For a black hole to physically exist as a solution to Einstein's equation, it must form an event horizon in finite time relative to outside observers. This requires an accurate theory of black hole formation, of which several have been proposed. In 2007, Shuan Nan Zhang of Tsinghua University proposed a model in which the event horizon of a potential black hole only forms (or expands) after an object falls into the existing horizon, or after the horizon has exceeded the critical density. In other words, an infalling object causes the horizon of a black hole to expand, which only occurs after the object has fallen into the hole, allowing an observable horizon in finite time. This solution does not solve the information paradox, however.
While black holes were a well-established part of mainstream physics for most of the end of the 20th century, alternative models received new attention when models proposed by George Chapline and later by Lawrence Krauss, Dejan Stojkovic, and Tanmay Vachaspati of Case Western Reserve University showed in several separate models that black hole horizons could not form.
Such research has attracted much media attention, as black holes have long captured the imagination of both scientists and the public for both their innate simplicity and mysteriousness. The recent theoretical results have therefore undergone much scrutiny but have not been invalidated. Since most of these results incorporate only well-tested theories such as statistical mechanics, general relativity, and quantum field theory, they may pose a significant challenge to the mainstream acceptance of black holes.
However, several alternative black hole models were shown to be unstable in extremely fast rotation, which, by conservation of angular momentum, would be a not unusual physical scenario for a collapsed star (see pulsar). The existence of a stable model of a nonsingular black hole is still an open question.