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Abstract: In this chapter I focus on asking and answering the following questions: 1What is a black hole? Answer: There are three types of black holes, namelymathematical black holes, physical black holes and astrophysical black holes.An astrophysical black hole, with mass distributed within its event horizon butnot concentrated at the singularity point, is not a mathematical black hole.2 Can astrophysical black holes be formed in the physical universe? Answer:Yes, at least this can be done with gravitational collapse. 3 How can weprove that what we call astrophysical black holes are really black holes?Answer: Finding direct evidence of event horizon is not the way to go. InsteadI propose five criteria which meet the highest standard for recognizing newdiscoveries in experimental physics and observational astronomy. 4 Do we havesufficient evidence to claim the existence of astrophysical black holes in thephysical universe? Answer: Yes, astrophysical black holes have been found atleast in some galactic binary systems, at the center of almost every galaxy,and as the central engines of at least some long gamma-ray bursts. 5 Will allmatter in the universe eventually fall into black holes? Answer: Probably -no-,because -naked- compact objects, if they do exist with radii smaller than theradii of event horizons for their masses but are not enclosed by eventhorizons, can rescue the universe from an eternal death by re-cycling out thematter previously accreted into astrophysical black holes. Finally I alsodiscuss briefly if we need a quantum theory of gravity in order to furtherunderstand astrophysical black holes, and what further astronomicalobservations and telescopes are needed to make further progress on ourunderstanding of astrophysical black holes.

Author: Shuang-Nan Zhang


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