If life can organize into sentient beings around stars other than our own, there are few assumptions we can make about the civilizations that would emerge. We’ve long ago given up on the idea that such creatures would look like us, just as we abandoned the concept of life on every conceivable astronomical object. William Herschel, among others, thought life might exist on the Sun, a notion that in different form may be coming back around, as witness the growing interest in panpsychism and stellar consciousness. But let’s talk about physical life forms rather than energy fields.
Since we have to assume something somewhere, let’s posit that any civilization would select as its top priority its own survival. That seems obvious enough. Survival demands energy, and that demand increases as the civilization grows through the Kardashev scale, gradually using more and more of the energy of its star and ultimately going beyond that to look for energy sources elsewhere. Dyson sphere thinking comes out of this realization, although we have yet to identify such structures if they do exist.
We need to (temporarily) forget Dyson and acknowledge that energy collection could take forms that are on the edge of our own fancy. Even science fiction at its best can’t imagine everything, and the vast hordes of astronomical data swelling our databases may contain things that we can’t identify as artificial constructs or activities. But we have a universe to play around in, and let’s take this cosmos to its logical conclusion. We know that it is not only expanding, but that the expansion is in fact accelerating.
The Hubble Constant, which relates the rate at which galaxies are receding from us as space expands to their distance from us, is not so constant after all. Or put another way, this value (currently considered in the range of 67.4 km/s/Mpc to 73 km/s/Mpc) actually varies over time. Those figures show the discrepancy between observations with the Planck satellite of the Cosmic Microwave Background and measurements using Cepheid variable stars. Thus the ‘Hubble tension,’ which is all about trying to resolve this difference. Whatever that result, current thinking is that as the universe ages, the expansion rate increases thanks to the interplay between gravity and dark energy.
Now since we’re already getting into mind-blowing territory here, let’s do what Dan Hooper did in 2018 and ask ourselves what a civilization with the vast skills of a Kardashev Type III civilization (one that can control the energies of an entire galaxy) might do to ensure its own future. We’re assuming a civilization that intends to maximize its energy use and must thus overcome the problem of spatial expansion. Because as Hooper points out in a paper in Physics of the Dark Universe, within 100 billion years, all matter not gravitationally bound to our own Local Group of galaxies will become what he calls ‘causally disconnected’ from the Milky Way.
Image: Dan Hooper, in an image provided by the University of Chicago.
Beyond This Horizon is the wonderful title of a Heinlein novel first serialized in Astounding in 1942 and later released in book form. Heinlein was interested in reincarnation, but in this case, the horizon we’re talking about is the ‘edge’ (forgive the metaphor) of the rapidly expanding universe, assuming that dark energy as we currently describe it is correctly understood. That last is the other key assumption that Hooper makes in his paper, the first being the need for civilizations to keep finding new sources of energy. And that’s it as far as assumptions go in this intriguing paper – two and no more.
Hooper (Fermi National Accelerator Laboratory, University of Chicago) points out that a Kardashev III civilization will understand that dark energy will increasingly dominate the total energy density of the cosmos, so that expansion goes exponential. Distant galaxies begin to cross this horizon, rendering them forever out of reach. Energy collection could involve collecting stars – the stellar harvest I mentioned in an earlier post, here seen at its most stupendous scale. The more stars that can be harvested – i.e., propelled toward the civilization’s center – the more stars will become gravitationally bound and thus rendered safe from being lost to the effects of this dark energy.
But we’re not talking about all stars. We need a subset of them. After all, stars aren’t static; they continue to evolve as we go after them. Listen to this, as Hooper considers two different rates of travel for the civilization to reach and ‘rescue’ stars:
…very high-mass stars will often evolve beyond the main sequence before reaching their destination of the central civilization, while very low-mass stars will oftentimes generate too little energy (and thus provide too little acceleration) to avoid falling beyond the horizon. For these reasons, stars with masses in the approximate range of M ∼ (0.2−1)M☉ will be the most attractive targets of such an effort.
