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Hossenfelder's deceitful babbling about dark matter, a MOND killer

One of the things that have always driven me up the wall about the postmodern would-be scientific media was their constant promotion of people who were or are absolutely self-evident hacks, crackpots, and scammers as if they were good scientists. Sabine Hossenfelder is one of the greatest examples I know.

In her newest rant she screams

No, that galaxy without dark matter has not ruled out modified gravity
and attacks the recent paper "A galaxy lacking dark matter" (TRF, Nature, arXiv). That paper has simply observed a (rather small) galaxy where objects seem to move exactly as general relativity predicts: there is no need for MOND or dark matter in that galaxy.




Now, if the dark-matter-like effects predicted for that small galaxy were strong enough to be observable, the conclusion is totally clear: Dark matter explains this Einstein-friendly galaxy well simply because each galaxy could have either kept its dark matter or gotten rid of it – the presence of dark matter is independent of the presence of visible matter.

On the other hand, the alternative models without dark matter – MOND or modified gravity – have a problem because when gravity is said to be modified, you can't turn these modifications off, so the behavior of each galaxy is predicted to disagree with general relativity. Of course, the problem is "how much". But let's believe the authors of the paper that the dark-matter-like effects are strong in that galaxy, too – Hossenfelder doesn't focus on this possible objection, anyway.




The authors of the Nature paper discuss this problem that their work poses for MOND, of course:
...In theories such as MOND [27] and the recently proposed emergent gravity paradigm [28] a “dark matter” signature should always be detected, as it is an unavoidable consequence of the presence of ordinary matter. In fact, it had been argued previously [29] that the apparent absence of galaxies such as NGC1052–DF2 [our new galaxy without dark matter effects] constituted a falsification of the standard cosmological model, and evidence for modified gravity...
The argument is clear. Imagine that native American scientists investigate (and disagree) whether bras on the European women are a genetic mutation of the women from a different continent; or something that may be added. Once a European woman showed the first strip tease for the audience of native Americans, the native American scientists knew: bras are just pieces of clothes, not a genetic aberration.

How stupid do you have to be not to get this simple point? Look at Backreaction:
...It is beyond me why so many astrophysicists believe that modified gravity is somehow magically different from particle dark matter, or indeed all other theories we have ever heard of. It’s not.

For both modified gravity and particle dark matter you have additional degrees of freedom (call them fields or call them particles) which need additional initial conditions...
Wow, so it's like a bra combined with the woman's ability to change her DNA – so that sometimes the bra mutation is there and sometimes, it isn't there.

Hossenfelder – because she has written several crappy papers about MOND – tries to defend the indefensible. MOND isn't in trouble, she writes, because like dark matter, "it also has new degrees of freedom".

No, it doesn't. MOND or modified gravity is still gravity, so it's some collection of Newton-style or Einstein-style fields or force laws that influence the motion of pre-existing masses. In other words, MOND is still gravity – massive objects move in a way that is calculable from the distribution and speed of other masses (i.e. from the stress-energy tensor produced by visible matter)!

You may use extra fields but as long as your theory may be classified as "modified gravity", those don't allow you to turn the effect on or off – just like ordinary Newton's gravity which also cannot be turned on or off. They're just a deeper formalism that allows you to calculate the modified Newton-style force.

The claim by that spherical bastard (if I use Zwicky's clever dark-matter-inspired technical terminology) that MOND comes with extra "fields which may have various initial conditions" and this claim supposedly allows the effect to be turned on or off is just complete hogwash. Even modified gravity still has to be a long-range force – the range is infinite or basically infinite relatively to particle physics scales, comparable to the size of galaxies or longer. It means that the dispersion relations for the waves are those of the massless (or almost precisely massless, relatively to the particle physics masses) particles.

In other words, any "lump" that you could add to these fields as a part of the initial conditions immediately flies away by the speed of light – or by some speed that is extremely close to the speed of light. No "lump" could stay within the region occupied by the galaxy for millions let alone billions of years. The only possible "lumps" that have the ability to stay in the galactic region are "lumps" enabled by the presence of the galactic matter, presence of the stars. The profile of these "static lumps" is fully dictated by the surrounding visible matter.

So after millions and especially billions of years, the dependence of the conditions in the galaxy on the initial conditions of the "new MOND fields" is guaranteed to have completely faded away – or flown away. If some "lumps" were capable of sitting inside the galaxy for billions of years, as well as flying away if they want, they would have to be called matter (matter is something that can sit, something that has a rest frame and/or a low speed). If they were invisible in optical telescopes, these "lumps" would be not just matter, they would have to be called dark matter. That's really what dark matter means. Theories of dark matter surely don't say that the dark matter particle cannot be described as a "lump" (e.g. a soliton). Dark-matter-like effects may be explained by some effects involving a very light axionic field but be sure that Edward Witten and collaborators call such a situation a theory of dark matter.

The whole point and motivation of MOND or modified gravity is that you don't need to make the additional assumptions about the presence or distribution of dark matter – new stuff – in each galaxy. MOND is "more predictive" – it really predicts that the dark-matter-like effects are always there and cannot be turned away because they're laws of physics – modifications of Newton's gravitational formula (that may be extracted from Einstein-style new fields or not, that plays no role for the verification of the predictions). MOND says that the distribution of the visible matter is everything you need – you must just use better (deformed) force laws than the original Newton's or Einstein's laws. So MOND is or was really more predictive i.e. easier to falsify – and indeed, that means that we may forget about it when a galaxy without the new, dark-matter-like effects is observed.

In the comment section, Andrew Thomas – who may be a dark matter cosmologist in Adelaide, Australia – politely points out that Hossenfelder's claims make no sense. She continues to repeat her demagogic meaningless fog in several comments in that section, too. It's really frustrating. It's obviously impossible to discuss modern scientific research with the likes of Ms Hossenfelder. But it's impossible to help them, either, they're just too pompous fools for that.

You know, this is not just some terminological disagreement. This is a discussion about validity of particular theories and various papers – Milgrom's, Verlinde's (and perhaps a paper by Hossenfelder) etc. – that have been called MOND. The papers have actually said something, promoted some ideas, some new universal laws which make predictions. If and when the predictions are falsified, you can't change the rules of the game and pretend that MOND meant something different. Only total idiots may be fooled in this way.

Now, every at least slightly decent cosmologist or theoretical physicist understands perfectly well that Hossenfelder's comments are just plain garbage. But most of them won't point this fact out clearly because they don't want to be seen as discriminating against women. Sorry, you are throwing your scientific integrity to the toilet, comrades.

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