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At a time long after the stelliferous there is a planet in orbit around a super massive black hole with almost all external heating coming from hawking radiation from the black hole it orbits. By this time the cosmic background radiation has red shifted to such an extent that it's negligible even when compared to the hawking radiation of a super massive blackhole.

I'm thinking about how this planet will be a fraction of a kelvin in temperature, but the black hole will last for so long that the current age of the universe will look like a brief time in comparison. Could this planet have life provided that the life had a very slow metabolism, and if so how might this life evolve over time?

Anders Gustafson
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    https://worldbuilding.stackexchange.com/q/22795/30492 – L.Dutch Aug 15 '19 at 19:12
  • Yep. Duplicate. – puppetsock Aug 15 '19 at 19:20
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    @stix No, it isn't. That previous question is about black holes small enough to provide illumination comparable to that of a star. This question is about dealing with the actual extremely cold Hawking radiation from astrophysical supermassive black holes. These are completely different regimes, and answers to one question do not apply to the other. – Logan R. Kearsley Aug 15 '19 at 19:30
  • Unrelated to the issue of the black hole, but getting colder than the cosmic microwave background would require running a heat pump. This puts the lowest possible temperature that can be achieved without such a device (whether somehow natural, or more likely constructed by intelligent, technologically capable beings) in our present-day universe at about 2.7 Kelvin. – user Aug 15 '19 at 19:30
  • @aCVn I mentioned that this planet is in the distant future, when the cosmic background radiation will be much colder than it currently is so that the planet can be colder than the current temperature of the cosmic microwave background radiation while still being warmer than the cosmic background radiation during its time. – Anders Gustafson Aug 15 '19 at 19:38
  • The question has been edited since flagged, however, the editing makes it nonsensical.

    Black holes don't cool off nor do they "redshift," they get brighter as they age. A black hole's temperature is inversely proportional to its size, and since black holes evaporate, a black hole's size decreases with age.

     – stix Aug 15 '19 at 19:40
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    @stix I don't know what question you're reading but there is nothing here about a black hole cooling off or redshifting. This talks about the CMB redshifting, which is perfectly standard cosmology. – Logan R. Kearsley Aug 15 '19 at 19:43
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    @aCVn That's not quite true. Astrophysical black holes already have effective temperatures lower than the CMB, no heat pump required. This is a result of gravitationally-bound systems having negative heat capacity, and it's why hawking radiation power is inversely proportional to black hole mass. – Logan R. Kearsley Aug 15 '19 at 19:46
  • Your planet won't get warmer than your "star", so it will be at single Kelvins above absolute zero at most. We can theorize about life at these conditions, but that won't be the life as we know it. – Alexander Aug 15 '19 at 20:09

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Almost certainly not (except in ways which violate the spirit of the question).

In order to make use of the extremely large wavelengths of Hawking radiation from a supermassive black hole, an organism would have to start out extremely large, for the same reason that radio antennae must be comparable in size to the wavelength of radiation they are designed to capture or emit. Much of the (already pitifully small) emitted energy would simply pass right through the whole planet. Thus, there is no way for small-scale structures to begin capturing energy to power chemical processes that could result in life, no matter how slow its metabolism is.

However, if life that started in the stelliferous era were to plan ahead and build large-scale structures capable of capturing the Hawking radiation from that black hole, then computational machinery could indeed continue to operate on the orbiting planet--at an extremely slow speed, sure, but also at extremely high efficiency, given the low operating temperature. So, although life could not evolve there, it could persist there, for a certain liberal definition of "life".

Now, given the possible existence of a computational infrastructure on this planet, you can then consider what kind of software that infrastructure might be running. The most straightforward option might be something like "simulations of the uploaded minds of the people who built it to give them immortality past the end of the stelliferous era", but another option might be "a fine-grained simulation of a habitable planetary environment from the stelliferous era"--in which case, life could indeed evolve in that simulation, on that planet orbiting that black hole.

Logan R. Kearsley
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