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arXiv 0909.0314gr-qc
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黑洞准正规共振:波动分析与几何光学近似

Black-hole quasinormal resonances: Wave analysis versus a geometric-optics approximation

Shahar Hod

摘要:

人们早就知道,零不稳定测地线与黑洞的特征模--所谓的准正态共振有关。其基本思想是用被困在不稳定圆轨道上的零粒子缓慢泄漏来解释程函极限下黑洞的自由振荡。复准正规共振的实部与不稳定零测地线处的角速度有关。共振的虚部与轨道的不稳定时间尺度(或逆李雅普诺夫指数)有关。虽然这种用扰动零{ \it射线}描述黑洞准正规共振的几何光学描述非常吸引人和直观,但通过直接分析支配黑洞时空中扰动{ \it波}动力学的Teukolsky波动方程来验证这种方法的有效性仍然非常重要。这是本文的主要目的。我们首先利用扰动一束不稳定零射线的几何光学方法,在程函近似下计算了近极值Kerr黑洞的共振态。然后,我们直接求解Teukolsky波动方程(辅以适当的物理边界条件),并表明,直接从波动分析得到的拟正规谱与从扰动零射线的几何光学近似得到的谱是一致的。

英文摘要:

It has long been known that null unstable geodesics are related to the characteristic modes of black holes-- the so called quasinormal resonances. The basic idea is to interpret the free oscillations of a black hole in the eikonal limit in terms of null particles trapped at the unstable circular orbit and slowly leaking out. The real part of the complex quasinormal resonances is related to the angular velocity at the unstable null geodesic. The imaginary part of the resonances is related to the instability timescale (or the inverse Lyapunov exponent) of the orbit. While this geometric-optics description of the black-hole quasinormal resonances in terms of perturbed null {\it rays} is very appealing and intuitive, it is still highly important to verify the validity of this approach by directly analyzing the Teukolsky wave equation which governs the dynamics of perturbation {\it waves} in the black-hole spacetime. This is the main goal of the present paper. We first use the geometric-optics technique of perturbing a bundle of unstable null rays to calculate the resonances of near-extremal Kerr black holes in the eikonal approximation. We then directly solve the Teukolsky wave equation (supplemented by the appropriate physical boundary conditions) and show that the resultant quasinormal spectrum obtained directly from the wave analysis is in accord with the spectrum obtained from the geometric-optics approximation of perturbed null rays.

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