
The Case Against Dark Matter - bilifuduo
https://www.quantamagazine.org/20161129-verlinde-gravity-dark-matter/
======
antognini
As a general principle I would advise a grain of salt whenever a physicist
claims to have solved an outstanding astrophysical problem. Verlinde is a
great physicist --- I remember reading Verlinde's paper on entropic gravity
when it came out and I'm still excited about that idea. But someone with a
background in pure physics often lacks the background in astronomy to know
whether their ideas will work. (I remember hearing a visiting physicist
present his ideas about stars producing axions, and having an astronomy
professor point out that if he was right, Cepheid variables would not pulsate
at the observed frequencies.)

The article mentions the Bullet Cluster as one observation that MOND has
difficulty explaining. Another observation that I'm not sure how MOND would
explain is the dramatic difference in dynamics between a globular cluster and
a dwarf spheroidal galaxy. Globular clusters are observed to have virtually no
dark matter, whereas some dwarf spheroidal galaxies have somewhere between
1000 to 10,000 times as much dark matter as ordinary matter [1]. These two
kinds of objects have comparable numbers of stars and are roughly the same
size (though dwarf spheroidals are admittedly larger by about an order of
magnitude). This discrepancy can be explained with cold WIMPs [2], but it's
hard to understand how some MOND-like process would produce such incredibly
different dynamics between the two kinds of galaxies.

I'll admit that I have not looked very carefully at Verlinde's paper. But the
evidence for dark matter is much more than galaxy rotation curves these days.
A credible alternative to WIMPs has to explain more than that.

All that aside, from a theoretical perspective the work is very interesting
because it provides a much-needed framework to understand why something like
MOND could exist. But I don't think that any of the direct detection
experiments should pack it in quite yet.

[1]: [https://arxiv.org/abs/1510.03856](https://arxiv.org/abs/1510.03856)

[2]: Well, sort of. Galaxy formation is still a big open problem, and globular
cluster formation is probably even less well understood. But with WIMP dark
matter it's very plausible that different formation mechanisms would produce
objects with different amounts of dark matter.

~~~
m_mueller
> Globular clusters are observed to have virtually no dark matter

That's a fascinating angle, first time I've read this.

> But with WIMP dark matter it's very plausible that different formation
> mechanisms would produce objects with different amounts of dark matter.

Could you expand a bit how WIMPs can explain that? Do dwarf spheroidals have a
strong central black hole while globular clusters haven't? Is the galactic
black hole needed to capture WIMPs from cosmic background, sort of like gas
giants capture lots of moons?

~~~
antognini
Dwarf spheroidals would form in the process of galaxy/structure formation. In
this process, it's really dark matter that drives growth, and ordinary matter
is just along for the ride. In the early universe the dark matter is diffusely
spread throughout the universe, with slight overdensities in various places.
These then collapse under their own gravity and form sheets, then filaments,
then finally fragment into different clusters. This would produce dark matter
haloes of a variety of sizes, some of which would be dwarf spheroidals. I
should note that there are problems with this picture --- the predicted
distribution of dark matter halo masses does not match the observed
distribution. In particular, standard galaxy formation theory predicts far
more dwarf spheroidals than are actually observed.

In the case of globular clusters, the cluster would form independently of dark
matter. Just as in an open cluster in the galaxy, you would perhaps have some
sort of a gaseous cloud of ordinary matter (mostly hydrogen), which would
collapse, fragment, and form stars. There are lots of open questions about how
globular clusters form, though, so this is just a very vague sketch.

~~~
m_mueller
Thanks!

------
bambax
> _Physicists do not like magic; when other cosmological observations seemed
> far easier to explain with dark matter than with MOND, they left the
> approach for dead._

But how is "dark matter" not magic?

To my untrained and pretty ignorant eye, dark matter looks like filler: the
observation does not match the theory, and so, instead of looking for a better
theory, we decided the observation was wrong and couldn't see what was there
(and not just a little bit, but the greatest part of it).

Isn't that the definition of superstition? Invisible forces that explain
events, in mysterious ways?

~~~
phaemon
> But how is "dark matter" not magic?

Because the theory made predictions that were then checked.

> instead of looking for a better theory, we decided the observation was wrong

Except they didn't. They looked for a better theory. In fact, they considered
multiple different theories. And the dark matter theory was the one that fit
the observations best.

~~~
edblarney
"Because the theory made predictions that were then checked."

? What predictions and made and then tested by 'Dark Matter'?

Dark Matter was conceived of the other way around - it helps confirm
predictions that don't actually work properly.

If Dark Matter theory made testable predictions, it would be accepted no?
Where are these experiments?

