Chasing the horizon

A variety of high-quality observations indicates that the Universe contains astronomical bodies that are both dark, in the sense that their direct emission is almost zero, and extremely compact. On the theoretical side, Einstein's General Relativity predicts the existence of black holes --- regions of spacetime from which no signal can reach distant observers. Are the dark and compact astronomical bodies that we observe really black holes in the sense of General Relativity? In this seminar I will describe in simple terms, mainly using pictures, the standard theoretical view on this question and an alternative view we have recently put forward in which black holes are substituted by, what we called, “black stars”.