Is dark matter's main rival theory dead?
theconversation.com
theconversation.com
The basic idea here, is that the reference frames used to measure galaxy rotation curves may not be inertial. that is, they might be influenced by rotational or other accelerative forces due to the galaxies' motion in the universe. Traditionally, it's assumed these frames are inertial, meaning they are free from such influences, which simplifies calculations but might overlook significant effects, and that's the key point that has been missed and is addressed by the paper, showing that you can't overlook those effects.
if we consider these non-inertial effects akin to acknowledging that galaxies might not be moving in straightforward, stable paths but could be "flying around" each other or a common center, then we can account for the observed rotation curves differently. They suggest that the forces arising from these complex motions could mimic the effects attributed to dark matter.
AFAIK, this has not been rigorously checked in the past. However, there are several other phenomenon that are explained by dark matter / dark energy besides this discrepency, so people in the field are not very eager to give up on dark matter just yet
So the math would be correct for a single galaxy, but this cannot happen for all galaxies at the same time.
What is interesting though, that they basically find a model with a single additional parameter per galaxy (angular velocity omega) that fits most of the rotation curves very well. However the most simple explanation how we get this omega is not by assuming fictitious forces from rotation frames, but simply real forces from dark matter.
We see some galaxies whose gravity is near the baryonic prediction and others where it is not.
- explaining this with dark matter is easy: some galaxies have a lot of dark matter and some have little. - explaining this with Mond seems impossible. If there is some gravity law we don't understand, how can it have different effects on different galaxies?
- how do we explain the famous bullet cluster finding?
Well, attributing it to dark matter is easy. It's not really explaining anything though, until you can say what dark matter is and why it's there. At this point it's basically just an abstract parameter that we assume, without any direct evidence, is in fact some kind of matter and not, well, something else. Maybe spacetime just have different geometry in different places? Maybe the assumption that it would be flat in the absence of matter is wrong? It could very well be that the neutral shape of spacetime in some places is more or less curved than we currently assume.
Then there are fun ideas like large masses permanently deforming spacetime. Going with the famous rubber mat analogy, what if the mat gets permanently stretched and even once the mass that stretched it is dispersed the sag remains?
That's of course assuming there even is such a thing as spacetime. Which, btw, sounds like a reasonable assumption. However, just because the math works doesn't mean it's a physical reality, it merely points to it.
There are rogue blackholes, maybe they leave spacetime deformations behind them that could be observed?
I wonder if that can explain some of the large scale filament type structures we see.
Hence: dark matter - because the effect we observe seems to behave that way. MOND is distinct because it declares what we're seeing is not a bunch of entities with that character.
It's worth noting we also talk about a lot of things like this even though strictly speaking they're not: i.e. electron-holes in semiconductors can be perfectly well modeled as their own form of matter - they can't just leave the semiconductor environment. But they do have momentum and position.
Dark matter could literally be the same type of effect, but if you're a wrinkle in space-time then it doesn't matter unless that description has some sort of experimental reality, and we currently do not observe it to do so. It is not like physicists would not rush to test any experimental formulation of dark matter as such that was accessible with current or near-future instruments.
That's the problem. It lumps unexplained phenomena into a luminiferous aether 2.0. The most likely explanation is our model of gravity isn't yet good enough and so perhaps RelMOND is worth a look.
It doesn't mention the Bullet Cluster though.
There are a few different theories on what dark matter is; there's axions, supersymmetry (from string theory), cold dark matter and more.