MG11 
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Universal BH thermodynamics from horizon symmetries

The laws of BH thermodynamics, in particular BH entropy and Hawking radiation, have been derived in numerous different ways and appear to be independent of the assumption on the underlying microphysics. This raises the question of a fundamental principle responsible for this fact. In this work we implement the horizon as an effective boundary of space-time and suggest the ensuing change of local symmetries as a candidate for such a principle. It is shown that the reduced boundary phase space of a horizon is smaller than the one of a generic boundary, as some of the physical degrees of freedom are converted into gauge degrees of freedom. The latter represent unobservable information and may be interpreted as origin of BH entropy. The appearant contradiction to recent results, where BH entropy appears due to Goldstone modes near the horizon, is discussed and resolved.