Tidal-charge effects on the superradiance of rotating black holes
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Regular Article - Theoretical Physics
Tidal-charge effects on the superradiance of rotating black holes Ednilton S. de Oliveiraa Faculdade de Física, Universidade Federal do Pará, Belém, Pará 66075-110, Brazil
Received: 1 July 2020 / Accepted: 16 October 2020 © The Author(s) 2020
Abstract The changes a (negative) tidal charge causes at the phenomenon of superradiance which occurs around rotating black holes are investigated. This is made by computing the amplification factors of massless scalar waves being scattered by the black hole. It is shown that the increase of the tidal-charge intensity leads to a considerable enhancement of energy extraction from near-extreme black holes. Such improvement results from the fact that extreme black holes with more negative tidal charges spin faster. Maximum amplification decreases with the increase of the tidal charge intensity if the angular momentum of the black hole per unit mass is fixed. The tidal charge may also change crucially the superradiance phenomenon of massless scalar waves causing maximum amplification to occur for m > 1 differently from the case of Kerr black holes.
1 Introduction A couple of decades ago it has been proposed the existence of black holes with tidal charge [1] in the context of the Randall– Sundrum brane-world scenario [2,3]. This charge results from the influence of the fifth dimension in our 4-dimensional observed universe, the brane. The exclusive effect of the tidal charge on the black hole physics has been quantified in several researches based on the study of static black holes [4– 9], among which our works treating about wave scattering from such black holes [8,9]. In these papers we have shown that black holes endowed with more intense (negative) tidal charges1 present bigger absorption cross sections, while their scattering spectra in the near-forward direction is not easily distinguishable from that of Schwarzschild black holes. This 1 We consider only black holes with negative tidal charges since these, as argued in Ref. [1], are physically more natural than the ones with positive tidal charges. a e-mail:
[email protected] (corresponding author)
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leads to the conclusion that the tidal charge interferes considerably in the dynamics near the black hole, but with an influence which can be neglected in the far region. When the tidal charge is combined with other properties, as angular momentum and electric charge [10], the black hole presents novel configuration compared with black holes predicted within General Relativity. Specially in the case of electrically neutral rotating black holes, the tidal charge modifies the limit of angular momentum such black holes can acquire allowing them to spin faster than extreme Kerr black holes. Also, some of these black holes possess a bigger ergoregion, what could allow more energy extraction from them than from Kerr black holes. Indeed, it has been recently shown that a negative tidal charge increases the efficiency of energy extraction from the black hole via the Penrose
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