If you think we have problems now, just wait a few billion years, when the accelerating expansion of the universe triggers an energy crisis of cosmological proportions.
Sounds grim, but as a new paper points out, an advanced civilisation faced with doom won’t have to go gently into that good night — there may very well be a way to rage against the dying of the light.
Owing to the inexorable influence of dark energy, the space in our universe is expanding at an accelerating rate. We don’t need to worry about this right now, but for those civilisations still around tens of billions of years from now, it’ll probably be a major headache.
By this stage, galaxies outside of our Local Group — a conglomeration of about 54 nearby galaxies — will be moving away from us faster than their light can reach us, making them completely unobservable, and by consequence, utterly inaccessible.
With all stars beyond the Local Group residing beyond the cosmic horizon, advanced civilisations, hereafter called civs, would be thrown into a colossal energy crisis. As pioneering physicist Freeman Dyson has speculated, advanced civs would likely collect energy by wrapping stars in solar panels, a megastructure known as a Dyson sphere. Without the ability to access new stars, civs would no longer be able to sustain themselves and expand.
The accelerating expansion of the universe, therefore, would eventually present an existential threat to highly technological intelligent life.
But as University of Chicago astronomer Dan Hooper explains in his new paper (available as a pre-print on the arXiv), advanced civs, whether they be our descendants or extraterrestrial intelligences (ETIs), could act proactively to deal with the situation.
To combat a universe dominated by dark energy, a civ that has graduated to Type III status on the Kardashev scale — a civ that’s harnessed the energy produced by stars throughout its galaxy — could use the energy collected by Dyson spheres to propel captured stars in the opposite direction of universal expansion. By doing, so a civ could collect as many stars as possible before it’s too late. As Hooper explains in his paper:
Given the inevitability of the encroaching horizon, any sufficiently advanced civilisation that is determined to maximise its ability to utilise energy will expand throughout the universe, attempting to secure as many stars as possible before they become permanently inaccessible. To this end, they could build Dyson Spheres or other such structures around the stars that are encountered, and use the energy that is collected to propel those stars toward the center of the civilisation, where they will become gravitationally bound and thus protected from the future expansion of space.
But advanced civs will have to be selective, as not all stars will be suitable for this megascale project. Hooper says stars that are between 20 to 100 per cent the mass of our Sun will make most sense; stars heavier than ours are too short-lived, and they’d likely expire before reaching their destination, while lighter stars would not be able to produce enough thrust.
If a civ started on this project today, and if it could move its Dyson spheres at about 10 per cent the speed of light, it could create a gravitationally bound conglomeration measuring 65 million light-years across (by comparison, the Milky Way is about 100 light-years in diameter).
This project would potentially increase the “total amount of energy that is available to a future civilisation by a factor of several thousand,” writes Hooper.
Interestingly, ETIs could be doing this right now, and it might be possible for us to see the signs. We just have to figure out the means of propulsion, and see if we can detect the spectral signatures produced by these mobile Dyson spheres. Also, we should be able to see it.
“If you noticed that a region of the universe was missing many or most of its intermediate mass stars, that might be a sign that they were being harvested in this way,” Hooper told Gizmodo.
This paper, which is slated to be published in the Journal of Cosmology and Astroparticle Physics, is predicated on some rather big assumptions. We don’t know if advanced civs would work to maximise access to usable energy to such a degree, or if our understanding of dark energy and its influence on the expansion of the universe is correct. What’s more, we don’t know how an advanced civ would move a Dyson sphere.
“Trying to speculate how exactly such a machine might work would be like a caveman trying to guess how a car works,” Hooper told Gizmodo.
“But what is important is that a star produces enough energy to accelerate a star to high speeds, though over a long period of time. How exactly they do it remains for the advanced civilisation’s engineers to sort out.”
Abraham Loeb, chair of the astronomy department at Harvard University, doesn’t love the new paper. In his recent essay, “Securing Fuel for Our Frigid Cosmic Future”, Loeb writes that “we do not know of any technology that enables moving stars around”, and “Sun-like stars only shine for about ten billion years (of order the current age of the Universe) and cannot serve as nuclear furnaces that would keep us warm into the very distant future”.
Fortunately, however, all is not lost. Instead of using the energy output from Sun-like stars, Loeb says advanced civs could migrate into rich clusters of galaxies.
“An advanced civilisation does not need to embark on a giant construction project as suggested by Hooper, but only needs to propel itself towards the nearest galaxy cluster and take advantage of the cluster resources as fuel for its future prosperity,” writes Loeb.
“The nearest cluster to us is Virgo, whose center is about fifty million light years away. Another massive cluster, Coma, is six times farther.”
Loeb says advanced civs throughout the Universe may be drawn to galaxy clusters similar to how ancient civilisations moved towards rivers and lakes. Once settled around a sufficiently large enough cluster, a “civilisation could hop from one star to another and harvest their energy output just like a butterfly hovering over flowers in a hunt for their nectar,” he writes.
These natural conglomerations of stars, which are gravitationally bound and immune to accelerated cosmic expansion, could sustain a civ for trillions of years, argues Loeb.
Regardless of which solution our descendants choose — should they still be around billions of years from now — it appears they’ll have some options. The expansion of the universe won’t be a problem for some time to come, but as both of these papers point out, survival will require some exceptionally long-term planning.