• Question: 73% of the mass of the universe is dark energy so why cant we harnece this energy and use it instead of fossil fuels?

    Asked by maximuspowers to Arttu, Ceri, James_M, Monica, Philip on 21 Jun 2011.
    • Photo: James M Monk

      James M Monk answered on 21 Jun 2011:

      We don’t even know what the dark energy is yet, so we’d need to know that for a start. More importantly, whatever the dark energy is, I would think it is already the vacuum energy of the Universe – the lowest energy level. So I’m not sure you could harness is because the Universe would already have decayed to a lower energy level (thus giving up the dark energy) if that were possible. If it *were* possible then I might want to be a bit careful about changing the vacuum state of the Universe. Dark energy also has the weird property of negative pressure, which I don’t fully understand, but is what causes the expansion of the Universe to accelerate.

      I did read once I think about a way of extracting energy using the Casimir effect and charged plates. If I have two metal plates (a bit like in a capacitor), then the space between them and either side of them is filled with virtual particles. Like a standing wave, there are only certain wavelengths that the virtual particles between the plates can have (because the wavefunction must be zero on the plate). On the other hand, there is no such limit on the wavefunction outside of the plates. This causes a pressure that can force the plates together. The force is tiny though – you would need enormous plates to get a decent amount of energy!

    • Photo: Arttu Rajantie

      Arttu Rajantie answered on 21 Jun 2011:

      It is actually not worth it. The total dark energy is quite high, but it is evenly spread over the whole universe, so the density is actually very low. If you calculate the total dark energy in a volume the size of the Earth, it is just 100 GJ. A typical nuclear power station produces that amount of energy in two minutes.

      As James says, if you still wanted to harness the dark energy, you would have to be able to take it out of the vacuum, leaving behind a state with a lower energy density (perhaps zero). It is not clear if such a state exists, and even if it does, extracting the energy would be very difficult.

      Furthermore, it would also be incredibly dangerous as a physicist called Sidney Coleman showed in 1977. If a large enough volume got somehow into this “true” vacuum state, it would trigger a chain reaction that would spread through the whole universe at the speed of light turning the whole universe into the “true” vacuum, and presumably destroying everything on its way.

      So, better to stick fossil fuels, then… 🙁

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