So we’re in Dyson sphere country after all, but a revised version of same. The author is talking about transporting usable stars by enclosing them to capture their energies and then applying that energy to the stars as thrust. We have to harvest stars, as the above quote makes clear, that are luminous enough to provide enough thrust, and avoid stars that are ‘fast burners’ and will not reach the central civilization in time to be useful. Hooper continues (again, notice that he’s running his calculations on two different velocities for moving stars; he deploys three velocities elsewhere in the paper):
A civilization that begins to expand in the current epoch, traveling at a maximum speed of 10% (1%) of the speed of light, could harvest stars in this mass range out to a co-moving radius of approximately 50 Mpc (20 Mpc). Unlike more conventional Dyson Spheres, these structures would not necessarily emit in the infrared or sub-millimeter bands, but would instead use the collected energy to propel the captured stars, providing new and potentially distinctive signatures of an advanced civilization in this stage of expansion and stellar collection.
Hooper plugs in a speed of transport of no more than 10% c. It’s an arbitrary choice that may raise an eyebrow or two given that we’re talking about civilizations that have had billions of years of experience in matters of science, technology and propulsion. But let’s take this conservative value for velocity, and the lesser ones he discusses as useful parameters. The idea is that the Dyson spheres being used can transfer all of their collected energy into the kinetic energy of the captured star.
What we wind up with is this: If we assume a maximum speed of 10% c, the advanced civilization could collect stars that are currently as far away from it as 65 Mpc. The numbers are spectacular. Remember, a megaparsec (Mpc) is one million parsecs, or 3.26 million light-years. There are a lot of stars to play with in that volume. Here’s Hooper’s Figure 2 portraying the results for the three velocities he considers in the paper.
Image: Figure 2. A summary of the prospects for an advanced civilization to transport usable stars to a central location, assuming that such efforts begin in the present epoch. Stars in the red (upper left) regions will ultimately fall beyond the cosmic horizon, while those in the blue (right) regions will evolve beyond the main sequence before reaching their destination, and thus not provide useful energy. The grey dashed lines denote the length of time that is required to reach and transport the star. We show results for transport that is limited to speeds below 10%, 1% or 0.1% of the speed of light, and assume that the Dyson Spheres transfer approximately 100% of the collected energy to the kinetic energy of the star (η = 1). The blue region in each frame has been calculated for the optimistic case of stars that are starting their main sequence evolution at the time that they are encountered… Credit: Dan Hooper.
The author then proceeds to adjust for stellar age, which takes into account the fact that we aren’t getting to each star at the beginning of its main sequence evolution:
Integrating our results over the initial mass function of Ref. [34] and the cosmic star formation rate of Ref. [35], we estimate that an advanced civilization (with vmax = 0.1c and η = 1) could increase the total stellar luminosity bound to the Local Group at a point in time 30 billion years in the future by a factor of several thousand relative to that which would have otherwise been available. Over a period of roughly a trillion years, the total luminosity of these stars will drop substantially, but will continue to produce substantial quantities of useable energy due to the longevity of the lightest main sequence stars.
So we can’t shut off dark energy but a sufficiently advanced civilization can extend its lifetime considerably. In the next post, I want to dig into the kind of technosignatures we might find if we happened across this kind of star harvesting in our observational data. In doing this, we’ll certainly not be limited to observations within the Milky Way, but have the entire visible universe to consider. Clément Vidal has examined pulsar imagery that could flag the kind of ‘spider’ pulsar engine described in a recent article here. The technosignatures involved in Hooper’s scenario may be even more tricky to find.
The paper is Hooper, “Life versus dark energy: How an advanced civilization could resist the accelerating expansion of the universe,” Physics of the Dark Universe Volume 22 (December 2018), pp. 74-79. Abstract / Preprint. In addition to several books and numerous scientific papers, Hooper has co-produced an excellent podcast called Why This Universe? whose archives are available here.