~~~
Synaesthesia
It's widely accepted. And I'm also a sceptic. More recently scientists have
started actually mapping out regions where dark matter is concentrated than -
from gravitic perturbations - and have discovered galaxies which contain more
dark matter than others, up to 99% as well as the mentioned globular clusters
which contain very little dark matter.

~~~
edblarney
"More recently scientists have started actually mapping out regions where dark
matter is concentrated than - from gravitic perturbations "

This is not evidence of Dark Matter - it's really just quantifying and
classifying the perturbations and anomalies derivations in gravity that we
see.

Basically, more detail to the observation that 'galaxies are not rotating the
way we think they should'.

That doesn't validate Dark Matter, it just more deeply quantifies the unknown.

------
drewhk
I am not a physicist, in fact, I am a complete dilettante, but can the
transition from a power law to an inverse law simply indicate that on large
scale the universe has less dimensionality? I mean, if we take a suitable
definition of dimension, likely along the lines of Hausdorff dimension
([https://en.wikipedia.org/wiki/Hausdorff_dimension](https://en.wikipedia.org/wiki/Hausdorff_dimension))
then we can see it as a number that defines how volume/surface changes with
changing diamater of an observed "sphere" (I know I oversimplify things here).
In other words, as we start to measure cosmic distances on a large enough
scale, volumes/surfaces covered don't grow quadratically anymore but with a
lower rate. This might be even in line with the assumption of a universe
curved into itself, i.e. closed. This would change the gravity power law from
having a constant 2 power to having a function as the power.

~~~
qubex
(Constant _negative two_ power to having a function as the power.)

~~~
drewhk
It depends on whether you imagine a fraction line there or not:
[https://wikimedia.org/api/rest_v1/media/math/render/svg/8c6e...](https://wikimedia.org/api/rest_v1/media/math/render/svg/8c6ee5510ba3c7d6664775c0e76b53e72468303a)
;)

------
marze
From the final paragraph:

“If somebody were to come to you and say, ‘The solar system doesn’t work on an
inverse-square law, really it’s an inverse-cube law, but there’s dark matter
that’s arranged just so that it always looks inverse-square,’ you would say
that person is insane,” he said. “But that’s basically what we’re asking to be
the case with dark matter here.”

Awesome point.

The McGaugh paper is really problematic for dark matter proponents. Amazing
that that work wasn't done earlier.

~~~
hvidgaard
They do state:

"They say the amount of dark matter in a galaxy’s halo would have precisely
determined the amount of visible matter the galaxy ended up with when it
formed".

It's not a far fetched idea, but it is problematic because we know of zero
such interactions.

------
PhantomGremlin
Rotation speed, rotation speed, rotation speed ... that's always been the
poster child for dark matter.

But do we _truly_ understand how galactic rotation speeds should vary with
distance? A typical galaxy has a hundred billion stars in it. Given such a
complex system, can we truly simplify the expected rotation speed to something
like "distance squared from the galactic center"?

The "three-body problem" is difficult enough, do we trust our approximations
to properly model the "100,000,000,000 solar system problem"?

~~~
EdHominem
There's a difference between "complex" and "has no closed-form solution".

It's actually a very simple calculation.

But fwiw, a lot of thought and tests have gone into showing that gravity works
the same way with feather and bowling balls. With big masses and many little
ones. If you have a reason to think it doesn't, discuss it and propose
tests...

------
biggerfisch
So is this really anything except an interesting way of deriving MOND? The
article says

> Verlinde thinks his theory will be able to handle the Bullet cluster
> observations just fine.

but doesn't actually show it doing that. As that's a major differentiator
between his system and MOND, shouldn't that have been looked at? Seems to easy
to otherwise dismiss this as a weird way of supporting MOND

------
phkahler
I'm gonna pull my hair out. I saw this:

>> The researchers analyzed a diverse set of 153 galaxies, and for each one
they compared the rotation speed of visible matter at any given distance from
the galaxy’s center with the amount of visible matter contained within that
galactic radius.

That just smelled like they think Keplers law applies (which I've been arguing
against for ages) so I googled "radial acceleration relation" and found this
paper:
[https://arxiv.org/pdf/1609.05917v1.pdf](https://arxiv.org/pdf/1609.05917v1.pdf)
The opening paragraph is this:

>> The missing mass problem in extragalactic systems is well established. The
observed gravitational potential cannot be explained by the stars and gas. A
classic ex- ample is that the rotation curves of disk galaxies become
approximately at (V=constant) when they should be falling in a Keplerian
fashion.

To say the gravitational pull at a given radius depends solely on the amount
of matter inside that radius is utterly bogus. It is entirely dependent on the
distribution of that matter. It is valid if the distribution is a uniform
sphere (or a sphere of concentric layers each of uniform density) but that is
not the distribution in a galaxy. Likewise, you can ignore all the matter at a
larger radius IF it's distributed as a shell (or series of shells) of uniform
density - but that is not the case when dealing with rings instead of shells.
In my mind, this whole debate has wreaked of a misapplication of the
Divergence Theorem from day one. If you thing galactic dynamics have anything
to do with Kepler I have a nice bridge to sell you.

