6 ms·

This was predicted by astronomer Stacy McGaugh on his super interesting blog as a natural consequence of modified Newtonian dynamics (https://tritonstation.com/

This was predicted by astronomer Stacy McGaugh on his super interesting blog as a natural consequence of modified Newtonian dynamics (https://tritonstation.com/2022/01/03/what-jwst-will-see/ https://tritonstation.com/2022/01/03/what-jwst-will-see/). McGaugh came to support MOND over dark matter reluctantly for just such reasons—MOND routinely predicts things in advance that dark matter struggles to justify ex post facto, usually by adding additional moving parts to the model. Whether or not you find the evidence persuasive, McGaugh's blog is worth a deep dive just to appreciate the subtleties and complexities of this kind of observational astronomy.

schiffern 4y ago
Any publications indicate whether Refracted Gravity (RG) predicts the same thing? RG has always fascinated me, since it neatly explains flat rotation curves[1] without messing up the equations for gravity too much. If some refractory effect (TBD) causes gravity to bend toward mass concentrations, then this effectively "confines" gravity into a plane. Then the propagation law becomes 1/r rather than 1/r^2. In effect it becomes a galactic analog of the SOFAR layer in the ocean.[2] Intuitively the "self feeding" effect on disk formation should accelerate the process, but I'd be interested to see a full treatment. [1] https://www.aanda.org/articles/aa/pdf/2020/05/aa35950-19.pdf https://www.aanda.org/articles/aa/pdf/2020/05/aa35950-19.pdf [2] https://en.wikipedia.org/wiki/SOFAR_channel https://en.wikipedia.org/wiki/SOFAR_channel
- whatshisface 4y ago
  >without messing up the equations for gravity too much. If some refractory effect (TBD) causes gravity to bend toward mass concentrations, then this effectively "confines" gravity into a plane. Then the propagation law becomes 1/r rather than 1/r^2. There are objects like globular clusters and dwarf galaxies that lie outside the galactic plane, but are not that far away. If our galaxy's gravity was confined to its plane, you would expect nearby external objects to move differently from how they are observed to move. See illustration: https://cdn.britannica.com/23/4423-050-964A0A08/Distribution-star-clusters-Galaxy.jpg https://cdn.britannica.com/23/4423-050-964A0A08/Distribution...
  - schiffern 4y ago
    I was hoping someone would mention globular clusters! I couldn't find a way to integrate it naturally in my post. I don't see any information about globular cluster movement in that image. Naturally the confinement wouldn't be 100%, and some amount of gravity would still "leak out,"[1] so the mere existence of globular clusters isn't proof one way or another. Surveying globular clusters offers a good test. RG might (or might not) explain why globular clusters are mysteriously 1) preferentially orbiting perpendicular to the primary galaxy's rotation, perhaps due to stability, and 2) preferentially aligned with each-other in their own plane, perhaps due to mutual (lensed) gravitation. Both of these observations are otherwise unexplained (or require numerous free parameters) under dark matter. We won't know until someone does a survey, and probably some simulation work. Sadly MOND soaks up most of the oxygen in the Modified Gravity field, leaving almost none for RG. [1] see Figure 16 https://arxiv.org/pdf/1603.04943.pdf https://arxiv.org/pdf/1603.04943.pdf
jquery 4y ago
It seems to me, as a layperson, if MOND was hypothesized first, it would be the dominant theory. It’s so elegant and simple with so much explanatory power. However, because it was hypothesized much later, the fact it matches past predictions is given no weight, despite its simplicity. I think this is a bit of a mistake. I believe that simplicity itself is evidence. Dark matter theory is starting to feel like the epicycles of Copernicus.
- nickelpro 4y ago
  If MOND completely described observable phenomenon it would supplant dark matter, but it doesn't. MOND still requires dark matter, just less of it. Or, to quote Milgrom, "some standard matter in some form that has not been detected"[1]. Which, IMHO, is nearly isomorphic to DM as far as statements go. So until we figure out what the missing mass is, MOND is widely viewed as an unnecessary complication. There is missing mass, if we figure out the nature of DM or discover this hitherto unknown "standard matter" then we can talk about modifying universal gravitation. [1]: https://web.archive.org/web/20160721044735/http://www.astro.umd.edu/~ssm/mond/moti_bullet.html https://web.archive.org/web/20160721044735/http://www.astro....
  - throwawaymaths 4y ago
    The explanatory power of dark matter is so weak though. With the right clumping of LCDM you could probably also explain various reports of poltergeists around the world. Then you could claim that reports of poltergeists are evidence of LCDM over MOND because it's another phenomenon that MOND can't explain.
    - ndsipa_pomu 4y ago
      However, there is plenty of evidence to suggest that it does exist. The distribution of dark matter being uneven is evidence that something functioning like dark matter has to exist as MOND doesn't easily replicate that. Colliding galaxy clusters show a distribution of matter that differs from the distribution of visible matter, so the simplest explanation is that there is invisible matter that reacts gravitationally.
ilitirit 4y ago
I've been out of the loop for a while. Can MOND satisfactorily explain gravitational lensing?
- hackernewds 4y ago
  It doesn't entirely, but Refracted Gravity does.
astroH 4y ago
So MOND does predict more galaxies at high redshift however it also predicts earlier reionization than LCDM which it turns out not to be true and the mass function of clusters is not what we see purely based on X-ray temperatures. So getting one thing right at the expense of many others doesn't make this particularly viable.

