
Dark Matter vs. Modified Gravity: A Trialogue (2012) - andrewflnr
http://www.preposterousuniverse.com/blog/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/
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tynpeddler
After reading that exchange, it's pretty clear that Sean Carroll has basically
ignored almost all the literature on modified gravity theories. It makes you
wonder why he's commenting on it.

First, Bullet Cluster is not a problem for modified gravity. There have been
numerous publications explaining why. In fact, it took much more effort to
reconcile the Bullet Cluster with cdm than it did with modified gravity.

Secondly, clinging to the Bullet Cluster without mentioning Abell 520 is
either ignorance or deception. Either way, there exist known celestial
phenomenon (like galaxy rotation curves, the phenomenon that originally
started the idea of dark matter) that are not adequately explained by dark
matter theories (the same is true of modified gravity btw), and ignoring them
in order to claim that we KNOW dark matter exists is intellectually lazy.

The problem with this discussion is that the intricacies of dark matter
theories are much better understood by the cosmology community. Modified
gravity on the other hand, it understood at best superficially by many
cosmologists. As a result, whenever new findings in astronomy are published,
there is a stampede of publications by people, who know dm but are ignorant of
modified gravity, explaining why this "proves" once and for all why dark
matter exists. The few modified gravity researchers out there are then left
running around refuting all the false statements made about modified gravity.

I think Stacy hit the nail on the head with this quote:

I also agree that this [modified gravity theories] is contrived. But we are
WAY into contrivance with LCDM(...). We’ve just gotten familiar with the
contrived parts so that they no longer bother us. That doesn’t make them any
less contrived.

~~~
andrewflnr
While he doesn't always come off very well here, I think you're being too hard
on him. He states outright in his earlier linked post[0] that he would prefer
that modified gravity were true. He's at least familiar with TeVeS: I watched
a video of a lecture he gave where he went into a fair bit of depth on it
(i.e. way over my head). And he also seems to have done some small amount of
modified gravity research himself (linked at the bottom of [0]).

My takeaway was that since the CMB spectrum seems to require dark matter, it's
going to be there even if some form of modified gravity is also true. I've
barely heard either side of the debate mention this in layman contexts. What's
the modified gravity perspective on that?

[0] [http://www.preposterousuniverse.com/blog/2011/02/26/dark-
mat...](http://www.preposterousuniverse.com/blog/2011/02/26/dark-matter-just-
fine-thanks/)

~~~
tynpeddler
The CMB spectrum does seem to require dark matter and it's a real problem for
MOND. The point most MOND physicists keep trying to make is that dark matter
has it's own share of problems but no one has discarded it as a theory yet.

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deepsun
Can anyone explain what's going on there?

As far as I see, one argument is that there can be multitude of explanations
for dark matter, both baryonic and modifications of known laws (gravity,
newtown dynamics). But the other commenter says that all other explanations
besides baryonic are ruled out by data. Did I get it correctly?

~~~
ISL
It is extremely challenging, perhaps impossible, to find a MOND theory that
fits all available observations.

For me, a modification of gravity would be more aesthetically pleasing than
dark matter, but that does not appear to be how Nature has chosen to be.

~~~
tynpeddler
>It is extremely challenging, perhaps impossible, to find a MOND theory that
fits all available observations.

All the commentators agree on this. What Ranier and Stacy are pointing out is
that the same thing is true of LCDM. Dark matter does not do a good job of
explaining galaxy rotation curves (even though this is the context which dark
matter was originally proposed for), bullet cluster, abell 520, or lots of
other galactic dynamics.

Of course, MOND has its own share of problems. Most importantly that Sean
points out is the 3rd peak on the CMB. Stacy sounds like he's holding out for
the possibility that relativistic formulations of MOND may explain this peak,
though he admits that may not happen. To support his position, Stacy makes
reference to other instances where MOND had unintuitive but correct
conclusions (like for the bullet cluster).

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zackmorris
From a bit of armchair research this Friday, here are some articles on the
connection between MOND and the Hubble constant:

[https://en.wikipedia.org/wiki/MOND](https://en.wikipedia.org/wiki/MOND)

[https://arxiv.org/abs/1204.6359](https://arxiv.org/abs/1204.6359)

[https://arxiv.org/abs/1703.06110](https://arxiv.org/abs/1703.06110)

[http://file.scirp.org/Html/1-4500356_51478.htm](http://file.scirp.org/Html/1-4500356_51478.htm)

[https://ned.ipac.caltech.edu/level5/Sept01/Milgrom2/Milgrom3...](https://ned.ipac.caltech.edu/level5/Sept01/Milgrom2/Milgrom3.html)

[https://academic.oup.com/mnras/article/438/2/1805/1016908](https://academic.oup.com/mnras/article/438/2/1805/1016908)

Basically what these are saying is that if the universe (and space around
galaxies) is expanding, then stars would be dragged along with that expansion
and move outward from the center of mass if they orbited at their expected
Newtonian speed (they would gain potential energy as their "altitude"
increases).

I'm wondering if this energy didn't go into the star (because it would violate
conservation of energy to see a star moving faster and faster over time) then
maybe the energy goes into space itself, showing up as "dark" energy, which
has gravity by mass-energy equivalence. So basically the farther away we are
from something, the heavier space appears in that region from our frame of
reference. Or maybe the extra speed of distant stars has more to do with how
long they've been pulled by the expansion of the universe than how much mass
is near them. That could maybe explain the pictures of colliding galaxies that
have uneven distributions of matter and dark matter that we don't see in
individual galaxies. And why we're unlikely to detect dark matter as a
particle locally (because the expansion of space is more easily observed over
large distances).

I guess one way to prove this theory would be to look for unevenness in the
Hubble constant (red shift), which would mean an unevenness in how fast (or
how much) space is stretching in different regions. The last link seems to
agree with something along those lines.

Anyway, I'm not a physicist, just throwing ideas out there! Feeling kind of
embarrassed to post this layman's analysis but what the heck.

------
memebox3v
"Science is dead" Poor Stacey. He is trying to be reasonable in the face of
ideology.