~~~
Tloewald
The use of Keplerian simply refers to the inverse square law, and if you think
the inverse square law isn't involved then maybe I should take that bridge off
your hands. You seriously think these guys don't know high school physics?

~~~
phkahler
The inverse square law applies to 2-body systems ONLY. I do in fact think they
know high school physics and are trying to misapply it. The use of that simple
physics can sometimes be used in other systems as a special case of the
divergence theorem (third year calculus) IF certain conditions are satisfied.
I'm just saying they keep applying by missing a key point in the justification
for doing so. Perhaps they're not actually using Kepler, but they sure do keep
bringing up his name - and I am correct that it's not OK to use that.

As I sometime say - the only dark matter is between the astrophysicists ears.
This assumes they're actually using Kepler as a reference for "expected"
behavior - some of those guys are smart beyond my comprehension.

------
mirimir
> One problem, said Brian Swingle of Harvard and Brandeis universities, who
> also works in holography, is that Verlinde lacks a concrete model universe
> like the ones researchers can construct in AdS space, giving him more wiggle
> room for making unproven speculations. “To be fair, we’ve gotten further by
> working in a more limited context, one which is less relevant for our own
> gravitational universe,” Swingle said, referring to work in AdS space. “We
> do need to address universes more like our own, so I hold out some hope that
> his new paper will provide some additional clues or ideas going forward.”

Woah. How can AdS model universes have less "wiggle room for making unproven
speculations"? There is no AdS universe to observe or experiment in. How could
speculations in such universes be "proven"? Or rather, disproved?

~~~
simonh
I think they're talking about mathematical proofs. AdS models are
mathematically simpler than the physical universe containing matter, so it's
easier to construct mathematical proofs.

~~~
mirimir
OK. But proofs about mathematical objects are entirely distinct from testable
hypotheses about the actual universe. So it seems odd to compare them. OTOH,
there is much "theoretical physics" that's not much more grounded in reality
than AdS models ;)

------
TheMagician0
Xenon1T's result is much anticipated:
[http://www.metaculus.com/questions/199/will-the-
xenon1t-expe...](http://www.metaculus.com/questions/199/will-the-
xenon1t-experiment-discover-wimp-dark-matter/)

------
benibela
Or MiHsC/quantised inertia:
[http://physicsfromtheedge.blogspot.de/2016/11/critique-of-
ve...](http://physicsfromtheedge.blogspot.de/2016/11/critique-of-verlindes-
gravity-1.html)

------
mabbo
Relevant recent xkcd comic: [https://xkcd.com/1758/](https://xkcd.com/1758/)

~~~
jkingsbery
I guess the state of the art is more complicated than the Department of
Astrophysics in the comic admits.

------
basicplus2
The concept of "Dark Energy" is as nebulous as "Dark Matter"

------
c3d
Dark matter and dark energy are ad-hoc adjustments to explain discrepancies
between observations and theory. The problem with such adjustments is that,
being mostly unobserved, they end up with too many degrees of freedom (e.g. a
distribution of invisible stuff where the density can be made "just right" to
fit the observations), so they are quite hard to falsify. It's a bit like the
"landscape" : when you construct a theory that can predict anything, you end
up predicting nothing. In my opinion, we leave "science" as defined by Popper
when we start doing that.

There is, however, an underlying assumption in all the existing theories of
physics, which I see too rarely challenged, and which is easily falsified.
It's the idea that the variable "x" in the theory is independent of the
physical measurement process being used. In other words, the "x" in MOND is
the same as the "x" in GTR and the "x" in quantum mechanics. The topology of a
space-time defined by a solid rod of metal in the Pavillon de Sevres (pre-1960
definition of the metre) is the same as the topology of space-time defined by
the movement of light in the vacuum (today's definition). And so on.