schiffern 4y ago

Any publications indicate whether Refracted Gravity (RG) predicts the same thing? RG has always fascinated me, since it neatly explains flat rotation curves[1] without messing up the equations for gravity too much. If some refractory effect (TBD) causes gravity to bend toward mass concentrations, then this effectively "confines" gravity into a plane. Then the propagation law becomes 1/r rather than 1/r^2. In effect it becomes a galactic analog of the SOFAR layer in the ocean.[2] Intuitively the "self feeding" effect on disk formation should accelerate the process, but I'd be interested to see a full treatment. [1] https://www.aanda.org/articles/aa/pdf/2020/05/aa35950-19.pdf https://www.aanda.org/articles/aa/pdf/2020/05/aa35950-19.pdf [2] https://en.wikipedia.org/wiki/SOFAR_channel https://en.wikipedia.org/wiki/SOFAR_channel

whatshisface 4y ago
>without messing up the equations for gravity too much. If some refractory effect (TBD) causes gravity to bend toward mass concentrations, then this effectively "confines" gravity into a plane. Then the propagation law becomes 1/r rather than 1/r^2. There are objects like globular clusters and dwarf galaxies that lie outside the galactic plane, but are not that far away. If our galaxy's gravity was confined to its plane, you would expect nearby external objects to move differently from how they are observed to move. See illustration: https://cdn.britannica.com/23/4423-050-964A0A08/Distribution-star-clusters-Galaxy.jpg https://cdn.britannica.com/23/4423-050-964A0A08/Distribution...
- schiffern 4y ago
  I was hoping someone would mention globular clusters! I couldn't find a way to integrate it naturally in my post. I don't see any information about globular cluster movement in that image. Naturally the confinement wouldn't be 100%, and some amount of gravity would still "leak out,"[1] so the mere existence of globular clusters isn't proof one way or another. Surveying globular clusters offers a good test. RG might (or might not) explain why globular clusters are mysteriously 1) preferentially orbiting perpendicular to the primary galaxy's rotation, perhaps due to stability, and 2) preferentially aligned with each-other in their own plane, perhaps due to mutual (lensed) gravitation. Both of these observations are otherwise unexplained (or require numerous free parameters) under dark matter. We won't know until someone does a survey, and probably some simulation work. Sadly MOND soaks up most of the oxygen in the Modified Gravity field, leaving almost none for RG. [1] see Figure 16 https://arxiv.org/pdf/1603.04943.pdf https://arxiv.org/pdf/1603.04943.pdf

jquery 4y ago

It seems to me, as a layperson, if MOND was hypothesized first, it would be the dominant theory. It’s so elegant and simple with so much explanatory power. However, because it was hypothesized much later, the fact it matches past predictions is given no weight, despite its simplicity. I think this is a bit of a mistake. I believe that simplicity itself is evidence. Dark matter theory is starting to feel like the epicycles of Copernicus.

nickelpro 4y ago
If MOND completely described observable phenomenon it would supplant dark matter, but it doesn't. MOND still requires dark matter, just less of it. Or, to quote Milgrom, "some standard matter in some form that has not been detected"[1]. Which, IMHO, is nearly isomorphic to DM as far as statements go. So until we figure out what the missing mass is, MOND is widely viewed as an unnecessary complication. There is missing mass, if we figure out the nature of DM or discover this hitherto unknown "standard matter" then we can talk about modifying universal gravitation. [1]: https://web.archive.org/web/20160721044735/http://www.astro.umd.edu/~ssm/mond/moti_bullet.html https://web.archive.org/web/20160721044735/http://www.astro....
- throwawaymaths 4y ago
  The explanatory power of dark matter is so weak though. With the right clumping of LCDM you could probably also explain various reports of poltergeists around the world. Then you could claim that reports of poltergeists are evidence of LCDM over MOND because it's another phenomenon that MOND can't explain.
  - ndsipa_pomu 4y ago
    However, there is plenty of evidence to suggest that it does exist. The distribution of dark matter being uneven is evidence that something functioning like dark matter has to exist as MOND doesn't easily replicate that. Colliding galaxy clusters show a distribution of matter that differs from the distribution of visible matter, so the simplest explanation is that there is invisible matter that reacts gravitationally.

ilitirit 4y ago

I've been out of the loop for a while. Can MOND satisfactorily explain gravitational lensing?

hackernewds 4y ago
It doesn't entirely, but Refracted Gravity does.

astroH 4y ago

So MOND does predict more galaxies at high redshift however it also predicts earlier reionization than LCDM which it turns out not to be true and the mass function of clusters is not what we see purely based on X-ray temperatures. So getting one thing right at the expense of many others doesn't make this particularly viable.