
Modified Gravity Theory Passes a Critical Test - georgecmu
https://www.quantamagazine.org/modified-gravity-theory-passes-a-critical-test-20200728/
======
ISL
The Quanta article doesn't appear to address the Bullet Cluster, nor the dark-
matter-rich dwarf galaxies.

Looking forward to having a gander at the paper itself.

Edit for context: the challenging thing for MOND is that ΛCDM explains a
number of precision observables and phenomena at once. MOND and TeVeS theories
can fit individual facts in isolation, but not the ensemble simultaneously.
ΛCDM fits those facts with only a single extra concept. It sounds like this
iteration claims to address the seminal observation one and one of the most-
tightly constrained observables simultaneously. There are several others. Will
have to read the paper before saying more.

~~~
kayaeb
I'm not positive that's true. I think the most challenging thing for MOND is
that basically no one will touch it unless they already have tenure because
it's a death sentence.

I saw Pavel Kroupa (big name in Milgromian gravity) present in Heidelberg (big
concentration of Astro), at the time Volker Springel (author of widely used
LCDM simulation code "GADGET") was there and Illustris simulation sets (LCDM
major project) had just been rolled out. And Pavel basically got heckled (in a
very erudite and respectable way, but constant interruptions from the LCDM
majority audience).

But Pavel had one slide, I can't find it now, but it was like 72 different
problems that LCDM had not solved (ok, 72 is an exaggeration; you can troll
his website for mentions of a lot of them [https://astro.uni-
bonn.de/~pavel/kroupa_SciLogs.html](https://astro.uni-
bonn.de/~pavel/kroupa_SciLogs.html)).

And like, Pavel's a big boy, he has some of the most cited papers in all of
astronomy, he's got tenure, and he's set, so he can take it and not care. But
grad students / postdocs I imagine would constantly have their work politely
ignored and get shunted into underfunded groups.

I'm just trying to say that LCDM has things it can't explain, and MOND has
things it can't explain, but the amount of resources in each theory is
seriously lopsided so LCDM can frequently "tweak" itself to solve problems
that MOND just doesn't have time or resources to to the same (for example disk
formation in LCDM models used to be impossible until they had the
supercomputing resources for the resolution required, and they found that the
feedback coefficient could a) not promote disk growth, b) promote disk growth,
and c) destroy disk growth, depending on how much they cranked it up. That's
NOT a triumph of LCDM making an amazing replication of the observation, that's
some grad student in a lab with enough CPU to tweak a meta-parameter until it
looks good.

Also the CMB is extremely tightly constrained... and multiple huge tightly-
constraining studies, WMAP, PLANCK, Gaia, are more than 3-sigma outside of
each other's results, so... perhaps too tightly constrained.

Edit: Found it, here is the great astronomical bloodbath of 2014: LCDM
(Springel and Rix) vs MOND (Kroupa). Great watch.
[https://www.youtube.com/watch?v=UPVGDXNSBZM](https://www.youtube.com/watch?v=UPVGDXNSBZM)

~~~
DiogenesKynikos
My experience from MOND talks is that a proponent of MOND gets up, explains
their theory, and then starts explaining how their theory explains the
rotation curve of galaxies.

They're then confronted with a bunch of really basic questions from the
audience: how does your theory explain the spatial spectrum of anisotropies in
the Cosmic Microwave Background? Is your theory consistent with Big Bang
Nucleosynthesis? Can your theory explain Weak Lensing measurements? How does
your theory deal with the Bullet Cluster? The answer is then generally, "I'm
not sure, but I'm working on it." That causes all the astrophysicists in the
room to lose interest. Standard cosmology explains all of these basic
observations with a minimal set of assumptions. If your theory can't or
doesn't explain the most basic set of observations, and there's another theory
that does, why should I care about your theory?

~~~
alvarelle
Imagine what proponents to heliocentrism such as Galileo had to face
explaining their theory and how it explain a simplified elliptic orbit for the
planets rather than the strange curly orbits known by geocentrists.

They'd be confronted with basic question from the audience: Why would things
fall down if it was not the center? (That was before Newton) Wouldn't we see
paralax in the stars (stars are much further away than what was believed at
the time). Wouldn't we feel it if earth turns so fast? "Maybe that explains
the tides" said Galileo (but it doesn't)

The previous model was also explaining the observations quite well at the
time. Why should we care for another theory? (God works in mysterious ways.)

I'm not saying that MOND is correct. ("they also laughed at Bozo the Clown".)
Just that the fact that there are some unexplained missing piece does not mean
one should reject it so quickly.

