
First test of rival to Einstein’s gravity kills off dark matter - edran
https://www.newscientist.com/article/2116446-first-test-of-rival-to-einsteins-gravity-kills-off-dark-matter/
======
jessriedel
Folks, the observation doesn't test Verlinde's idea, this tests a very general
class of models, such as the well-known MOND scheme.

> Milgrom, however, supports the work. He also points out that according to
> his own 2013 analysis of gravitational lensing data in galaxies, MOND
> produces similarly impressive results as Verlinde’s gravitational model does
> in Brouwer’s study.

(Also: [http://motls.blogspot.ca/2016/11/verlindes-de-sitter-mond-
is...](http://motls.blogspot.ca/2016/11/verlindes-de-sitter-mond-is-
highly.html) )

This data cannot distinguish between Verlinde and other explanations for MOND-
like behavior. We've long known that MOND seems to fit a _subset_ of all
observational data better the dark matter, in particular the Tully–Fisher
relation

[https://en.wikipedia.org/wiki/Tully%E2%80%93Fisher_relation](https://en.wikipedia.org/wiki/Tully%E2%80%93Fisher_relation)

However, for the rest of the data (e.g., the CMB, large-scale structure, the
bullet cluster), MOND is generally considered to give a worse fit than dark
matter, or to be silent (because it's not obvious how to extend MOND to a
complete cosmological theory). That's why experts think the totality of
evidence supports dark matter.

Nothing has changed.

~~~
DennisP
Seems like it's pretty easy to make dark matter a good fit, because you can
just adjust the presumed distribution of dark matter to fit the data.

~~~
jessriedel
Nope, this is only an issue on galactic scales and below. On supergalactic
scales, where most of our data comes from, the DM distribution is determined
by theory. That includes all the stuff I mentioned: LSS, CMB, bullet cluster,
etc.

~~~
DennisP
So, my impression from popular articles is that physicists don't actually know
what dark matter is, and have a fair number of candidate ideas. According to
the theory that makes good predictions, what is dark matter made of?

~~~
jessriedel
Dark matter (DM) is typically modeled like electrons are modeled: as a species
of massive fermion, described by a quantum field. The parameters that go into
the model (both DM and the electron) are the mass of the particle, plus its
various couplings to the other known particle species.

In this sense, the only difference in our knowledge of DM and electrons is the
DM mass and the DM coupling constants. For the electron, these numbers are
known to high precision. For DM, they could lie anywhere in a range over many
orders of magnitude (although they are generally bounded from above, because
we have not seen evidence of strong interactions).

At supergalactic scales, the predictions of the DM model are not very
sensitive to it's mass or couplings. On galactic scales, various nonlinear
processes in normal matter (such a supernovea) and possible DM interaction
effects become important. Since we don't know those things, the theory has
free parameters.

Folks definitely consider other DM models, e.g. bosons, but these are only
considered to be favored over MOND insofar as they agree with the same
observational data. In other words, the set of all theories of DM which
produce the same observational predictions forms an "equivalence class"; and
it's the _equivalence class_ , not a particular choice of DM mass and
couplings, that is favored over MOND. This is basically what people mean when
they talk about "cold dark matter", it's the CDM in the LambdaCDM model, the
standard model of cosmology.

[https://en.wikipedia.org/wiki/Lambda-
CDM_model](https://en.wikipedia.org/wiki/Lambda-CDM_model)

~~~
amluto
In my intro cosmology class, we modeled dark matter as a perfect fluid with
some classical density field that interacted with everything else until some
very short time after the Big Bang and then stopped interacting except by
gravity. We set the equation of state to match that of normal matter: if you
put 1 gram of dark matter in a box and slowly expand the box to double its
volume, you still have 1 gram.

What makes fermionic dark matter be preferred over (massive) bosonic dark
matter? I'm not sure why it should make any difference.

~~~
jessriedel
> I'm not sure why it should make any difference.

It doesn't (modulo theoretical prejudices against bosons). I was just picking
something concrete and popular so I could talk about how it's behavior becomes
uncertain on small scales.

------
beambot
> String theorist Lubos Motl savaged Verlinde’s ideas in a recent blog post:
> “I wouldn’t okay this wrong piece of work as an undergraduate term paper.”

I find this rudeness and pedantry in academia infuriating. There's no
justification for this level of nastiness -- not in private, public, peer
review, or science as a whole. Seeking out the unknown and creative
explanations thereof is the hallmark of good science; check your ego at the
door. I know nothing about him... but in my book, Lubos Motl can go pound
sand.

~~~
bsder
> I find this rudeness and pedantry in academia infuriating.

So you'd rather people quietly knife you in review? Because that's what
happens if you penalize people for public rudeness. Personally, I'd rather
have a loud critique to my face than a polite one behind my back or, even
worse, no engagement at all.

I don't need your politeness or friendship when I'm putting forth some new
theory. I need accurate, engaged criticism and the number of people who will
do that is vanishingly small. And a lot of the ones who will do that have
social issues almost by definition.

