Astrophysicist Nikolai Kardashev first proposed these three phases (called Level I, II and III) of galactic expansion — which he referred to as the three "types" of technologically advanced civilizations — in 1962 as a way to measure the energy consumption of increasingly powerful societies. Dyson proposed that really advanced civilizations would re-engineer their solar systems, perhaps dismantling planets to form a shellof satellites around their star to capture its energy. Solar sails could remain in place by balancing against the pressure of light from the sun. This net wouldn't be made of twine or even metal, but of satellites — a swarm of millions of solar-powered satellites known as "Dyson spheres." Such a colossal cloud of harvesters could permanently hover around a star, beaming energy back to a nearby planet — or, as Hooper proposed in his new paper, actually use that star's energy to accelerate the whole ball of fire back toward the planet that wanted to use it. Carl Saganestimated that Earth rates about 0.7 on the scale. But some scientists think there could be alien civilizations thousands, or even millions, of years older than ours who are already well into their Level III, star-harvesting phase.
– Watch out for stars moving across galaxies or losing power—it could be aliens gathering energy before the lights go out. Dan Hooper, an astronomy and astrophysics professor at the University of Chicago, posted a paper in the preprint journal arXiv.org arguing that aliens in far-off galaxies may be sucking energy from stars to offset risks posed by an expanding universe, LiveScience reports. "The presence of dark energy in our universe is causing space to expand at an accelerating rate," writes Hooper. "As a result, over the next approximately 100 billion years, all stars residing beyond the Local Group [of galaxies including the Milky Way] will fall beyond the cosmic horizon and become not only unobservable, but entirely inaccessible, thus limiting how much energy could one day be extracted from them." Therefore, he argues, an advanced civilization may use something like "Dyson spheres"—an imagined solar-powered satellite dating back to 1930s science-fiction, notes LiveScience—to harvest a star's energy or use that energy to move it closer to home. Hooper hinges his case on the work of Russian astronomer Nikolai Kardashev, who argued in 1962 that technologically advanced civilizations would expand via three steps: harvesting resources of their planet, then the nearest star, then all stars in the galaxy and nearby galaxies. So how to spot energy-grabbing aliens? Massive stars emanate certain light wavelengths, which will appear in galaxies' light signatures if stars are drained energy—that is, when humans have powerful enough instruments to detect it.