~~~
DiogenesKynikos
Comparing oneself to Galileo is generally viewed as a sign of crackpotery,
especially if it's used as a response to legitimate criticism.

> Just that the fact that there are some unexplained missing piece does not
> mean one should reject it so quickly.

It's not just one missing piece. It's a whole series of basic properties of
the observed universe. Most MOND theories are tailored to match one particular
observation, but fail to match everything else. Until there's a MOND theory
that matches a basic set of observations (like CMB anisotropies and the large-
scale structure of the Universe, the ratios of abundances of the light
elements, weak lensing measurements, etc.), MOND is simply uninteresting to
most astrophysicists.

------
zenethian
Help me understand why this is so controversial in the scientific community.
I'm neither a physicist nor do I study any other relevant fields, so I'm
asking as a "layman" as it were.

A cursory search shows that "dark matter" hasn't been observed at any point in
time. It looks like it's a sort-of mathematical placeholder for the
"something" that's missing that we can't see (or can't see _yet_.) Am I
correct in my understanding that these alternative theories are trying to
prove that there's a way to explain these discrepancies without inventing
something unobserved?

Why is that so controversial? To me, again as an outside observer, it feels so
counter-intuitive to _invent_ a new type of matter you can't observe than to
just say that your calculation is close but not right and to start over. Is it
not a crutch?

If I were to take this approach into another field that has an equal amount of
controversy where there is a huge gap in my proof, and say that the cause is
an "unseen force" that I just invented to explain how my theory works despite
the fact that I can't prove in any way that the key part of my theory exists,
would I not be laughed out of a room? How did dark matter become accepted
while being invented instead of observed?

(I hope my tone isn't coming off as mocking. I'm genuinely trying to
understand without a lot of knowledge on the subject, and I don't intend to
discredit the current accepted theory nor support the alternative ones, only
to understand why the scientific community came to accept the one that is
accepted.)

~~~
pdonis
_> Am I correct in my understanding that these alternative theories are trying
to prove that there's a way to explain these discrepancies without inventing
something unobserved?_

I would say MOND is just postulating a different "unobserved" thing--an
alternate theory of gravity, or an additional "field" that looks like an
alternative theory of gravity, instead of dark matter. I don't think MOND has
any advantage in the Occam's Razor department.

Or, to put it another way, the word "unobserved" is not quite correct. We have
actual observations that can't be accounted for, as best we can tell, with our
current theories of physics, without adding _some_ new element. Dark matter
and MOND are just two different choices for what new element to add. Neither
one can say it's not adding _anything_ new.

 _> Why is that so controversial? To me, again as an outside observer, it
feels so counter-intuitive to _invent_ a new type of matter you can't observe
than to just say that your calculation is close but not right and to start
over_

MOND is not just doing a different calculation that is claimed to be more
accurate within existing physics. It is adding new physics. See above.

"Starting over" would be something like finding errors in the calculations
based on existing physics (GR and the observed distribution of visible matter)
that, when corrected, removed the discrepancy between observations and theory.
Nobody has done that, and the calculations based on existing physics have been
checked every which way, so it seems highly unlikely that there is an error
lurking there that hasn't been found.

 _> why the scientific community came to accept the one that is accepted_

My understanding is that MOND, in general, does not close the gap between
observation and theory as well as dark matter does. The paper referenced in
this article appears to be claiming that its version of MOND "catches up" with
dark matter in terms of closing that gap. I haven't had a chance to read the
paper in detail so I can't say how credible I think that claim is at this
point.

