Black Hole Appetite
Energy-hungry galactic empires might skip tapping stellar energy and simply go to extracting energy from black holes. These collapsed islands of space-time are the universe's ultimate Energizer Bunnies. They are far more efficient at converting mass to energy than are the fusion engines of stars.
But more than that, says Vidal, is the ability to control the microcosm as well. Today we can manipulate individual atoms via nanotechnology. But advanced alien physicists would tinker with elementary particles and the very structure of space-time itself.
Having mastered control over space-time, a super-civilization might want to fabricate their own black holes for a variety of tasks: waste disposal, starship propulsion, hyper-computing, maybe even time travel.
On the macro-scale, super-civilizations might re-engineer stars using black holes. The quest for immortality beyond a star's lifetime would be a big motivation.
Suppose extraterrestrials manufacture a black hole to accrete material from a burned-out star, a white dwarf. This would provide abundant energy beyond the star's fusion-burning stage.
But how would you prove the black hole was artificial? Tell-tale evidence would come from measurements that showed the black hole weighed less than 3.5 solar masses. That's the minimum mass for crushing matter into a black hole via a supernova core-collapse.
Some low-mass X-ray binary stellar pairs might be artificially constructed, Vidal speculates. Or, aliens might come up with ways to control natural X-ray binaries - just as our ancestors extracted energy from waterfalls.
Curiously, NASA's Chandra X-ray telescope has found an overabundance of these systems within several light-years of the galactic center. The central bulge of our galaxy should be the home to the oldest civilizations because of its ancient stellar population.