The Quiet Galaxy

Explanations · They are there and we cannot see them

Aestivation: advanced civilisations are asleep, waiting for the universe to cool so that their thinking will go further on the same energy

Proposed by Anders Sandberg, Stuart Armstrong and Milan Ćirković, 2016 fc smaller

Some animals sleep through the summer and wake when conditions improve. The aestivation hypothesis says that the oldest civilisations may be doing the same on a cosmic scale. Every act of computing has an energy cost that depends on the temperature of the surroundings, and the universe is cooling as it expands. A civilisation that cares mostly about how much thinking it can do with the energy it has gathered would get vastly more done by waiting trillions of years until the universe is cold, and then running everything at once. On this view the galaxy is quiet because its inhabitants have banked their resources and gone to sleep, leaving caretakers to stop anyone else from spending the inheritance.

The three authors, all of them affiliated with Oxford's Future of Humanity Institute, published the idea in 2016. It starts from a result in physics known as Landauer's principle: erasing a single bit of information, which any computation must do, releases a minimum amount of heat, and that minimum is proportional to the temperature of the environment. The cosmic background temperature today is a little under three degrees above absolute zero. As the universe expands it will keep falling, and in a very distant future it could be lower by a factor of ten to the thirty or more. For a civilisation whose goal is to maximise the total amount of computation it ever performs, whether to think, to build simulated worlds, or to do whatever such minds care about, the same stock of energy spent then would buy an enormously larger amount of thought than it would now.

The rational strategy for such a civilisation, the authors argued, is to gather resources early, while stars are shining and matter is easy to come by, then switch off and wait. During the wait it would need to protect its stock from being used up by others, which could explain the absence of visible expansion: not an empty galaxy but one whose owners are present, dormant, and would act only to prevent waste. The authors suggested a few things one might look for: the galaxy should have little sign of ongoing large-scale engineering, and processes that squander resources on cosmic scales, such as stars burning out in ways that could have been prevented, should be rarer than expected if a dormant civilisation were tidying up.

The bearing on the Drake Equation is on the share of intelligent civilisations that are detectable, which this view lowers to nearly zero for the oldest and most capable ones, precisely because they are capable. It leaves the earlier factors untouched and, like the zoo hypothesis and the dark forest, belongs to the family of explanations in which the galaxy may be full and chooses to look empty.

The main objection came in 2019 from the physicist Charles Bennett, Robin Hanson and Jess Riedel, who argued that the energy accounting is mistaken. The original argument assumes that the heat given off by erasing information can only be dumped into the cosmic background, so a colder background is worth waiting for. Bennett and his co-authors answered that the universe today already holds places that can soak up that waste heat at any time, so a civilisation has no reason to wait, and that the original paper gave no worked-out model of the effect it claimed. In their comment they thank Anders Sandberg for confirming their summary of the argument, but say it should not be read as endorsed by the original authors. A second objection is the one that applies to the whole family: sleeping civilisations that nevertheless guard their resources would still have to prevent newcomers from spending them, and there is no sign anyone is preventing us.

What it says about the equation

Every explanation is a claim about one or more of the seven pieces that are multiplied to give the number of detectable civilisations. This one bears on 1 piece:

The test: what would strengthen or weaken it

Would strengthen it: Evidence that large-scale wasteful processes in the universe are being suppressed, such as an unexplained shortage of certain kinds of stellar death; discovery of dormant artificial structures; a resolution of the energy-accounting debate in the original authors' favour.

Would weaken it: Detection of an old civilisation that is active and expanding now; confirmation that the computational advantage of waiting is small or negative; evidence that the universe's resources are being consumed freely without interference.

These two lines are what the record is read against when a new paper or discovery is logged. A result of the kind named here, published with its evidence, is what would settle it; a press claim on its own settles nothing.

The last reading

Once a week the records are read against the news, each against the test above. Open a line to see what that week held. Every reading ever made of this record is on the reading page.

  • 2026-10-16 Logged, no change By hand

    No evidence of suppressed stellar death or dormant structures. Garrett (2026-08-31) works out what the waste heat of a civilisation computing in the cold would look like and proposes a submillimetre search for it, noting that such civilisations may instead be asleep; that is the first concrete observational test near this explanation since the 2017 paper. Logged.

    Garrett, 2026 source

How seriously it is taken

A recent and much-discussed idea from serious researchers, admired for making a definite physical argument; its energy argument was challenged in print in 2019 by authors who argue that waiting for a colder universe gains nothing, leaving the hypothesis as an interesting possibility rather than a strong contender.

The papers that bear on it

Books, films and other work that take it up

Read next

Notes

Checked 2026-10-16. Bibliographic details borne out: Journal of the British Interplanetary Society 69(11), 406-415, 1 November 2016 (Oxford University Research Archive record, https://ora.ox.ac.uk/objects/uuid:2836b807-21ce-431a-9271-289e837f10be); arXiv:1705.03394 posted 2017-04-27; the paper's own figure is a 10 to the 30 gain in achievable computation, set against a present background of about 3 kelvin (https://arxiv.org/abs/1705.03394); NASA's COBE measurement gives 2.725 kelvin today (https://lambda.gsfc.nasa.gov/product/cobe/about_firas.html). The 2019 comment is by Bennett, Hanson and Riedel, Foundations of Physics 49, 820-829 (arXiv:1902.06730). Corrected: the page had summarised the comment's points wrongly (it said the results could be computed now and stored, that the advantage of waiting was merely smaller, and that waiting forgoes the energy of stars; the comment says instead that waste heat can be dumped into existing reservoirs now, so there is no advantage in waiting), had said the original authors published a reply (none was found; the comment says only that Sandberg confirmed its summary), and had said two of the three authors were at the Future of Humanity Institute (the paper lists all three). The standing line no longer says most physicists regard the challenge as correct, which no source supports.

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