Who needs dark matter when you can have... negative matter !

"This implies that our Universe is just one of those things that happen on occasion..."

That's one of the memorable lines in this very interesting paper that tries to replace conventional dark matter with something more exotic : negative matter. The other wonderful line is the extremely astute remark, "Observations clearly indicate that the Universe is not empty." That's one thing we can surely all agree on. But is regular dark matter too mainstream for you ? Is doing away with dark matter too contrarian ? Then this is the paper for you !

The press release was quite interesting but left me very puzzled so I decided to read the paper, even though this kind of thing is well outside my area. Here's my take on it for whatever that's worth.

I get the distinct impression that the author was having great fun writing this. The overall tone is - by the standards of academic research at any rate - frivolous and playful. I doubt it's supposed to be taken entirely seriously; he's clearly not saying that he's definitely overturned all of physics. Taken as a curious piece of speculation I wholeheartedly approve - taken as anything more than that and the guy's a nutter. Which is a nice summary of the some of the very best papers, really.

Anyway, the key point is figure 1, which illustrates how matter with negative mass accelerates due to gravity. As everyone's familiar with, normal (positive) matter accelerates towards other normal matter under gravity. Negative matter would accelerate away from itself. Weird, but okay. That might leave you wondering how in the world negative matter could possibly be a substitute for dark matter : observations indicate the presence of large amounts of unseen positive mass. How could stuff which tends to push itself apart possibly be a substitute for stuff that's supposed to be pulling everything together ?

Well, where it gets really strange is the interaction between positive and negative matter. Positive matter would accelerate away from negative matter, but negative matter would accelerate towards positive matter. Stick a mass of negative matter at the end of an equal mass of positive matter in free space and in principle they both accelerate forever, eventually reaching - says Farnes - lightspeed. He's quite explicit about equalling lightspeed, not just merely coming close to it. Conservation of energy is not violated; I'm guessing (but only guessing !) because a moving negative mass has negative kinetic energy. He notes that such claims have been labelled as "preposterous", however.

My guess is that if you have negative matter instead of positive matter, you don't need an equal amount of it to reproduce the effects of "ordinary" dark matter. It only has to provide that extra push to reproduce the apparent effects of unseen positive mass (e.g. galaxy rotation and motions in clusters). In very hand-waving terms I can sort-of see how negative matter could then act as a replacement for conventional, positive-mass dark matter : it wouldn't be a direct substitution.

A lot of the paper describes how this model can unify dark matter and dark energy. The latter is much easier to see (though I won't for a second claim to understand the maths) : if you've got some substance pushing everything apart, then sure, that could drive the acceleration of the expansion of the Universe. It's more difficult to understand how negative matter could be responsible for the evidence typically interpreted as large amounts of missing positive matter, at least in an intuitive way.

Farnes presents code, simulations and analytic formulae to address all of the major underlying objections one might raise. He notes that negative mass may or may not equate with antimatter; at least in principle the two need not annihilate each other. He shows that structures can form in a negative matter-dominated universe, even more rapidly, in face, than in one dominated by positive matter. Runaway accelerating particles do not occur, presumably because random motions dominate (also gravity is a very weak force). And he demonstrates how negative mass may lead to flat rotation curves, although to be honest I didn't really understand how that works.

He doesn't address gravitational lensing though (God knows how that would work with negative mass) or say much about the CMB power spectrum, which IIRC has been interpreted as very strong evidence for dark matter independent of galaxy rotation curves. Also, this theory requires a continuous creation of negative matter.

So as I said, don't get too excited. It's an interesting bit of speculation but it's also bloody weird. To mind mind a constant amount of missing positive mass, plus some other, unrelated component for dark energy, still feels like a simpler explanation. I hope there will be more serious and detailed responses from more knowledgeable people. It is Christmas, after all...

I haven't tried the code provided but I hope someone will.
https://arxiv.org/abs/1712.07962