The idea of moving stars as a way of concentrating mass for use by an advanced civilization – the topic of recent posts here – forces the question of whether such an effort wouldn’t be observable even by our far less advanced astronomy. In his paper on life’s response to dark energy and the need to offset the accelerating expansion of the cosmos, Dan Hooper analyzed the possibilities, pointing out that cultures billions of years older than our own may already be engaged in such activities. Can we see them?
I like Centauri Dreams reader Andrew Palfreyman’s comment that what astronomers know as the ‘Great Attractor’ is conceivably a technosignature, “albeit on a scale somewhat more grand than that cited.” An interesting thought! And sure, as some have pointed out, nudging these concepts around on a mental chess board is wildly speculative, but in the spirit of good science fiction, I say why not? We have a universe far older than our own planet with possibilities we might as well imagine.
If we turn our attention in the general direction of the constellation Centaurus and then look not at the paltry 4.3 light year distance of Alpha Centauri but 150–250 million light years from Earth, we encounter a region of mass concentration that folds within the Laniakea Supercluster. The latter is galactic structure at an extraordinary level, as it takes in some 100,000 galaxies including the Virgo Supercluster, and that means it takes in the Local Group and the Milky Way as well.
What’s happening is that this hard to observe region (it’s blocked by our own galaxy’s gas and dust) is evidently drawing many galaxies including the Milky Way towards itself. The speed of this motion is about 600 kilometers per second. Bear in mind that the Shapley Supercluster lies beyond the Great Attractor and is also implicated in the motion of galaxies and galaxy clusters in this direction. So the science fictional scenario has a civilization clustering matter at the largest scale to avoid the effects of the accelerating expansion that will eventually cut off anything that is not gravitationally bound. Cluster enough stars and you maintain your energy sources.
Image: Located on the border of Triangulum Australe (The Southern Triangle) and Norma (The Carpenter’s Square), this field covers part of the Norma Cluster (Abell 3627) as well as a dense area of our own galaxy, the Milky Way. The Norma Cluster is the closest massive galaxy cluster to the Milky Way, and lies about 220 million light-years away. The enormous mass concentrated here, and the consequent gravitational attraction, mean that this region of space is known to astronomers as the Great Attractor, and it dominates our region of the Universe. The largest galaxy visible in this image is ESO 137-002, a spiral galaxy seen edge on. In this image from Hubble, we see large regions of dust across the galaxy’s bulge. What we do not see here is the tail of glowing X-rays that has been observed extending out of the galaxy — but which is invisible to an optical telescope like Hubble. Credit: ESA/Hubble & NASA.
Recall the parameters of Dan Hooper’s paper, which posits the collection of stars in the range of 0.2 to 1 solar mass as the most attractive targets. The constraint is needed because high-mass stars will have lifetimes too short to make the journey (Hooper posits 0.1 c as the highest velocity available) to the collection zone. The idea is that the civilization will enclose lower-mass stars in something like Dyson Spheres, using these to collect the energy needed for propulsion of the stars themselves. Not your standard Dyson Sphere, but astronomical objects using propulsion that may be detectable.
Hooper doesn’t wade too deep into these waters, but here’s his thought on technosignatures:
From our vantage point, such a civilization would appear as a extended region, tens of Mpc in radius, with few or no perceivable stars lighter than approximately ∼2M☉ (as such stars will be surrounded by Dyson Spheres). Furthermore, unlike traditional Dyson Spheres, those stars that are currently en route to the central civilization could be visible as a result of the propulsion that they are currently undergoing. The propellant could plausibly take a wide range of forms, and we do not speculate here about its spectral or other signatures. That being said, such acceleration would necessarily require large amounts of energy and likely produce significant fluxes of electromagnetic radiation.
This is a different take on searching for Dyson Spheres than has been employed in the past, for in the ‘star harvesting’ scenario of Hooper, the spectrum of starlight from a galaxy that has already been harvested would be dominated by massive stars, with the lower mass stars being already enclosed. On this score, it’s also interesting to consider the continuing work of Jason Wright at Penn State, where an analysis of Dyson Spheres as potential energy extractors and computational engines is changing our previous conception of these objects, resulting in smaller, hotter observational signatures.
In the near future we’ll dig into the Wright paper, but for today it’s useful indeed, because it points to why we speculate on such a grand scale. Let me quote from its conclusion:
Real technological development around a star will be subject to many constraints and practical considerations that we probably cannot guess. While we have outlined the ultimate physical limits of Dyson spheres, consistent with Dyson’s philosophy and subject only to weak assumptions that there is a cost to acquiring mass, if real Dyson spheres exist, they might be quite different than we have imagined here.
And the key point:
Nonetheless, these conclusions can guide speculation into the nature of what sorts of Dyson spheres might exist, help interpret upper limits set by search programs, and potentially guide future searches.
But back to Hooper and the subject of Deep Time. For Hooper’s calculation is that all stars that are not gravitationally bound to the Local Group (which includes the Milky Way and Andromeda, among other things) will move beyond the cosmic horizon due to accelerating expansion on a timescale of 100 billion years. It will be autumn among the galaxy clusters, meaning that their energies will need to be harvested or rendered forever inaccessible. Our hypothetical advanced civilization will need to begin moving stars back toward their culture’s central hub. Hooper sees a civilization conducting such activities out to a range of several tens of Mpc, which boosts the total amount of energy available in the culture’s future by a factor of several thousand.
This is an application of Dyson Spheres far different from what Freeman Dyson worked with, and I agree with Jason Wright that technologies of this order are probably far beyond our current imaginings. But as Dyson himself said in a 1966 tribute to Hans Bethe: “My rule is, there is nothing so big nor so crazy that one out of a million technological societies may not feel itself driven to do, provided it is physically possible.”
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. The Wright paper is “Application of the Thermodynamics of Radiation to Dyson Spheres as Work Extractors and Computational Engines, and their Observational Consequences,” The Astrophysical Journal Volume 956, No. 1 (5 October 2023), 34 (full text). I drew the Dyson quote from Wright’s paper, but its source is Dyson, Perspectives in modern physics: Essays in Honor of Hans A. Bethe on the Occasion of his 60th birthday, ed R. Marshak, J. Blaker and H. Bethe (New York: Interscience Publishers) July, 1966, p. 641.
