. Some consequences of the modified Maxwell equations are investigated. The results show that for reasonable parameters the modification does not affect existing experiments and observations. However, the field equations with a curvature-coupled term can be testable in astrophysical environments where mass density is high or the gravity of electromagnetic radiation plays a dominant role in dynamics, e.g., interior of neutron stars and the early universe.

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