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Comment by sandworm101 | Hacker News Reader

>>> about 5 centimeters across. Consequently, it is almost impossible to spot with a telescope.

If it is 5cm across, then it should be rather bright (Hawking radiation). While still tiny, it wouldn't be black.

This calculator[1] indicates that the temperature for a 2.5 cm radius black hole should be 0.007 K. So below the temperature of the cosmic background (2.7 K).

[1] https://www.vttoth.com/CMS/physics-notes/311-hawking-radiati...

Does this mean that the cosmic background could be adding energy to the blackhole (Maybe helping with stability?)? I'm not sure how temperature and heat transfer would work in this context.

Edit: For those interested, there is an entire article on "Black Hole Thermodynamics" on Wikipedia (https://en.wikipedia.org/wiki/Black_hole_thermodynamics)

You are correct: a black hole whose Hawking temperature is less than that of the cosmic background radiation is “colder” than the space it is immersed in and consequently absorbs energy (and mass-energy equivalence) from the heat bath it is immersed in. The easiest way to envision it is that the black hole swallows up photons of the microwave background radiation and is emitting less energy as Hawking radiation, leading to a net imbalance in the black hole’s favour.

Of course eventually the expanding universe will drop the temperature of the cosmic background radiation further and the process will reverse.

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