On the Necessity of Horizons – Disputing the Existence of Hawking Radiation from Horizon-Less Objects

Abstract

As an attempt to complete black hole thermodynamics, Stephen Hawking showed in 1974 that black holes emit thermal radiation. This result has since been vastly disputed, both because it violates the classical notion that black holes are regions of spacetime from which nothing can escape, but also because it leads to a paradox: The thermal radiation which causes the black hole to evaporate contains no information about the black hole. Thus, information appears to be lost – despite the deterministic nature of the physical theories describing the phenomenon. As an attempt to solve this paradox, some researchers have suggested that horizon-formation is avoided in stellar collapse models due to the presence of a so-called pre-Hawking radiation. This proposal is based on claims that Hawking-like radiation also occurs in collapse models where a horizon never forms. It has further been proposed that this radiation may prevent black holes from forming at all. On the other hand, others claim that such a radiation is too weak to play a crucial role in the course of stellar collapse and that its existence leads to serious physical inconsistencies. The question of whether a horizon is needed in order for Hawking radiation to occur therefore seems to be at the very heart of this discussion. This further seems to be closely related to the questions of where and when the Hawking particles are created. Because of the global nature of event horizons these latter concerns are intrinsically difficult to address. We do not hope to solve this problem in this thesis. Rather, we aim to narrow down the discussion to its essentials. In the attempt to do just that, we study the possibility of Hawking radiation from a subclass of exotic compact objects (ECOs) which look exactly like black holes to distant observers. From our highly idealised models we do not find evidence of Hawking radiation from horizonless objects. However, more advanced and realistic models are needed in order to properly conclude that event horizons are necessary for particle creation, and thus the dispute remains unresolved.