I believe that our current understanding of physics now has become so precise
that we need to take into account subtle differences between this and that
measurement process, and rewrite physics in the context of a "more general"
relativity, where the laws of physics are to be described independently not
just from, say, the state of movement, but more generally independently of the
physical process being used. Just like GTR forced us to deal with quantities
such as metrics, this more-general relativity would force us to deal with
arbitrary transformations between two methods to measure the same thing.

A simple example is when we relate time as measured by the rotation of the
earth (definition of "days"), the rotation of earth around the sun (definition
of "years"), the decay of populations of C14 ("radiocarbon dating"), and our
current cesium-decay based atomic clocks (the definition of "second"). The
four laws are obviously correlated, they all measure time. But the "fit curve"
between them is a distribution in itself, similar to the "metric field" in
GTR.

I may be wrong, but I suspect that if we introduce this kind of complexity,
the problem of "dark matter" will have not one, but multiple solutions,
possibly different equations, depending on the measurement you are
considering. This does not mean we should not look for equations that predict
observations, but rather that we should be cautious when correlating
measurements that correspond to different physical processes. It is not
obvious to me, for example, that we should take for granted that we can safely
deduce the distribution of masses from the distribution of luminosities (even
if, I'll admit, I don't see any good alternative...)

In quantum mechanics, we do little else with renormalization. When you shift
from one scale to another, you need to renormalize to fit the data, because
the raw equations would otherwise yield to infinities or other abnormalities.
"Shut up and calculate" is frustrating, but if we rephrase that as "observe
the universe, it's the ultimate test", it really means the same thing, but is
philosophically more grounded.

In the end, MOND may just be the shape of the relation between two
measurements of mass at large scale, and Verlinde's paper one first step in
understanding how this relationship emerges.

Or not. There are some interesting objections to "statistical emergence" of
gravity, or "entropy-based gravity". One of them is how interference patterns
are not destroyed by gravity, at scales where, if we were to trust Verlinde's
approach, interactions along different paths would affect probabilities enough
to be sensible in interference patterns. I've not done the calculations myself
to validate if this objection is solid, though. Intuitively, it seems like
there is a flaw (the effect of per-state populations on interference should be
at most of the same order as the effect of gravitation itself, which is in the
"barely observable" category), but I need to spend more time to understand
both sides.

~~~
raattgift
'and our current cesium-decay based atomic clocks (the definition of
"second")'

Cs-beam and Cs-fountain frequency standards rely on the transitions between
the principal microwave resonances of Cs-133 atoms in their ground state. You
pop a gas of Cs-133 atoms prepared in one of the hyperfine states into a
tunable microwave cavity coupled to a crystal oscillator in a feedback loop
which maximizes the transitions of the Cs-133 atoms to the other hyperfine
state. The longer the exposure time to the ~ 3.26 cm microwaves and the colder
the gas, the better the signal with which to steer the microwave frequency
towards the frequency in the SI second. Ideally you have a gas at 0 kelvins
spending an infinitely long time in the microwave cavity, and your feedback
loop does careful electronic division or multiplication of the tuned frequency
to produce a round output signal, often at 1, 5 or 10 MHz exactly.

Decay has nothing to do with it. Cs-133 is stable.

I'm afraid I don't understand your commentary on General Relativity vs MOND at
all, especially not the part about the '"fit curve" between them is a
distribution in itself, similar to the "metric field" in GTR'.

~~~
c3d
The phrase "cesium-decay" was a rather inadequate shortcut. I was thinking of
the excited state "decaying" back to the ground state, but you are right, it
was not the right word.

The metric field in GTR is defined by the distribution of matter. So starting
with GTR, to make another shortcut, we had to take the distribution of matter
into account when "converting" from one set of coordinates (measurements) to
another. In other words, the space of "transfer functions" from one set of
measurements to another is already quite rich.

Similarly, when you transfer from one measurement of time to another, the
"transfer function" is also quite rich. If you want to convert precisely from
atomic clock to earth movements, you have to take into account interaction
with many planets and other celestial bodies. So again, this transfer function
is rather "arbitrary".

My point here is that the laws of physics are no less good or less precise
when expressed in years than they are when expressed in oscillations of Cs-133
atoms. Some physical systems, e.g. atomic interactions, will be easier to
describe relative to Cs-133. But others (e.g. meteorology) are expressed much
more simply if you base them on celestial movements.

