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The vastness of the universe suggests advanced alien civilisations, or at least evidence of them, ought to be out there. The signs 鈥� such as megastructures and communications 鈥� should be obvious. Instead, astronomers are confronted with a silent universe beyond our own planet.

This 鈥渆erie silence鈥�, as cosmologist Paul Davies puts it, inspired the great physicist Enrico Fermi to ask: 鈥淲here are they?鈥�

Many have proposed solutions, but now another is on the table. What if aliens are out there after all, only they are sleeping, awaiting a glorious future when the universe is cooler, providing the right conditions for them to fulfil their ultimate ambitions?

According to this , alien civilisations emerged and flourished in the early universe, growing in size and developing technology to harness the energy from stars and galaxies. Eventually, after material expansion, these aliens would turn to more philosophical musings, pondering the big questions of the cosmos.

Addressing these will need considerable thinking, not only with their organic brains, but also with synthetic brains, like Deep Thought, the immense supercomputer built to ponder the ultimate question of life, the universe and everything, dreamed up by writer Douglas Adams.

Energy flow

But there鈥檚 a limitation in the thermodynamics of computation. Such immense computations require a flow of energy, from hot to cold, and this is ultimately limited by the background temperature of the universe. Early on this was a few tens of kelvin above absolute zero. But each degree of cooling creates potential for an immense quantity of additional computation.

Eventually, the aliens having grown tired of their material world and seeking to answer their big questions, decide to 鈥渁estivate鈥�, or literally slumber through the cosmic summer, waiting for the universe to cool to a point where conditons are right to perform their extraordinary calculations.

There is one troubling caveat though. This cooling is slow, taking many billions of years, and so, like a squirrel storing nuts for winter, our aliens would also have to prepare. While they sleep, stars will evolve and puff off their outer layers, and galaxies will collide.

To counter this dispersal of resources, they will need to engineer their environment to ensure that when they awaken, the matter and energy they need to thrive will still be on hand. That implies mega-engineering, which we have yet to see. The authors acknowledge this by saying aestivation would have to be 鈥渓argely invisible鈥�.

How seriously should we take this possible solution to Fermi鈥檚 paradox? It is little more than guesswork and speculation. There is nothing wrong with a bit of speculation, it can inspire new thinking and new solutions, but it should be taken with an appropriately sized pinch of salt.

Geraint Lewis is a professor of astrophysics at the University of Sydney, Australia