To combat an expanding universe, aliens could hoard stars

Survivalists prep for disaster by stocking up on emergency food rations. Aliens, on the other hand, might hoard stars.

To offset a future cosmic energy shortage caused by the accelerating expansion of the universe, a super-advanced civilization could pluck stars from other galaxies and bring them home, theoretical astrophysicist Dan Hooper proposes June 13 at arXiv.org.

It’s a far-out idea, tackling a dilemma in a future so distant that human beings can hardly fathom it: 100 billion years from now, each neighborhood of the universe will be marooned as if on a cosmic island, with resources from the rest of the universe inaccessible. “We’ll be in this very dark, lonely place where we won’t be able to see other galaxies,” says theoretical astrophysicist Katie Mack of North Carolina State University in Raleigh. That isolation is thanks to a mysterious “dark energy” that is causing the universe to expand faster and faster (SN: 4/7/01, p. 218).

Advanced societies might be able to harness the energy of stars by surrounding them with giant, hypothetical structures called Dyson spheres (SN: 4/24/10, p. 22). But the expansion will eventually make it impossible to reach stars outside the civilization’s home turf. Aliens that possess such technology might want to maximize energy reserves by sending spaceships to retrieve stars before the cosmic isolation sets in. Each star’s energy could be captured with a Dyson sphere, and that energy would then be used to propel the star homeward.  

The study doesn’t specify exactly how a civilization might move a star, or what it would do with the energy once captured. It’s hard to speculate about beings so powerful that these extreme feats would be possible, says Hooper, of Fermilab in Batavia, Ill. “It’d be like asking a caveman to figure out how my automobile works.”

The effort would do best to focus on stars that aren’t too big or too small, Hooper calculates. Big stars live fast and die young, so during the tens of billions of years needed to transport the stars home, they would fizzle out. The least massive stars, on the other hand, wouldn’t emit enough energy to fuel their own propulsion; they wouldn’t be able to outpace the universe’s expansion.

We have no evidence that such an advanced civilization exists (SN Online: 1/3/18). But if aliens are harvesting stars, says Hooper, “this would not be a subtle activity.” Scientists might be able to spot signs of the stars being propelled across the universe. A dearth of certain types of stars in particular cosmic neighborhoods would also be a tip-off.

Theoretical astrophysicist Avi Loeb of Harvard University suggests, however, that it wouldn’t be necessary to collect stars because “nature did it for us.” Large clusters of galaxies are already richly populated with stars. Plus, because the clusters are bound together by gravity, they would remain intact as the universe expands. So rather than improving their home galaxy by collecting stars, a civilization could simply move to greener pastures. “You just need to hop from one to another,” Loeb says. (Never mind the fact that interstellar travel still evades us mere humans.)

Mack, likewise, notes that uprooting to a galaxy cluster might be easier for a civilization, but “maybe they have really strong sentimental attachments to their home galaxy.”

Either way, E.T. won’t be able to stave off the end forever. Eventually, in about 100 trillion years, stars will stop shining altogether. Gathering stars would be a way to accomplish as much as possible, Mack says, “before all the stars die out and the universe is cold and dark and empty.”