If the transfer function between, say, radiocarbon dating and cesium clocks
can be practically anything, what tells us that the transfer function between
light densities and matter densities can safely be assumed to be practically
an identity?

And if it's not an identity, then why should the laws of gravity as expressed
using light distributions match the laws of gravity as expressed using matter
distribution? I think this is an oversimplification that we need to get rid
of.

Hope I'm making more sense to you ;-)

~~~
raattgift
I'm sorry, no, this is even less clear to me.

GR has coordinate freedom built into it; you can apply (almost) any set of
coordinates (locally) on spacetime and calculate against them as you wish. The
metric, however, is fixed by the geometry itself, and it is the metric that
determines the interval in _any_ set of coordinates. That's why you can write
down e.g. the Minkowski metric in cartesian or spherical coordinates and come
up with the same dS^2 -- the spacetime interval -- or as another example the
Schwarzschild metric in Schwarzschild coordinates, in- or outgoing Eddington-
Finkelstein coordinates, Kruskal-Szekeres coordinates, etc etc and come up
with the same dS^2 even though the line element is written very differently in
each case, and even though _coordinate_ singularities appear in some systems
of coordinates but not in others. (That they vanish in _any_ valid coordinate
system shows that they aren't physical singularities; the singularity in a
Schwarzschild black hole that does not vanish under at least valid coordinate
system is a strong argument that it _is_ physical, although that remains a
subject of research.)

One of the main features of GR is that it gives you a clear and mathematically
provable mechanism for translating from one set of valid coordinates to
another. That's one of the main reason it's called _General_ : the coordinate
systems need not be in uniform motion with respect to one another, and even
more crucially, they do not even have to be in the same tangent space. There
is nothing at all arbitrary about how one "transfers" from one system of
coordinates to another: the Bianchi identities on the Einstein Field Equations
give the degrees of freedom necessary for a gauge transformation.

The physicality of singularities in some exact solutions of the Einstein Field
Equation (like Schwarzschild's as mentioned above) and the work of Kenneth G
Wilson has led most general relativists to think of GR as an effective field
theory (EFT) valid everywhere just outside of irremovable singularities. That
means GR is accurate even _inside_ the event horizons of black holes. Really
only the singularities themselves are the problems with GR -- in particular
it's the problem of its canonical quantization, and thus the driver for a
theory of quantum gravity that works much closer to the irremovable
singularities (or better still, makes them vanish).

But because GR is easy to consider an EFT, it is even easier to treat regions
of spacetime that are not exactly flat as if they are exactly flat with some
perturbations. So, in fact, I do not at all agree with you that "... you have
to take into account interaction with many planets and other celestial bodies
..." since in a perturbative approach, their contributions can be removed as
irrelevant. Even _nearby_ celestial bodies can be removed as marginal. Here
"marginal" and "irrelevant" are vocabulary right from Wilson, and translate
very nicely into a Feynman diagram approach by counting loops of gravitons.

Or alternatively, I could _technically_ agree that you have to take into
account those bodies you mention, but since you do so simply by ignoring them
explicitly on the grounds that their contributions to the metric provably do
not matter for the local system under consideration, that contradicts your
assertion in the paragraph "Similarly...".

I don't understand how your fourth last to second last paragraphs inter-
relate. The first of those approaches an argument about coordinate freedom;
the second I cannot decipher at all (it certainly isn't a General Relativity
argument; I think you're looking to argue with the Standard Model instead, and
that's defined on flat spacetime, and by the EFT argument above, flat
spacetime applies in any radiocarbon dating experiment you choose to do on
Earth -- and when you repeat the same experiment, the secular variation in the
proximity of the the moon or sun or other planets will make not even a
marginal difference). And your second last paragraph is I think a question
about multimetric gravity -- we have strong evidence for the universal
coupling of everything in the Standard Model to the single metric, and if you
want to introduce a second metric for some sector of matter, you have to go
beyond the Standard Model, and you have the problem of making the coupling
just universal enough that you reproduce GR's matches with observation. There
are ways of doing that, but most of them have the extra metrics decay
extremely early in the history of the universe so that they do not produce ANY
observables. Afshordi and Magueijo's recent paper is interesting because they
believe they can produce an observable in spite of a decaying extra metric to
which light couples. That comes at a cost though: you would have to introduce
a zoo of gauge particles in order to make this work at all, which hardly makes
the theory more simple than Cosmic Inflation.

