This was described in 1964 by Russian astronomer Nikolai Kardashev who hypothesized that a so-called Type III civilization would control the entire energy output of a galaxy. (We don't even reach Type I status because we have failed to harness nuclear fusion or build a constellation of solar power satellites.)
But why should a super-civilization be so energy voracious? And how in the heavens do they tap the energy of an entire galaxy?
First, a far-advanced society would need a lot of energy to support a rapidly growing wave of colonization, ambitious astroengineering projects, and burgeoning populations. Green technology can only go so far.
Secondly, super-smart extraterrestrials have far more than the total stellar energy output of the entire Milky Way at their fingertips. They could tap into the mother of all storage batteries: the supermassive black hole in the core of our galaxy. This gravitational engine is vastly more efficient at converting matter to energy than stellar nuclear fusion.
A huge amount of radiation is generated by the million degree accretion disk of trapped gas whirling around the 4 billion solar mass black hole at our galaxy's heart.
Makoto Inoue of the Institute of Astronomy and Astrophysics in Taipei, and Hiromitsu Yokoo of Chiba University are proposing that advanced civilizations might pool their resources to construct a ring of "power stations" at the galaxy's core. They would orbit the central black hole just beyond its solar system-sized accretion disk.
Some fraction of the radiation seething from the disk would be reflected and focused onto the power plants. Each power plant would transmit collected energy as a collimated microwave beam from a 100-mile diameter antenna.
Or, aliens might use molecular interstellar clouds to fashion an efficient transmission system, by means of a maser. A maser (an acronym for Microwave Amplification by Stimulated Emission of Radiation), creates an intense coherent beam of radiation when atoms or molecules in a gas, liquid or solid medium, force an incoming mix of wavelengths to work in phase, or, at the same wavelength. This would amplify radiation from the accretion disk and make a sharp collimated beam.
Nature has already fashioned mega-masers in clouds near galactic black holes, and therefore radiation pumping by artificial means may not be so difficult to set up, say the authors.
A consortium of super-civilizations might pool resources to build a chain of power stations encircling the black hole. It would be the heart of a robust and fault-tolerant energy grid connecting numerous worlds like a fantasy scene out of the film "Tron."
However, I think it is more likely that a federation of expanding space colonies, spawned from a single mother civilization, would work together to maintain their viability. This wouldn't run into the thorny question of how two or more independent but similarly co-evolved species manage to contact each other and work out a practical energy infrastructure.
But could we detect evidence of such a mega-engineering project?