I found an interesting article by Edward Harrison [1] who proposed a way to harness energy from spacetime expansion by attaching a string to a receding cosmic object (like a galaxy). ​ However, one could not extract unlimited energy as the string would break once the object goes beyond the Hubble sphere (Similar to how a rope would break if we let the attached object fall into the event horizon of a black hole). ​

I was thinking that perhaps one could avoid the problem by attaching a string to an object, let the string unwind to get as much energy as we can from the receding object until it reaches the Hubble length, then use part of the energy that we obtained from the unwinding string to create a new object with the same mass and at the same distance as the previous one and repeat the process indefinitely. But I have been told that the energy that you would gain due to the tension of the string attached to the receding object upon reaching the cosmological horizon would be the same to the required for creating a new object with the same mass.

Also, I am not sure if the energy you get is lower than the predicted due to gravitational redshift, i.e. the same way this paradox is resolved.​

Perhaps one way to avoid this would be to attach the string to a star, so even if the energy we would get from it due to the Hubble flow would be the same amount needed to make a new star at the same distance as the previous one initially, we would gain energy from the star itself as heat and brightness. Would this work? Or would energy be conserved somehow? ​

​ [1]: https://adsabs.harvard.edu/full/1995ApJ...446...63H

  • $\begingroup$ Interesting idea. I think the civilization that could do this would have far easier and more plentiful ways to harvest energy, such as Dyson Spheres, Penrose Process machines around black holes, or directly converting the mass energy of matter. Also, it's not obvious what your question is. $\endgroup$
    – RC_23
    Sep 5, 2022 at 21:27
  • $\begingroup$ The star you are attaching the string to would have to be outside our galaxy or else it wouldn't be participating in the Hubble flow. $\endgroup$
    – D. Halsey
    Sep 6, 2022 at 0:49
  • $\begingroup$ @D.Halsey Of course, but this is a gedankenexperiment, so I assume that the star is at enough distance from use to be affected by the Hubble flow $\endgroup$
    – vengaq
    Sep 6, 2022 at 0:53
  • $\begingroup$ It probably will not work as iopscience.iop.org/article/10.1086/509250/meta suggested that the redshifted star by dark energy will have less dense core hence become colder, result in energy loss $\endgroup$
    – Secret
    Sep 6, 2022 at 2:33
  • $\begingroup$ @Secret The energy that you would get by the unwinding string mechanism of the star would be the same as the energy needed to create another star. There, we would not gain or lose energy, it would remain constant. However, the extra input of energy from the star itself, would be extra energy, even if the star loses some energy due to redshift. So, wouldn't that be adding energy to the system? $\endgroup$
    – vengaq
    Sep 7, 2022 at 12:44

1 Answer 1


This should be equivalent to the question of whether you can do an infinite computational process, and that has been studied previously:

Dyson, Time without end: Physics and biology in an open universe, Reviews of Modern Physics 51 (1979), pp. 447–460, doi:10.1103/RevModPhys.51.447.

Krauss and Starkman, 1999, Life, The Universe, and Nothing: Life and Death in an Ever-Expanding Universe, http://arxiv.org/abs/astro-ph/9902189

Katherine Freese and William Kinney, 2002, The ultimate fate of life in an accelerating universe, http://www.arxiv.org/abs/astro-ph/0205279

For current standard cosmological models, I think the result is that you can't do an infinite computation. If you could extract an infinite amount of mechanical work from cosmological expansion, then surely you could do an infinite computation.

Since the result is different for zero versus nonzero cosmological constant, I don't think you're going to get a valid answer through any thought-experiment argument that doesn't take that into account.


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