Dark energy, exotic matter and properties of horizons in black hole physics and cosmology

We summarize recent results on the properties of near-horizon metrics in different spherically symmetric space-times, including Kantowski-Sachs cosmological models whose evolution begins with a horizon (the so-called Null Big Bang) and static metrics related to black holes. We describe the types of matter compatible with cosmological and black-hole horizons. It turns out, in particular, that a black hole horizon can be in equilibrium with a fluid of disordered cosmic strings ("black holes can have curly hair"). We also discuss different kinds of horizons from the viewpoint of the behavior of tidal forces acting on an extended body and recently classified as "usual", "naked" and "truly naked" ones; in the latter case, tidal forces are infinite in a freely falling reference frame. It is shown that all truly naked horizons, as well as many of those previously characterized as naked and even usual ones, do not admit an extension and therefore must be considered as singularities. The whole analysis is performed locally (in a neighborhood of a candidate horizon) in a model-independent manner. Finally, the possible importance of some of these models in generating dynamic, perturbatively small vacuum fluctuation contributions to the cosmological constant (within a cosmological Casimir-effect approach to this problem) is discussed.