Fascinating. So this MOND theory works in this case. If I understand correctly, scientists still prefer the dark matter theory because it applies in more cases at galactic scale, and this is just one occurrence of the MOND theory working fine?
The biggest piece of evidence for DM is the BAO patterns in the CMB. Forget all the other numerous mountains of evidence, that is the biggest one. MOND has no good explanation for this without introducing something that's effectively DM.
DM = Dark Matter, BAO = Baryon Acoustic Oscillations, CMB = Cosmic Background Radiation, MOND = Modified Newtonian Dynamics
https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to...
MOND also predicted early galaxies, and a group seeking to disprove MOND by disproving EFE changed their mind because they found evidence of EFE.
> it fits the data best.
It's easy to fit the data when you can conjure a parameter to explain anything. What if I told you that GR is wrong and there's a ball of dark matter orbiting the sun that distorts Mercury's orbit.
You wouldn't be able to prove me wrong.
The claim that DM requires conjuring up parameters is completely baseless. There are one or two parameters (besides a handful other parameters from LambdaCDM) that determine the statistics of the DM distribution, and observations match well with simulations based on those parameters. There are small deviations called the dwarf galaxy problem and no one is inventing parameters to explain those, so what are you talking about?
And yes, baryonic distribution is absolutely a parameter, but it's not a free parameter (or it's a less free parameter) because it's value is constrained to a measurement that is orthogonal to the quantity inferred (light vs rotation curve). Meanwhile, dm density is a free parameter. It could be zero, or, 10x the baryonic mass, or anything in between.
> "DM theory needs to make predictions we can then test"
That is what scientists do: Start from the hypothesis of a dark matter dominated universe, from the beginning (or soon after the big bang), then turn on time and physics (gravity plus gas physics in a computer simulation), and galaxies form as gravity causes matter to clump together. The properties of those theoretical galaxies are testable predictions.
Even so, there are predictions low-parameter DM models seemingly can't make, like: what percentage of galaxies have zero dark matter?
You assert that it is impossible for a model of a dark matter dominated universe to "predict" galaxies without dark matter, but that is exactly what people are looking at here in a large scale cosmological simulation: https://arxiv.org/abs/2202.05836
That interpretation of GR assumes that Mercury isnt perturbed by some form of dark matter. Go back and redo all the equations with a dark matter that obeys the right rules before claiming that GR is a better explanation.
1. you observe a galaxy doing something strange
2. whatever it does, you add just enough dark matter to account for that behaviour.
Such process seems fishy, because it can explain ANY observation.
If we would go back 130 years ago where GM was not a thing yet, and GM would be competing with DM theory, occams razor would point to DM, because it is simpler one - it fits with newton nicely.
(not an expert though, just repeating stuff I heard on youtube; I’m happy that experts work on all kinds of angles)
I know you are already aware, but that is literally the entire premise of MOND -- the M is for "MOdified Newtonian Dynamics".
No one is doing that, though. What cosmologists do is parameterize the statistics of the DM distribution. That's one or two parameters. Then we compare observations to simulations to determine how likely the observed distribution is given the statistical properties. For example, a few galaxies with almost no dark matter would be expected due to the dynamics of clusters and galaxies. You could in principle calculate how often that should be the case, and if we were to observe it much more often than we should then there would be a problem with DM. No one is suggesting that the DM distribution can assume any arbitrary shape.
No, variation in galaxy properties is an output, not an input, of the model.
You could decide to quantify and catalog different galaxies with one or more parameters that describe their properties. You could then compare whether that catalog is statistically consistent with the output of the model (and must take into account all uncertainties in the model and the observations).
By analogy, you can measure that different people have different heights, but it does not mean that the specific height of each individual person is a unique input parameter in any fundamental model of biology.
That's what I mean by "it's a per-galaxy parameter". For each galaxy, to explain the behavior of that galaxy, you're saying "it must have X amount of dark matter".
The focus on "per-galaxy parameters" is like expecting to be able to predict how tall Tom Cruise should be after reading a textbook on the theory of evolution.
For example, see:
https://arxiv.org/abs/2001.10538
And they still fail to explain other observations which require additional arguments as to why the dark matter is once again distributed in just the right way to give the rotation curve... that is already successfully predicted by MOND in most cases with just one universal parameter.
Second, what makes you think there isn’t great fame in disproving dark matter?
I meant I can see why dark matter is appealing to more researchers in general and why MOND is in the minority.
If dark matter is right, we can find it.
If MOND is right, we’ve just been doing incomplete math this whole time.
One of those is way more exciting.