Besides, really advanced civilizations—as Marvin Minsky pointed out—would consider radiation at any temperature above the cosmic background temperature of 2.7 K to be wasteful. Perhaps an ETC advanced enough to construct a Dyson sphere is advanced enough to squeeze every last drop of useful work out of a star’s radiation, leaving waste heat at a few kelvin. Perhaps Dyson spheres are common but we should be looking for them by searching for points in space that possess a small temperature excess over the microwave background?
