Because of dark energy, our universe is expanding at an increasing rate. Over the next 100 billion or so years, that expansion will mean that any stars apart from those nearby will not only be impossible to see but also entirely unreachable.

That means that it will be difficult to extract energy from them. That is probably going to be very difficult for any advanced civilisations that rely on those stars to harvest that energy.

Now Dan Hooper, a senior scientist with Fermi National Accelerator Laboratory, has written a paper exploring what those civilisations might do in response to that event.

He claims that civilisations could expand quickly outwards, build huge structures around those stars, and use the power generated to direct them back towards the civilisation.

Such megastructures have long been seen as a potential mark of advanced civilisations. Though it is very difficult to imagine what a future alien race might do, it is likely they will search out ways of harnessing renewable energy, such as building power plants around stars – those structures, known as Dyson spheres, have never been seen but are the object of keen astrophysical investigation.

But the new paper makes clear that could only work for so long, and would become impossible once the universe filled with dark energy and the stars were too far apart. That would force them to reach out, grabbing stars before they become inaccessible.

"Given the inevitability of the encroaching horizon, any sufficiently advanced civilization that is determined to maximize its ability to utilize energy will expand throughout the universe, attempting to secure as many stars as possible before they become permanently inaccessible," the paper reads.

To make sure they didn't get lost, the civilisations would be able to move the stars closer to their home – with the hope that they would eventually become stuck in their own orbit, and would not stray too far away to become useful.

Having worked out what such activity might look like, the paper then aims to look at how we might be able to see if that activity is happening already. If it were, we would be able to look for stars that seem to be moving in an unexpected way, or to spot holes in galaxies where a star might normally be expected.

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