No, he's correct that each galaxy does need tuning. Specifically, the amount of DM and it's specific distribution is tuned to match each observation, which is what the OP said.
> but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM.
I think this "strong case" for LCDM is often oversold. See this review of the evidence for LCDM compared to modified gravity:
From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm
This is simply not true.
No cosmological theory tries to explain why each star is in its exact observed location. They try to predict the statistics of galaxies, star formation, etc.
If you run a simulation of the universe, beginning with very simple, random initial conditions, and then run physics forward for 13 billion years, you get a universe that statistically looks almost identical to our own. You get the same mix of spiral and elliptical galaxies as observed, clustered together in the same way, with the same star formation history, etc. You don't get exactly our universe, with the Milky Way exactly here and Andromeda exactly there, because you begin with random starting conditions. However, you get a universe that is statistically identical in most ways.
Note that this is not the case for Milgromian theories. So far, no one has managed to create a theory without dark matter that can achieve this. Alternative theories generally get one or a few things right, at the cost of getting a whole host of other things wrong.
It's not that astrophysicists a priori like dark matter. It's just that dark matter is a very simple paradigm that explains a huge number of phenomena with only a handful of free variables, and none of the alternatives work so far.
Nice strawman.
> Note that this is not the case for Milgromian theories. So far, no one has managed to create a theory without dark matter that can achieve this.
LCDM also fails at this (see Dwarf galaxy problem, for one example), so this is not a point in your favour.
> It's just that dark matter is a very simple paradigm that explains a huge number of phenomena with only a handful of free variables, and none of the alternatives work so far.
LCDM doesn't work that great either. See the review I linked.
I assume you're referring to the "missing satellites problem." This problem is already essentially solved:
1. Newer, more sensitive surveys have detected more Milky Way satellite galaxies.
2. Reionization and supernova feedback quench star formation in small dark matter halos, meaning that the smallest dark matter halos never form galaxies.
> See the review I linked.
I think we've talked about this elsewhere on HN before, and I pointed out that the paper you're linking to gets a lot of things wrong and is published in an obscure journal (presumably because it wouldn't survive peer review at any of the major journals).
This is actually exactly wrong. If you run LCDM for 13 billion years you don't get galaxies till much later than what we observe via say JWST. However, with MOND you predict early galaxies due to the higher force of gravity at distances causing earlier collapse.
I would not take all the recent claims of extremely high-redshift galaxies, found in initial JWST data, at face value. Several of these claims have already been shown to be false.
...and? If we look at the way a star wobbles, then tune the amount and distribution of planets around it to fit, isn't that just science?
With DM the rationale goes the other way around. What means it's not a solid explanation.
That is perfectly fine, as long as proponents don't go saying it's settled knowledge and start fighting the people looking for alternatives. When they start doing that, they stop being scientists, and start being believers.
No it doesn't. We have strong priors that the movement of bodies is affected by gravity, and gravity comes from matter. When we see movement we cant' explain given the matter we can see, the most obvious explanation is matter we cannot see for some reason.
That is science because gravity has been extensively tested and confirmed in that regime.
Gravity on the scales where dark matter is relevant has not been tested to this extent, so we're just assuming our theory is correct in this new regime and thus inferring the DM distribution based on that assumption and then saying, "this must be the case because we understand gravity". Except as I said, our understanding in that regime has yet to be confirmed, so that's ultimately a circular argument.
By contrast, modified gravity theories can match many of the predictions of DM theories using only direct observations of visible matter. It actually made some successful predictions before observations were made, where those observations required adjusting DM models to fit. Successful prediction with fewer parameters is exactly what you want from a scientific theory, and that's what MOND has done. See the review I linked for more information.
At this time, neither theory is fully satisfactory as discussed in that review. Superfluid dark matter models were compelling for awhile because they combined MOND and dark matter in a compelling way, but the early models were also inconsistent with some other observations. This is far from a solved problem and the LCDM model gets too much credit and attention given its problems.
Couldn't we just as easily say the same about EM or any of the other forces?
The reason we think it is some kind of matter is because we know that matter interacts with gravity in ways that would explain our observations, as long as we assume that matter isn't visible for some reason. And it isn't like there aren't candidates even within the current standard model that might have the right properties.
> By contrast, modified gravity theories can match many of the predictions of DM theories using only direct observations of visible matter.
...and curve fitting. They keep having to add parameters to MOND to make it fit what we see and currently attribute to dark matter, like the bullet cluster. That's the physics equivalent of code-smell.
I agree, but isn't that basically what MOND does too? It made one small tweak to gravity, and all of a sudden we can successfully predict a range of galaxy properties with a pretty tight margin for all galaxies, where LCDM can't make such precise predictions because each galaxy's DM distribution can only be tuned after its been observed, and the number of free parameters needed for tuning means the margin is very wide. That's great for MOND but doesn't bode well for LCDM.
From direct observations, MOND is strong where LCDM is weak and vice versa. I think LCDM just gets too much credit and more alternatives need to be tried. There's almost certainly something to MOND because the observations it matches and the successful predictions it made would be too coincidental otherwise.
MOND gets one or two properties of galaxies correct, at the cost of messing up huge numbers of other observed facts about the universe (such as the abundances of chemical elements or the CMB power spectrum).
Also, you should realize that MOND in itself is not actually a theory of nature. You can't just "hack" Newton's 2nd law, because that law is a consequence of other, more fundamental principles. If you want to modify gravity, you have to modify General Relativity. There have been many attempts to modify GR in order to create a MOND-like theory, but they tend to fail in various ways (again, by predicting things that are empirically false). So far, GR passes every empirical test of gravity physicists have come up with.
As John von Neumann said:
With four parameters I can fit an elephant to a curve. With five I can make him wiggle his trunk
Not true. The LCDM predicts the DM distribution with a typical DM profiles, and then the amount of baryons is heavily dependent on star formation, etc. Therefore each galaxy of the same luminosity will have somewhat different dark matter halo. Those are not tuning parameters at all.
Regarding the linked article, it an article written by proponents of MOND and is not objective. That is fine, but at the moment I don't believe MOND is a serious alternative, and the majority of the astro community would agree. Just the fact that MOND still requires dark matter anyway to reproduce CMB is major problem for it. But if someone develops the version of MOND that has equal predictive power to LCDM, I'll be happy to accept it, but we are not there.
Then explain how MOND is able to make predictions that don't need such considerations. This suggests either that dark matter is superfluous, or that dark matter has some underlying order that reproduces MOND in some limit, thus eliminating the tuning needed. This is the approach taken by some superfluid dark matter models, which at least is not ignoring the problem.
> Regarding the linked article, it an article written by proponents of MOND and is not objective
Right, because anti-MOND or dark matter reviews are totally objective?
> Just the fact that MOND still requires dark matter anyway to reproduce CMB is major problem for it
It's not at all a problem to say we need DM like sterile neutrinos, since DM alone is insufficient to adequately explain all observations anyway.