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In the far future, humans are struggling with the idea of whether or not to use dark energy's repulsive-gravity capabilities to surround Mars in a sphere of it (regardless of how, let's assume they have energy manipulation) to simulate Earth's gravity. Disregarding cost and more accessibility, would this be feasible?

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    $\begingroup$ Sorry for the confusion, but I think your original question was much closer to what you wanted to ask. You are interested in dark matter, not dark energy, as dark matter is what would add to the gravitational force. The mistake in your original question was the bit about "repulsive" gravity. You want the standard attractive gravity. You don't want dark energy. I've suggested an edit. $\endgroup$ – conman May 8 at 20:03
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    $\begingroup$ If they can magically manipulate energy with handwavium, they can presumably just directly control gravitons as an easier solution. $\endgroup$ – jdunlop May 8 at 20:13
  • $\begingroup$ I just realized that I may have misunderstood your question due to its ambiguity. You mentioned the repulsive force of dark matter. I assumed you meant the attractive force of dark matter (which would work well, hence my answer). But if you meant the repulsive force of dark energy (which I'm now thinking you meant), then that is a much less straight-forward question. $\endgroup$ – conman May 8 at 20:25
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    $\begingroup$ VTC OT:Unclear. Are you talking about this dark energy? You can never rely on everybody knowing what you're talking about. You need to bring the basics of a current theory (if that's what you're referring to) into your question. Links should only be used for supporting information. Is this a reality-check? If so, present a complete scenario for us to evaluate. Are you simply asking if a theoretical (read: fictitious) concept could be used the way you describe? Be very specific about what you're asking about. $\endgroup$ – JBH May 8 at 20:27
  • $\begingroup$ I removed the dark-matter tag since you just made that one up (and it no longer applies anyway). I added in one for mars and for gravity. If there's an existing tag for the type of energy, I'm not sure what it would be. $\endgroup$ – Cyn says make Monica whole May 8 at 20:33
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Dark energy does not matter on small scales.

This would not work because dark energy is quite insignificant on small scales - after all, there's a reason we don't see its effects on the motions of bodies in the Solar System or even on stars within the Milky Way. Its density is approximately $8.8\times10^{-30}\text{ g cm}^{-3}$ based on Planck data, which is tiny compared to the density of regular matter or energy in, say, a planet. In fact, a region of space the size of Mars would contain the equivalent of about a milligram and a half of dark energy. That's quite small.

There is expansion on human-sized (or planet-sized) scales, but it's small. In the absence of other gravitational effects, the universe expands according to Hubble's law: $$v=H_0d$$ where $H_0$ is the Hubble constant and $d$ is the distance to the other object. There is an ongoing (and exciting) dispute about the value of $H_0$, but it's thought to be in the vicinity of $68\text{ km s}^{-1}\text{ Mpc}^{-1}$, meaning that if $d=1\text{ Mpc}$, $v=68\text{ km s}^{-1}$. Even that is quite small compared to normal motions of planets, stars and galaxies. You need to go to distances of dozens or hundreds or megaparsecs before the expansion of the universe - or dark energy - becomes at all relevant.

Let's say you want to change all of that and assemble a ball of dark energy. For an order of magnitude estimate of how unfeasible this is, consider the difference in mass between Earth and Mars - about 0.89 Earth masses. Dividing this by the ambient dark matter density yields a volume of $6\times10^{56}\text{ cm}^3$ - encapsulated by a sphere 5.5 light-years in radius. You would have to sweep up all the dark matter in a 5.5 light-year radius to get an energy density comparable to the one you require.

Regardless of whether or not this is possible, it's highly inefficient.

There's also the slight large problem that dark energy will not increase the gravity of Mars, as its effect is repulsive.

Dark matter has its own problems.

There's been some confusion about whether you want dark matter or dark energy. Presumably, more dark matter would indeed increase Martian surface gravity to an Earth-like figure. However, you have more problems with that - some new, some old. First, the local dark matter density is tiny, roughly $10^{-19}\text{ g cm}^{-3}$. This is about ten orders of magnitude higher than the dark energy density, but small all the same. Again, dark matter is simply inefficient.

You have an even larger problem though: dark matter is collisionless, meaning it cannot form planetary- (or star-) sized structures. Clumping can happen on galactic scales via a number of different mechanisms, but not on Mars-sized scales. Any clump of dark matter of the size you want will dissipate.

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No, dark energy does not work that way as far as we know.

  1. The repulsive effects of dark energy are directly proportional to the distance between the two objects.
  2. The attractive effects of gravity are inversely proportional to the distance between the two objects.
  3. This means that the dark energy's repulsive "force" will only exactly counter the gravitational force at a very specific distance where the two forces are equal, and then only if the dark energy is actually between the two objects.

So even if you were to put Mars under a blanket of artificially concentrated dark energy it would act on the people on the surface in exactly the same way that it would act on the planet itself. So there would be no net change for the inhabitants.

P.S. You would actually do better if you did use dark matter and just concentrate it in the core of mars and have it just use the actual gravity of the dark matter to increase the gravity on mars. Or better yet, just add real mass to mars (it's probably easier).

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Careful what you wish for.

  1. You here assert dark energy has repulsive gravity. Mars is in a sphere of it. Presumably instead of pulling down on things from below as gravity seems to do, this effect would push down on things from above. Ok, cool. We are heavy again. Mmm.

  2. You have a lot of this repulsive gravity stuff, and it is all around Mars. It is enough to effectively increase the perception of gravity to that of Earth. Earth has 10x the mass of Mars. That means a push down from above of 90% earth's gravity.

  3. Inside the sphere on the surface of Mars, the vectors of Mars gravity and repulsive gravity are additive, combining to make us heavy. On the far side of the sphere there is repulsive gravity too. But now the repulsive gravity counteracts the gravity of Mars.

  4. Mars was counting on its gravity to keep going around the Sun. But Mars now has an effective mass of -90% that of Earth.

We are heavy down here on Mars, and it is getting dark. And cold.

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  • $\begingroup$ Unfortunately, the sphere of anti-gravity producing matter wouldn't even push down on people inside it. Inside a hollow sphere, the forces cancel out exactly at every point: the part of the sphere above you is closer but smaller than the part of the sphere that's below you, they exert equal force on you in opposite directions, you feel nothing. This happens for any force that drops off with the square of distance, like gravity, magnetism or electric charge. $\endgroup$ – Robyn May 8 at 22:16
  • $\begingroup$ @Robyn: I knew that about hollow earth; very cool to realize it would be true for hollow reverse gravity shell too. Antigravity dome it is! $\endgroup$ – Willk May 9 at 1:22
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Dark matter doesn’t have repulsive gravity — you may be thinking of dark energy, which is a property of space-time rather than a substance that you can put somewhere. Dark matter has attractive gravity, just like regular matter. Putting a lot of dark matter inside rather than around Mars would indeed increase its gravity, if there were any way of getting it there.

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    $\begingroup$ The OP may have used the dark-matter tag, but I'm 99% sure they're not asking about dark matter at all. $\endgroup$ – JBH May 8 at 20:29

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