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Galaxies 2014, 2(1), 62-71; doi:10.3390/galaxies2010062
Revisiting Vaidya Horizons
Received: 12 December 2013 / Revised: 29 January 2014 / Accepted: 29 January 2014 / Published: 10 February 2014
(This article belongs to the Special Issue Aspects of Black Hole Physics)
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Abstract: In this study, we located and compared different types of horizons in the spherically symmetric Vaidya solution. The horizons we found were trapping horizons, which can be null, timelike, or spacelike, null surfaces with constant area change and also conformal Killing horizons. The conformal Killing horizons only exist for certain choices of the mass function. Under a conformal transformation, the conformal Killing horizons can be mapped into true Killing horizons. This allows conclusions drawn in the dynamical Vaidya spacetime to be related to known properties of static spacetimes. We found the conformal factor that performs this transformation and wrote the new metric in explicitly static coordinates. Using this construction we found that the tunneling argument for Hawking radiation does not umabiguously support Hawking radiation being associated with the trapping horizon. We also used this transformation to derive the form of the surface gravity for a class of physical observers in Vaidya spacetimes.
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