Consider also the number of mass extinctions that have occurred in Earth's past. It is unlikely that the same number of these would occur on another Earth- like world at exactly the same time and with the exact same frequency. Finally, consider the cultural developments in Earth's history,...
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Keywords: gamma-rays: observation; gamma-rays: theory; general relativity; particle acceleration; stars: black holes1. Introduction It is commonly accepted that every active galaxy harbors a supermassive black hole (BH), whose mass typically ranges between 106 Mʘ and 109.5 Mʘ, in its center...