Aestivation hypothesis
Proposed in That is not dead which can eternal lie: the aestivation hypothesis for resolving Fermi’s paradox (2017), by Anders Sandberg, Stuart Armstrong, Milan Ćirković.
[Aestivate: to sleep through the summer, or a period too hot for living. The opposite of "hibernate".]
Idea: Aliens are sleeping until the far future, when the universe is cold enough, to maximize computation. The reason is that cold = more efficient use of energy for computation, according to the Landaur limit: $E\ge kT\ln(2) \text{Joule}$ need to be dissipated for an irreversible change of one bit of information.
... this can produce a $10^{30}$ multiplier of achievable computation. We hence suggest the “aestivation hypothesis”: the reason we are not observing manifestations of alien civilizations is that they are currently aestivating...The phrase "That is not dead which can eternal lie" comes from a poem by H. P. Lovecraft, a philosopher ahead of his time who really understood how human thoughts matter little in a cosmic viewpoint.
A civilization would aestivate if:
