[throwawaymaths]

DM can't explain renzo's rule, or the tully-fisher relationship, or why the milky way has a keplerian return (efe from the magellanic clouds), or why elliptical and lenticular galaxies don't seem to have dark matter. All these are explainable by MOND.

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.

[isthatafact]

>> It's easy to fit the data when you can conjure a parameter to explain anything.

I know you are already aware, but that is literally the entire premise of MOND -- the M is for "MOdified Newtonian Dynamics".

[AnimalMuppet]

Not the same. MOND adds a parameter - the non-newtonianness which is purely a function of the masses and the distances. DM lets you add a new parameter (the DM density) at each point in space. That's effectively an infinite number of parameters, whereas MOND has very few.

[mr_mitm]

> DM lets you add a new parameter (the DM density) at each point in space. That's effectively an infinite number of parameters, whereas MOND has very few.

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.

[AnimalMuppet]

That's at the universe-sized level. At the galaxy level, as you state, we say "oh, that galaxy has almost no dark matter". That's a per-galaxy parameter. At the Bullet Cluster, we say "the dark matter must be here and here". That's a point-by-point distribution.

[isthatafact]

> "That's a per-galaxy parameter."

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.

[AnimalMuppet]

Let me change your analogy. You take each person, and measure their height. You also "measure" how tall they "should be". You then show that the differences between their actual height and the height they should have had fits a model. That's nice, but for each person, you still assigned a value for the difference between how tall they are and how tall they should have been.

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".

[mr_mitm]

I wouldn't have to prove you wrong. If your stance is that Mercury's perihelion shift is explained by DM, then I'll counter that GR explains that, gravitational lensing, gravitational waves, black holes, the CMB, the helium abundance, and then some. By Occam's razor, GR would be the preferable theory.

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?

[falserum]

Process is (laymans understanding):

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)

[throwawaymaths]

There is a parameter per galaxy, and it can be wildly different. Some galaxies have no dm, some galaxies are "only dm". Hardly "small deviations"

[mr_mitm]

Those are measurements, not parameters. Just like the exact baryonic matter distribution is not a parameter of GR. You have an initial matter distribution, which is a random sample of a probability distribution (that is a part of the model) and then it starts clumping together over time.

[throwawaymaths]

You're wrong. They are parameters if you're using them to bestfit another value (rotation curves). This is basic high school science/stats.

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.

[cryptonector]

They're not measurements. The measurements are rotation rates at various distances from the galactic center, then you plug that into a model and the model tells you where the DM is in that galaxy, then you say "DM explains it all for this galaxy!", but no, the amount and distribution of the DM is an output of the model and that cannot prove anything. There is never an explanation for why DM amounts and distribution vary so much. DM theory needs to make predictions we can then test, not produce model outputs.

[isthatafact]

What you are calling a model is simply a way of quantifying a measurement of a galaxy to describe (or model) its mass distribution. There is no physics involved.

> "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.

[throwawaymaths]

You're confusing the broad scale cosmological predictions of DM with explaining rotational parameters of individual galaxies.

Even so, there are predictions low-parameter DM models seemingly can't make, like: what percentage of galaxies have zero dark matter?

[throwawaymaths]

> then I'll counter that GR explains that, gravitational lensing, gravitational waves, black holes,

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.