~~~
dogma1138
Dark matter (at least any of the non cold baryonic matter and standard model
WIMP candidates) adds new physics too it’s just adds it in different field of
physics that often is much less related to cosmology than gravity is.

~~~
pdonis
_> Dark matter (at least any of the non cold baryonic matter and standard
model WIMP candidates) adds new physics too_

I never said it didn't. The point is that _both_ MOND and dark matter add new
physics.

------
Beldin
The reason work on alternative theories of gravity is important:

We have one reality R, for which we have various possibly imperfect theories
T(R) and a large set of possibly imperfect observations O(R).

For gravity, we know that general relativity doesn't perfectly match current
observations -- GR(R) != O(R). Since R is the same in both sides, this is not
supposed to happen. There are three ways to resolve this:

\- GR() is incomplete,

\- O() is incomplete,

\- both are incomplete.

Dark matter focuses on the second, MOND on the first. We shouldn't give up on
any option until there's either a solution or a counterexample.

~~~
beervirus
What observations differ from general relativity?

~~~
pdonis
Observed galaxy rotation curves (the speed of rotation of stars around the
galaxy's center as a function of distance from the center) cannot be accounted
for using GR and the observed distribution of visible matter in the galaxies.
You need to postulate some extra matter that is not visible ("dark matter") to
account for the observed rotation curves using GR. That is the discrepancy
that originally prompted physicists to postulate dark matter. Since then the
same postulate has also been used to improve the match between theory and
observation in other areas, notably our model of how the early universe
evolved.

------
CydeWeys
It's worth pointing out that modified gravity has less acceptance than even
string theory. It's worth working on, as a somewhat plausible alternative to
the standard theory, but it's not very likely to end up being correct. It's
worth ruling out fully, though (the same can't be said of e.g. flat Earthism).

This particular work seems like adding on epicycles, or overfitting as you'd
call it in machine learning.

~~~
cultus
To be fair, from the outside at least, a lot of string theory looks like
adding epicycles as well.

~~~
CydeWeys
Yes, that is a common view amongst most physicists.

~~~
auntienomen
I disagree, it's not a common view. It's common for physicists to think string
theory might be empty theorizing, but it doesn't look at all like epicycles.
With epicycles, you start with an existing model and than add complexity.
That's not strings. Strings starts with something entirely new, that's already
complex.

------
buddyp450
How is this level of math even possible... like... how are people hard-working
enough to reach this level of understanding?

~~~
laichzeit0
If the general hacker news sentiment is anything to go by, then it’s just hard
because of the archaic mathematical notation. If we could just use better
notation then a simple mathematical bootcamp would make mathematics accessible
to all the brilliant self-taught programmers as well /sarcasm.

~~~
pa7x1
If only those pesky physicist and mathematicians would name their variables
correctly everything would become self-evident.

~~~
jjk166
Newton, why do you want to name this the 6th dimensional electron-graviton
interference coefficient? Just call it G for gravity!

------
micheles
Actually this is classical general relativity, the math is not particularly
advanced. When I was doing Theoretical Physics I would have considered this
paper "easy", at least relative to most papers on hep-th.I never believed in
MOND and even this paper is not convincing, since the lagrangian is built ad-
hoc rather than derived from a clean principle.

------
tigerlily
Posting this here in case anyone finds it rehabilitating:
[http://www2.phys.canterbury.ac.nz/~dlw24/universe/](http://www2.phys.canterbury.ac.nz/~dlw24/universe/)

~~~
ncmncm
What has been the response to this?

------
bawana
Is the distribution of dark matter correlated to regular matter in a way that
suggests one is more causal to the other than to itself?

------
sandgiant
Very interesting work! Great to see people are still putting effort into
alternative theories of gravity.

------
paulpauper
The problem with MOND theories is they require the existence of a fifth force
to account for the deviations of Newtonian gravity .

~~~
bufferoverflow
Isn't a cosmic-scale force much more likely than invisible untouchable matter
that's almost (but not) everywhere?

~~~
meroes
You mean like neutrinos?

~~~
dnautics
... except that neutrinos are detectable. Kind of an important distinction.

~~~
cthalupa
We decided neutrinos almost certainly existed long before we could detect
them.

~~~
dnautics
This not the same; at the time when they were postulated due to mass defect in
nuclear reactions, there was a reasonable mechanism of detecting them, if
nothing else the principle of microscopic reversibility. With current dark
matter there literally is no known way of detecting them that doesn't beg the
question, and all of our attempts to identify them are literally "shots in the
dark".

