The era of black holes

After the protons decay, the universe grows even darker and more rarefied. At this late time, roughly when the universe is older than 1045 years, the only stellar-like objects remaining are black holes. They are unaffected by proton decay and slide unscathed through the end of the previous era. These objects are often defined to be regions of space-time with such strong gravitational fields that even light cannot escape from their surfaces. But at this late epoch, black holes will be the brightest objects in the sky. Thus, even black holes cannot last forever. They shine ever so faintly by emitting a nearly thermal spectrum of photons, gravitons, and other particles (Hawking, 1974). Through this quantum mechanical process - known as Hawking radiation - black holes convert their mass into radiation and evaporate at a glacial pace (Fig. 2.2). In the far future, black holes will provide the universe with its primary source of power.

Although their energy production via Hawking radiation will not become important for a long time, the production of black holes, and hence the black hole inventory of the future, is set by present-day (and past) astrophysical processes. Every large galaxy can produce millions of stellar black holes, which result from the death of the most massive stars. Once formed, these black holes will endure for up to 1070 years. In addition, almost every galaxy harbours a supermassive black hole anchoring its centre; these monsters were produced during the process of galaxy formation, when the universe was only a billion years old, or perhaps even younger. They gain additional mass with time and provide the present-day universe with accretion power. As these large black holes evaporate through the Hawking process, they can last up to 10100 years. But even the largest black holes must ultimately evaporate. This Black Hole Era will be over when the largest black holes have made their explosive exits from our universe.

Continue reading here: The Dark Era and beyond

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