A US physicist suggests the accelerating universe and some clever aliens will eventually combine to produce tell-tale evidence that we are not alone. Andrew Masterson reports.
Fast forward 100 billion years and humanity – or perhaps another technically advanced species living on a planet in another galaxy entirely – will face a terrible problem.
By then, notes Dan Hooper of the US Fermi National Accelerator Laboratory and Centre for Particle Astrophysics, all the stars in the specified Local Group (which, in the case of humans, comprises the Milky Way and 53 other nearby-ish galaxies) will have succumbed to the accelerating force of dark energy and zoomed off a very long way, away.
They will have travelled so far, Hooper notes in a paper lodged on the preprint server arXiv, that they “fall beyond the cosmic horizon and become not only unobservable, but entirely inaccessible”.
For a sufficiently high-tech civilisation such an eventuality not only means the night sky will be very boring to look at, it also signals major problem.
It is assumed that any sufficiently developed civilisation – and given 100 billion years humanity itself might move beyond seeing Apple watches as the epitome of technological achievement – will eventually work out how to build Dyson spheres and thus derive its energy supply from one, two, or many encased stars.
That’s all well and good, but as the accelerating expansion of the universe drives more and more candidate stars forever out of reach, at some point any Dyson-dependent civilisation is going to hit a crunch-point.
Hooper, however, has worked out a solution to the problem – something for which his impossibly distant descendants may one day thank him.
The first thing to do, he notes, is for the civilisation to expand rapidly outward through its Local Group of galaxies. During this manoeuvre, intrepid outriders would undertake the massive construction task of building Dyson spheres, or similar devices, around some of the stars they encounter.
The energy then harnessed by the spheres could be used to propel the star itself away from the approaching cosmic horizon, back towards the centre of the local group.
Hooper estimates that this this method of harvesting stars and punting them into a new position could work over distances as big as “several tens” of megaparsecs. A single megaparsec measures 3.3 million light-years.
So far, so simple, but Hooper cautions that the expansion of the universe won’t be defeated by just encasing any old star. Pick the wrong one, he calculates, and it will be just a hiding to nowhere.
The ability to transport usable stars, he writes, is limited by two factors: the speed of the approaching cosmic horizon, and the lifespan of the stars themselves.
He warns: “If an encountered star is either of very low mass (and thus slow to accelerate), very high mass (and thus short lived), and/or very far away, it may be impractical to construct a Dyson Sphere with the goal of transporting it to the central civilisation.”
Hooper readily admits that his 100-billion-year timeframe is anthropocentric. The universe, he adds, is a very big place and it is thus entirely possible that for some other fantastically advanced civilisation a long way from the Milky Way the crunch-point has already been encountered.
This, he suggests, could be a very useful thing for Earth-bound astronomers because it suggests a new and novel way to look for signs of extraterrestrial life.
“Many of the past searches for Dyson Spheres have focused on detecting the presence of structures around individual stars within the Milky Way,” he writes.
“Here, we are instead considering galaxies and groups of galaxies in which many or most of the stars are surrounded by Dyson Spheres (and may have been removed from the galaxy), leading to very different observational strategies and signatures.”
A galaxy wherein usable stars have been encased in spheres, he notes, will look odd when observed because it will appear to be full of only massive stars – ones too big or too short-lived to be of use.
Also, he notes, stars that are travelling to a chosen central location may well be trailed by tell-tale propellant exhaust – although he admits that there is no way of knowing what such an fuel might be, or how, thus, to look for its spectral signature.
“That being said,” he adds, “such acceleration would necessarily require large amounts of energy and likely produce significant fluxes of electromagnetic radiation.”