[orclev]

Another ELI5 by someone not at all qualified to understand this stuff (I.E. a layman).

We know from measurements of the leftover energy (CMB) approximately how long ago the big bang happened, and how fast it appears to be expanding. Our most favored model of the universes physics, General Relativity makes certain predictions that mostly match up with reality up until you get to galaxy scales at which point they start to diverge. In order for our measurements to work under General Relativity our galaxies need to be more massive than they appear to be based on all the stuff we can actually see in them. This needed excess mass is called dark matter, but even with dark matter the universe appears to be expanding faster than it should. The theory proposed in the paper is that the universe as whole isn't actually expanding faster, but due to a quirk of where we are in the universe it only looks like it is when we look at nearby galaxies. Unfortunately for that to be true we would need to be in a void in the structure of the universe which General Relativity predicts shouldn't be possible.

An alternative theory of universal physics exists called MOND. MOND is similar to General Relativity, but rather than solving the problem at the galaxy scale through theoretical dark matter, it instead just assumes that gravity works differently once you reach a certain cutoff point. This aligns with observations of actual galaxies (not entirely unsurprisingly because the cutoff point was chosen in order to align with those observations) without needing dark matter to exist. From the perspective of the paper there's another nice property of MOND which is that simulations based on it allow for the kind of void to form that the paper predicts would be necessary to explain the locally observed expansion.

Basically, General Relativity can't explain how fast galaxies spin without Dark Matter, nor how fast the universe appears to be expanding. MOND combined with our galaxy being in the middle of a big void can explain both. Both theories, General Relativity and MOND require a certain amount of hand waving in order to align with reality. MOND requires a bit less but is highly suspect because it's solution is basically "gravity just acts different sometimes" which is suspiciously close to "it's that way because it is".

As for the actual math involved in all of this, beats me, we'll need to wait for someone who's actually in this field to look it over and explain what if anything is wrong with it all.

[marcus_holmes]

So General Relativity is a theoretical framework that has been proven to match observation[0], while MOND's details have been chosen to match observation, without any theoretical basis? Is that right? Or is there some mechanism proposed as to why gravity's cutoff point is where it is?

[0]up to a certain point, and if you include Dark Matter, which we still don't understand (the original assumption was that it was WIMPs, but as we still haven't found a WIMP that would do this, then we don't know what it is).

[orclev]

So, first thing, I'm not a astrophysicist nor even a physicist of any type, so I might be misunderstanding things here, but this is how I interpret all this. Hopefully if I've gotten something significantly wrong someone will correct it.

General Relativity matches observations to a point. The issue is that it stops matching observations once you reach galactic scales. In order to explain why that doesn't work you need to start hand waving, and the start of that is dark matter. MOND was thought up not so much as an alternative to General Relativity but as an alternative to dark matter. It tweaks some of the math used in General Relativity to assume that gravity behaves differently at different levels. Basically once you have a strong enough gravitational field it behaves like the gravity we know, but until you hit that point its effects diminish at a different rate. Doing that explains why galaxies behave like they do. For the bulk of the galaxy gravity is strong enough that it behaves exactly like General Relativity says it should, but out near the edges of the galaxy gravity has grown weak enough that it behaves differently. It's sort of hand wavy and leaves a bit of a bad taste in the mouth since there's no real explanation of why gravity should behave that way. On the other hand it doesn't require some phantom matter that we have no observational data to back up.

Either theory falls far short, and both of them require a lot of fudging around the edges to align with galactic scale observations, although MOND once you get past the arbitrary change to gravity seems to require less hand waving. Importantly for the linked paper it also seems to line up with the proposed theory and predict the kind of void the paper is predicated on which would be a strong point in favor of MOND.

Of key point to the proposed theory, General Relativity predicts that in the first moments after the big bang that the universe was essentially uniform, that everything spread out more or less evenly, and it wasn't until much later when things started to form the likes of planets and stars that we started seeing significant variation in matter distribution of the universe. MOND on the other hand allows for variation in that initial expansion. That's important for the paper because there simply isn't enough time in the General Relativity model to explain a void the size that their theory predicts would be necessary to form. MOND allowing for more variability early on on the other hand does allow enough time that a void of the necessary size could exist.

Basically General Relativity on its own doesn't work for things galaxy size and bigger. MOND on its own doesn't work at galactic cluster levels and above. The theory proposed in the paper could explain the discrepancy we see in the rate of expansion of the universe, but doesn't seem to be possible under General Relativity, but is possible under MOND. Both General Relativity and MOND rely on the presence of things not observed yet in order to match with our observations once you scale things back far enough, and neither on its own can explain why the universe seems to be expanding faster than they predict it should. The paper proposes one theory for that, but it's only possible with MOND.

[nn3]

As far as I understand the problem with dark matter is also that there isn't a single dark matter theory, but lots of different ones with a lot of very variable tunables. So the problem is that dark matter by itself isn't that predictive.

[orclev]

The term dark matter itself is kind of misleading in the first place. The math in general relativity just doesn't add up when applied to certain observations. They've essentially gotten 2 + 2 = 5. In order to fix that they just assume that one of those 2's was actually a 3 somehow, and the extra 1 it picked up got labelled as dark matter. In other words, dark matter is just a term for some missing numbers somewhere in the calculation. Based on the different places where the extra numbers might be included they think it's something with mass, but really that's just a guess based on the existing formula and the changes that would be necessary to make it match the observation.

[marcus_holmes]

thanks :)

[SCHiM]

Isn't there also observational data that disfavors MOND?

I remember it had something to do with galaxies colliding and the dark matter staying behind or something along those lines.

[orclev]

Not sure honestly. That might have something to do with the vHDM theory mentioned in the paper. I didn't really follow a lot of it, but I think (major grain of salt here) it's saying that that theory predicts some very light neutrinos exist and tend to collect inside of galactic clusters, but not galaxies themselves and that it's those neutrinos that make up the missing mass at universal scales that general relativity explains using dark matter.

Either case seems pretty hand wavy honestly. When it comes to galaxy and universe level physics it all seems pretty weak compared to the sort of particle physics and classical physics that we can actually measure and test on Earth. It's all just a bunch of theoretical math with relatively few actual measurements to pin it all down. I don't think we're anywhere near having a solid theory of the universe so it's mostly an exercise in trying to prove which theory is the least wrong at this point, rather than which one is correct.

[dnautics]

The bullet cluster is an n of one and the dark matter estimates come from redshifting/lensing, which could have come from other sources in such an unusual scenario.

Some of the other crazy observations like ultradiffuse galaxies are, on recalculation, not as extreme as initially guessed, and predicated on an indirect empirical estimate of mass (number of globular clusters) with no mechanistic confirmation.