[itishappy]

I agree with most of what you've said here, particularly the following line which I'm copying for emphasis because I think it's incredibly important.

> These simulations have a lot of value, they demonstrate that standard cosmology and normal gravity has no problem forming voids and filaments.

That being said, I think the author intends for this article to be more of a call to action than an actual result. Simulations aren't cheap, somebody needs to actually do the work. The point that there aren't any simulations without dark matter is an important one too.

[thicktarget]

One can do simple simulations on a laptop which show the cosmic web. It's not really an excuse for not having tried. There are lots of claims in the article which need to be justified, and in science that comes before making big claims.

https://alvinng4.github.io/grav_sim/examples/cosmic_structur...

These simulations take their simple initial conditions from the Cosmic Microwave Background fluctuations, but models without dark matter fail to match the observed CMB. There are no major baryon-only simulations because cosmology doesn't work without DM, and you have nothing to start from. You need a quantitative model which works on some level to even begin, people have tried with modified gravity models.

[itishappy]

We need a model that includes electromagnetism. The author isn't the only one making this claim. When we do magnetohydrodynamic cosmological sims we consistently find surprising effects. The recent simulation showing that black hole accretion disks are supported by magnetism comes to mind.[0][1]

Apologies, I know this is typically considered bad form, but have you gotten to the following section in the article?[2] It appears to directly contradict your claims.

> MOND’s also been around since the early 1980s, but, in 2021, it finally developed a model – the Aether-Scalar-Tensor framework, or AeST – which ALSO maps perfectly onto the acoustic peaks revealed by WMAP and Planck. (It does it by proposing a new vector field and scalar field that duplicate the effects of Cold Dark Matter in the early universe...

[0] https://astro.theoj.org/article/93065-an-analytic-model-for-...

[1] https://www.caltech.edu/about/news/cosmic-simulation-reveals...

[2] https://theeggandtherock.com/i/158515951/more-matter-or-less...

[thicktarget]

Many state of the art galaxy formation simulations use MHD, it does not however affect the formation of the Cosmic Web.

> MOND’s also been around since the early 1980s, but, in 2021, it finally developed a model – the Aether-Scalar-Tensor framework, or AeST – which ALSO maps perfectly onto the acoustic peaks revealed by WMAP and Planck. (It does it by proposing a new vector field and scalar field that duplicate the effects of Cold Dark Matter in the early universe

This doesn't contradict what I said. These models are not simply removing DM, I said people have tried with modified gravity models like these. First the models which fit the CMB were engineered to do, and each model has many more free parameters than dark matter. And note they have replaced invisible matter with an invisible matter-like field, it's not a simplification. Remembering that cold dark matter predicted these features, with fewer parameters and it is a physical model, not merely a fit. People have run simulations with the more basic MOND models, to find it cannot form realistic structure. Generally it forms structure more quickly than standard cosmology. Finding they need to add dark matter to their already modified gravity models to get something reasonable.

https://arxiv.org/abs/2007.00555

https://arxiv.org/abs/2305.05696

https://arxiv.org/abs/1309.6094

[itishappy]

> Many state of the art galaxy formation simulations use MHD...

> Generally [MOND] forms structure more quickly than standard cosmology.

Magnetohydrodynamics, not just hydrodynamics. It would need some type of restoring force similar to how the black hole simulation was found to behave when it included magnetic fields.

> People have run simulations with the more basic MOND models, to find it cannot form realistic structure.

Neither does ΛCDM though, right? It matches the CMB, sure, but it doesn't match the many of the observed properties of the universe either. The observed voids and over-densities are significantly larger than predicted, nor is the observed distribution of dark matter in galaxies what we predict.

> And note they have replaced invisible matter with an invisible matter-like field, it's not a simplification.

Great point, and I'm happy to admit I missed this.

But I'm not actually a proponent of MOND, nor even an opponent of ΛCDM. It's the best we got. I just feel it's not quite as accepted as current consensus seems to, so I'm glad there's people out here still trying to poke holes in it and find alternatives.

[scotty79]

I think the point is that for filaments to form in simulations you have to assume dark matter to be so abundant that it stops explaining other things that are explained with it. Basically you can adjust dark matter parameters to explain everything but you can't find a single set of parameter values to explain all the things at the same time.

[thicktarget]

That's not the case. The hydrodynamical simulations mentioned use the same cosmological value set by CMB data. The ratio is not the same in every galaxy, because matter and DM behave differently on small scales, but this variation comes out naturally in simulations.