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arXiv 2307.02589gr-qchep-th
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克尔黑洞周围试验体的运动

The Motion of Test Bodies around Kerr Black Holes

Adrien Druart

AI总结:

本文研究克尔黑洞周围具有自旋诱导四极矩的致密试验体运动,系统综述测地线运动、推导MPD方程及其守恒量,并建立协变哈密顿表述,在自旋一阶下通过Hamilton-Jacobi方程重现运动常数。

摘要:

本论文旨在探索在小质量比近似下,有限尺寸、致密试验体围绕克尔黑洞运动的性质。小物体被建模为克尔几何的扰动,忽略其引力反作用,但通过允许其具有非平凡的内部结构来包含对纯测地线运动的偏离。这样的物体可以通过一条带有多个多极矩的世界线来精确描述。此后,我们将始终考虑在四极阶截断的多极展开。此外,只考虑自旋诱导的四极矩,从而排除任何潮汐型变形的存在。对于天体物理学上现实的对象,这种近似与将运动方程展开到物体自旋大小的二阶是一致的。本文结构如下。第一部分致力于对克尔时空中测地线运动的扩展综述,包括哈密顿表述和类时测地线的分类,特别强调高自旋黑洞的近视界测地线。第二部分介绍了一般弯曲时空中扩展试验体的运动方程,也称为 Mathisson-Papapetrou-Dixon (MPD) 方程。第三部分讨论了克尔时空中 MPD 方程的守恒量,并限制在上述四极近似下。最后,给出了弯曲时空中试验体运动的协变哈密顿表述,并推导出一个重现自旋诱导四极 MPD 方程的哈密顿量。结果表明,在自旋大小的一阶求解 Hamilton-Jacobi 方程时,前一部分得到的运动常数直接出现。关于二阶计算的若干预期结束了讨论。

英文摘要:

This thesis aims to explore the properties of the motion of finite size, compact test bodies around a Kerr black hole in the small mass-ratio approximation. The small body is modelled as a perturbation of Kerr geometry, neglecting its gravitational back-reaction but including deviations from a purely geodesic motion by allowing it to possess a non-trivial internal structure. Such a body can be accurately described by a worldline endowed with a collection of multipole moments. Hereafter, we shall always consider the multipole expansion truncated at quadrupole order. Moreover, only spin-induced quadrupole moment will be taken into account, thus discarding the presence of any tidal-type deformation. For astrophysically realistic objects, this approximation is consistent with expanding the equations of motion up to second order in the body's spin magnitude. The text is structured as follows. The first part is devoted to an extended review of geodesic motion in Kerr spacetime, including Hamiltonian formulation and classification of timelike geodesics, with a particular emphasis put on near-horizon geodesics of high spin black holes. The second part introduces the equations of motion for extended test bodies in generic curved spacetime, also known as Mathisson-Papapetrou-Dixon (MPD) equations. The third part discusses conserved quantities for the MPD equations in Kerr spacetime, restricting to the aforementioned quadrupole approximation. Finally, the covariant Hamiltonian formulation of test body motion in curved spacetime is presented, and an Hamiltonian reproducing the spin-induced quadrupole MPD equations is derived. It is shown that the constants of motion obtained in the previous part directly arise while solving the Hamilton-Jacobi equation at first order in the spin magnitude. Some expectations regarding the computation at quadratic order close the discussion.

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