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If gravity is repulsive at long distances, would this suffice in explaining "dark energy"? Was there any reason why a new concept (dark energy) was introduced rather than just assuming that gravity becomes repulsive over long distances?

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    $\begingroup$ The cosmological constant aka "dark energy" makes gravity sort of "repulsive" at long distances... it's been part of the theory since the early days, we just didn't know what to use it for. Now that the measurements are indicating that we need it, we are using it. Whether that's the correct answer is still up for debate. $\endgroup$ – CuriousOne Jun 28 '16 at 23:38
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    $\begingroup$ Such an assertion would be somewhat difficult to prove. @CuriousOne , do you have any insight on what measurement or evidence would provide such proof? $\endgroup$ – David White Jun 28 '16 at 23:59
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    $\begingroup$ Variation of this theory have been proposed in the past. They mostly go under the moniker Modified Newtonian Dynamics (MoND). They were never particularly popular and have been mostly been put aside under the combined weight of difficulty making them covariant and the gravitational micro-lensing data from the bullet cluster. $\endgroup$ – dmckee Jun 29 '16 at 0:08
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    $\begingroup$ @DavidWhite: Prove? Well... cosmology with cosmological constant seems to be consistent with the acceleration data, at the moment. Would I consider that strong evidence for anything except that we don't have a clue what's going on, at this moment? Probably not, but that's just my personal opinion about the current state of cosmology. I don't know what precision observations can or will reveal in the future. Maybe we will need "more", maybe we won't. $\endgroup$ – CuriousOne Jun 29 '16 at 0:19
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    $\begingroup$ @CuriousOne , the cosmology constant strongly reminds me of the numerous "fudge factors" that are sprinkled throughout chemical engineering curricula. The fudge factor works, or it wouldn't be useful. Whether or not that fudge factor can be tied into some kind of "first principles" formulation is what I am asking about. $\endgroup$ – David White Jun 29 '16 at 2:27
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Gravity, per general relativity (GR), is normally attractive. Normally means that the sources of the gravity, and thus the sources that determine the geometry and curvature of spacetime, have positive energy density, and obey other positive energy conditions. The pressure and other factors that enter into the stress energy tensor that is the source of the curvature of spacetime can, however, be negative for so called exotic matter, or exotic fields. In those cases those factors contribute to a repulsive effect in gravity.

But it is not repulsive then when far away, it is then repulsive everywhere. The negative pressure contributes everywhere, not just at a distance.

That is a somewhat simplistic description of what happens. Let's try to be more specific. To be a little more accurate, GR is described by an equation that reads, in non-mathematical terms,

Einstein Tensor = (Cosmological constant term) X Curvature Scalar X metric tensor + constant X Stress Energy tensor

The left hand side represents the curvature while the right hand side is the stress energy tensor representing matter and energy, and the cosmological term. The cosmological term is said to represent the dark energy.

Nobody know what the dark energy is, it's a constant value of dark energy per unit volume, and it is very very small. It seems to be constant, measurements indicate no variation on what is observed further away than closer in (i.e., from radiation emitted a long time ago or more recent). The hypothesis is that it is some exotic quantum field, similar to the field that caused the inflation soon after the Big Bang, but different, with negative pressure, causing the accelerated expansion of the universe. Some theorize that it might vary some over space and time (theory called quintessence, appropriately), but none of that has been observed. Fact is that we don't know, but if it is due to a quantum field it would be an exotic one. Others theorize that GR needs to be modified, but none of the modifications proposed have found any evidence in their favor, so far.

It is worth noting that the cosmological term is so small that it has in essence no effect on gravity's effect for anything except cosmological distances. Black Holes form everywhere in the universe, and that little repulsive effect makes no difference.

The reason it makes a difference cosmologically is that if the term is a constant energy and repulsive pressure per unit volume, as it seems to be from observations, as the universe and thus the volume expands, there is more of that energy and negative pressure. When it becomes large enough it becomes more of an effect than that of positive energy matter, pressure, and energy. That is what's happened in the universe, where now dark energy contributes 68% of the total energy content of the universe, and so dominates. As a result we are accelerating in the expansion. But it is NOT an effect ONLY a long distance away, it is everywhere in spacetime. 10-12 billion years ago, and until about 5 billion years ago, matter dominated, everywhere. Farther back in time radiation energy dominated, again everywhere.

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There are a few problems I can think of with this idea -

  1. Gravity has to, at some distance turn from positive, to zero, to negative. It would be interesting to know that distance.

  2. Will the repulsion increase, or decrease with increasing distance? Dark energy hypothesis indicates repulsion would go up with increasing distance. Which does not make sense - what would make the repulsion increase on the way? If something (say space) causes it to increase, then it is not gravity in true sense, because gravity is caused by mass.

  3. Gravity is understood as curvature of space(time), caused by mass. How mass is expected to cause a reverse curvature at larger distances? that even, increasing with distance. Meaning no limit, the repulsion can be infinite at infinite distances. We can be positive that there is some mass at infinite distances from every mass in the universe. That means every mass should be repulsed with infinitely large strength.

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  • $\begingroup$ This answer isn't helpful because it doesn't present relevant data $\endgroup$ – D J Sims Jun 29 '16 at 5:14
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    $\begingroup$ This answer basically says "the OP's idea does not bode well with GR". Discarding a competing theory/hypothesis just because it doesn't fit in your theory's framework is not good science. $\endgroup$ – Martin Argerami Jun 29 '16 at 5:17
  • $\begingroup$ These are simply the problems with idea of "gravitational repulsion at large distances". The idea needs to fit GR, otherwise, you need to redefine a lot of gravity stuff at short distances. $\endgroup$ – kpv Jun 29 '16 at 5:26
  • $\begingroup$ @kpv. 1) Do the math. Lambda is always repulsive, the rest attractive, figure out where. For cosmology it's irrelevant because distance in GR, from what? It's everywhere, the FLRW solution shows you the values of the matter density attracting vs lambda repulsing, and when dark energy became dominant. 2)No, you are wrong. Dark energy as used and calculated is constant density always. The Planck CMB gave data that it is not varying, within the uncertainties. 3) No again, you are wrong. It is what the equation solution shows. EXTRA) read and on the model, you're way off. Dodelson is good. B $\endgroup$ – Bob Bee Sep 5 '16 at 1:24
  • $\begingroup$ @BobBee: The difference is you are thinking in mathematics and I am thinking in common sense. Mathematics can be absolutely perfectly correct, or, it can be totally inapplicable to a situation. With the confidence you have said "you are wrong", It seems there are no doubts about what dark energy is and whether there is repulsion, and what causes it etc. I do not think that is the case. I mostly respond to questions where no agreed answered in the scientific community and these topics are still subject of speculation. I would not stand in front of you if it is a well settled subject matter. $\endgroup$ – kpv Sep 5 '16 at 2:04

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