[FRIAM] new studies confirm existence galaxies almost-no-dark-matter

[Eric Smith]

Hi Glen and Gil, 

What you have below, Glen, is right I think.  To begin with the summary, and put the TLDR afterward, it looks like these diffuse-galaxy results say that gravity stays a clean theory, and we need to identify the origin and nature of dark matter as a separate thing.  While a hard problem, it is a problem that respects the structure of physics as we have been using it.  Gravity is gravity, we can treat matter as “living on it” at all the energies where we have ever done physics, and we need to figure out how they unify, which we don’t really have a theory for at all, but which we have good reason to believe only comes into play at extremely high energies.  

The longer version:

We know Einstein’s GR not only changes the picture of gravity from Newton’s, but for comparable predictions (locations and rates of orbits, their stability, etc.) it requires corrections from Newtonian gravity in the strong-field regime.  The calculations become complicated and hard to do with pencil and paper, but this is okay, because once it is in its geometric language, the Einstein version is in a conceptual sense “cleaner” than the Newtonian version.  

The above view says that Newton becomes a better and better approximation to Einstein the weaker the field gets.  On the whole, galaxy dynamics on the large scale is governed by very weak fields.  So for the radius-dependence of orbital velocities to deviate far from the Newtonian prediction (as they do in most known galaxies) requires either ordinary gravitation with out-of-the-ordinary matter, or a _different_ deviation from Newton, which would exist in the weak-field limit, but only become visible on very large scales.  Since Einstein -> Newton in the very weak field limit, the latter possibility would require a deviation from Einstein too.  I am not sure that could be done conceptually “cleanly” in the same way GR is clean.

So to find that the diffuse galaxies lacking dark matter go back to orbital predictions that converge to weak-field Einstein with no Dark Matter, which is also weak-field Newton with no DM, favors the interpretation that gravity really is just gravity, and that we have to figure out where some additional matter is coming from, just as the accelerating expansion tells us we have to figure out where some “Dark energy” is coming from.  The cosmological constant is an important lynchpin because it is the only observation about the structure of the vacuum for which we really don’t have a “theory” at all.  Anything else we can measure is handled well by standard model physics, though with still some unexplained parameters.  

In a way, this result is the one that could have been expected.  There are now lots of images from gravitational lensing that show “clouds” of DM off-center from galaxies that we can see in the visible.  This especially happens when galaxies collide.  So DM was behaving like matter already, and it is not very surprising to see that maybe it could be all-but-stripped from a galaxy, leaving only a scattering of visible matter.  It would not surprise me if at some point somebody can show that it was a long-ago collision that did this stripping, and much later the diffuse ball of stars re-settled to an ellipsoid.  

Keep in mind, in all of this, that the strong-field limit of GR is getting better and better constrained with the gravitational-wave detections, in addition to all the astrophysical stuff that it has successfully modeled for decades.  So some muddying of GR that only shows up at weak fields would be strange.  

Finally, n.b. that my understanding of this doesn’t qualify as professional — I got off the train too soon.  But I think everything I have said above is a correct account.

All best,

Eric


> On Apr 1, 2019, at 5:55 PM, glen∈ℂ <gepropella at gmail.com> wrote:
> 
> I'm not sure how magnetism plays into all this.  But it is interesting that these are ultra diffuse galaxies.  Maybe there is something wrong with how we extrapolate the rules in flat space to the rules in very bent space, where everything gets so weird.  It seems (to me) that a regular galaxy would be more like a colloidal solution, with lots of little clumps of bent space (heavy things like brown dwarves[†] and such).  Such a pock-marked, bristly, region of space must be more difficult to model than something relatively well-behaved like an ultra diffuse galaxy. Right?  In the vicinity of "almost singularities" (very heavy objects), any measurement or calculation error will have more of an impact on the result.
> 
> [†] I forgot to turn off the real-time spell checker on this new-to-me computer and, lo and behold, "dwarves" is not the plural of "dwarf"!  WTF?  https://grammarist.com/usage/dwarfs-dwarves/ tells me it's a neologism popularized by Tolkien.  So, by using it, I'm wearing my Dork on my sleeve.
> 
> On 3/31/19 4:24 PM, Gillian Densmore wrote:
>> So it's possible that what we think of as dark matter could be more to do
>> with a whole lot of magnets/lots and lots and lots of gravity energy and
>> makes things go weird?
>> Or weirder?
>> On Sun, Mar 31, 2019 at 3:25 PM glen <gepropella at gmail.com> wrote:
>>> 
>>> https://news.yale.edu/2019/03/29/new-studies-confirm-existence-galaxies-almost-no-dark-matter
>>> 
>>>> The finding was highly significant because it showed that dark matter is
>>> not always associated with traditional matter on a galactic scale. It also
>>> ruled out several theories that said dark matter is not a substance but a
>>> manifestation of the laws of gravity on a cosmic scale.
>>> --
>>> glen
> 
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