# Can dark energy be used for interstellar propulsion?

I wonder whether dark energy can be used for interstellar propulsion? For instance, will this setup theoretically work?

• Put a massive object far ahead of a spaceship.

• Connect it with a rope to the spaceship.

Now, due to space expansion the both spaceship and the heavy object experience force directed from each other. But the force is proportional to mass, so it is greater for the heavy object. So it will pull the spaceship via the rope against local stars in the spaceship's neighborhood.

A disadvantage is that this would require astronomically big distance between the object and the spaceship, and similarly long rope.

Are there other methods of using dark energy for propulsion?

You need to tether two galaxies together. It would require a mighty long tether. If you consider the expansion of the universe according to FLRW metric you get for flat space the Newtonian-like result I derived in this Stack Exchange post. The energy equation $$\left(\frac{\dot a}{a}\right)^2 = H^2 = \frac{8\pi G\rho}{3}.$$ can be used to extract energy. The change in scale parameter $\dot a$ on a tether that is a huge solonoid with a magnet means as a magnetic is moved through the solonoid the magnetic flux variation through solonoid windings is proportional to $\dot a$. Some form of magnetic induction system could be arranged to extract energy.

It's theoretically a good idea, and the great advantage ( for me :) in being virtually clueless about cosmology is that I can guess wildly.

If space expands, I believe your rope, astronomically long as you say, (I appreciate you don't literally mean a rope, I hope) or any other mechanically connected system, will also stretch at the same rate, leaving you with no effective pulling power.

As far as other methods are concerned, unless we know, even in vague terms, what dark energy actually is, I will have to leave to those people more experienced to suggest realistic means of harnessing it.

You might extract energy, or pull the two bodies closer or father apart, but you would not be getting closer to anything else. Everything is expanding away from you and your tethered big mass. Remember the dark energy is just another way to talk about expansion and acceleration of it (until we actually know anything about the actual physics of dark energy), and in cosmological expansion everything goes away from everything else.

• So it cannot be used as a reaction medium to transfer some momentum to it? No way? – Anixx Dec 10 '16 at 8:05
• I don't see how, but I don't think my argument is a full proof. Just from the expansion and acceleration it seems not, but feel free to come up with a way. – Bob Bee Dec 10 '16 at 19:46
• should not dark energy produce particles from vacuum by breaking virtual particle pairs? Also, I have heard about de Sitter radiation (that is very tiny but nonetheless...). If we can use CMB for propulsion (can we actually?), we should be able to use de Sitter radiation as well. – Anixx Dec 10 '16 at 21:35
• Not dark energy. de Sitter radiation is like Hawking radiation, but coming from the de Sitter horizon. It is a very small effect, like Hawking's. Yes, close to that horizon it'll push you away a little. Not clear how that helps in intergalactic propulsion. It is not dark energy that breaks virtual pairs (virtual pairs is an equivalent way to look at tunneling of the radiation across horizons). None of that has to do with dark energy. It does have to do with the strength of the gravitational field and only happens from horizons, not just anywhere. – Bob Bee Dec 11 '16 at 1:56
• As for CMB, it is isotropic, from all directions. How would you use it for propulsion? – Bob Bee Dec 11 '16 at 1:58

I only base my answer on Hubble Law. On this we can say that if we rigidly tether our vehicle to a very far massive object, let us say a galaxy, it will drive us past the galaxies that were in between at start.