Out of the Dark

by Brian Koberlein | 30 March 2018 | astronomy

This is a companion discussion topic for the original entry at https://blog.briankoberlein.com/out-dark/

Brian, you mentioned “some galaxies that are almost entirely dark matter”. Are they ultradiffuse galaxies as well?
Also, is it possible to detect whether DF2 has a SMB at it’s centre or not?

I know of Dragonfly 44, which is mostly dark matter, and is diffuse. It might be possible to detect a supermassive black hole, but if it’s there, it isn’t active.

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Thank you for posting about this. Most of the articles I saw on this topic would just say that this galaxy was missing dark matter, that it was first one discovered like this and all the “scientists” were completely baffled. Your post on this is so helpful. This is the level of info I was looking to hear. It explains things so much better. Thank you very much.

We have some animations done on real data and from our galaxy center. There is some dosen sun masses spinning around BH. So. If BH is made out of sub matter sort of energy, near dark matter but more condensed, is it possible something like this.
In some rare occasion BH can be overloaded with ‘too much matter’, congested and destructed in such tremendous explosion that it has swept of all sub matter in one wave?
Is there any speculations how this happens?

Reading a bit more about MOND recently, I recognized even MOND includes an ‘External Field Effect’, which under certain conditions (surroundings) could cause a normal baryonic gravitational behaviour. I don’t know the surroundings of this galaxy and whether this could apply.
CMB based analysis anyway seems to need some form of DM. But in light of several difficult to explain effects and missing findings of DM candidates I think interpretations should be very careful. I still think, that some form of DM will turn out as the solution, but this form seems to be much weirder than the just weakly interacting cold gas typically assumed. With few hints to the specific properties of DM and Quantum Gravity being open there might be more explanations than just two to consider.

Brian, I read in the post that normal matter is 12% of the total matter in the universe, I had always thought barionic matter is around 4%-5%. Am I wrong?

Baryonic matter is about 4% of the total “stuff” of the universe (matter, dark matter, dark energy). It is about 12% of the matter, the rest being dark matter.

Cristal clear.

How likely do we think it is that ‘dark matter’ is a correction for a flawed gravitational model, and there are better foundations waiting for us that do not require dark matter?

As a stretch question, are there other effects that might be ficticious due to an inadequate (or ‘effective’) models/theories. E.g. (without doubting good work done so far) coulomb force, Yukawa potential, Electro-weak force, even gravitation?

At this point the evidence for dark matter is pretty strong, and the modified gravity models don’t fit the data nearly as well. So it likely isn’t a correction for a flawed gravity model. New evidence can always change that, though.

Um, sorry Brian, this blog post is definitely not up to your usual standards.

First, the van Dokkum+ (2018) paper makes only a very weak case that NGC1052–DF2 is DM-poor (and there is, what, already an arXiv preprint pointing out just how, um, non-robust their statistical analysis is). Second, as Sabine Hossenfelder makes clear in one of her usual excellent posts (she’s “Bee” in BackReaction), there’s widespread ignorance on what various modified gravity theories actually are, much less how well (or not) various observations are inconsistent with them.

Once I figure out how to properly post links, I’ll provide a few, to back up the above.

Here goes …

“Current velocity data on dwarf galaxy NGC1052-DF2 do not constrain it to lack dark matter” (Martin+ 2018)

Peter Coles (“telescoper”), 16 April: The Dark Matter of Astronomy Hype

10 April: No, that galaxy without dark matter has not ruled out modified gravity

26 March: Modified Gravity and the Radial Acceleration Relation, Again

See my last comment. There’s widespread ignorance of what “theories of modified gravity” there are, in the professional literature, and even more about what observational tests they have, and haven’t, passed. Worse, as a close reading of many papers which supposedly consider - and reject - models based on such theories shows, even some theoretical astrophysicists who should know better seem comfortable with what - to mere layperson me - looks like lazy dismissals that reveal poor understanding of such theories (I’m being polite).

You may be thinking, “but the Bullet Cluster lays waste to all such modified gravity theories, right?” Here’s what Bee has to say about this:The Bullet Cluster as Evidence against Dark Matter

Fair point. And yes, I recommend everyone check out Hossenfelder’s excellent post on this: http://backreaction.blogspot.com/2018/04/no-that-galaxy-without-dark-matter-has.html