~~~
Frondo
Accurate, engaged criticism is great, that stuff belongs everywhere.

"I wouldn’t okay this wrong piece of work as an undergraduate term paper" is
not accurate, engaged criticism, it's mean-spiritedness under a veneer of
toughness.

It's easy and fun to be rude and insulting, and when you can get away with it
because of the subculture you're writing within, why not, right? Because being
a dick is being a dick no matter who or where you are, that's why, and being a
dick doesn't advance anything beyond your own momentary joy.

Every industry, every field, everywhere, will be better off once people get
past being dicks just for fun, and give accurate, engaged criticism while
maintaining civility.

~~~
quanticle
_Every industry, every field, everywhere, will be better off once people get
past being dicks just for fun, and give accurate, engaged criticism while
maintaining civility._

I disagree. The blunt people (who come off as "dicks" to the outsiders) are
like a community's immune system. They protect the community against the sort
of people who take accurate engaged criticism, make one or two changes to fix
specific issues, and then resubmit their work, wasting the community's time
and resources, driving off the people who are making _good-faith_ efforts to
help the community. They're not being dicks "just for fun". They're protecting
the time and attention of other community members by saying bluntly what many
other people are often thinking.

Communities that have too many of these people become closed and ossified.
Communities that have none of these people evaporate, as the core contributors
just walk away and find better things to do than deal with people who are more
interested in getting fame and attention for themselves than making good-faith
efforts to advance the project of the community. The trick is to find a good
balance.

------
pavel_lishin
I'd be kind of bummed to lose dark matter as a concept. I want to hold onto
the idea that it's just as rich as our matter, with its own quantum
chromodynamics and an equivalent to the electromagnetic force, leading to a
"shadow" universe with their own planets, life, etc. that exists side by side
us, trying to figure out why their gravity measurements are just slightly
off...

~~~
logfromblammo
You can also have the idea that there are one-way interactions between the
normal-matter universe and the dark-matter universe.

It isn't too difficult to formulate a mathematical system where two subsets
are completely identical when considered in isolation, but when interacting,
one subset operates as if the other doesn't exist, while the other is easily
perturbed by any values in the mirror subset. I'm not sure if you can do it
without at least one dimensional basis unit that squares to zero, though.

Imagine, if you will, that there are dark-matter people. We have no evidence
whatsoever that they exist, any they are constantly annoyed by all the real-
matter people that wander between their dark-couches and dark-televisions, and
the real-matter bicyclists that pedal through their dark-bedrooms at the worst
possible times.

~~~
pavel_lishin
Wouldn't there be some sort of energy exchange for this to happen? Wouldn't we
notice that there's an unexplained loss of energy whenever we do anything?

Or did I just invent a crappy layman-author's explanation of where inertia
comes from?

~~~
logfromblammo
Conservation of energy/mass is a rule for the known universe. If we knew all
the rules for the dark part of the universe, we wouldn't even be having this
discussion.

Anyway, the way the math works out, it is impossible for a known-universe
person to even measure the effects the known-universe has on the dark-
universe. The numbers can't come back to the known-universe without an
additional input from it, and they would be explainable with another
hypothesis that did not include a dark-universe. It way be that all effects we
observe to be random are actually perfectly predictable to a dark-universe
observer.

------
bmh100
Please change the submission title. It is downright misleading. This test did
not "kill off" dark matter. To do so would imply overwhelming evidence against
the concept of dark matter as a whole. Instead, this theory, which eliminates
the need for dark matter in certain situations, has passed its first test. The
sentence "... test ... kills off dark matter" is simply false.

~~~
acqq
Moreover, it's not even a complete "theory" in the sense that it's still not
known if it can even repeat the results of the General Relativity.

[https://news.ycombinator.com/item?id=13197595](https://news.ycombinator.com/item?id=13197595)

~~~
raattgift
It's not complete for other reasons, several of which are detailed in
Verlinde's paper itself; however, it wholly reproduces GR in the EFT limit by
design. Indeed, Verlinde starts with standard dS as his unremovable background
on which he puts strings. What in GR we would consider perturbations of dS and
IR corrections of GR (at ~ galactic scales) emerge from the behaviour of the
strings and how they form long-distance entanglements.

However, it's not outrageously wrong, and it is noteworthy that an
accomplished string theorist has decided to tackle gravity in a universe like
ours rather than leaning on AdS/CFT arguments.

~~~
acqq
Thanks for the correction and clarification.

For others who, like me, aren't in the field, the "EFT" mentioned is
explained, for example, here:

[https://arxiv.org/abs/gr-qc/0311082](https://arxiv.org/abs/gr-qc/0311082)

"The problems of quantizing gravity within the experimentally accessible
situations are similar to those which arise in a host of other non-
gravitational applications throughout physics. As such, the size of quantum
corrections can be safely estimated and are extremely small. The theoretical
framework which allows this quantification is the formalism of effective field
theories."

dS is "de Sitter space":
[https://en.wikipedia.org/wiki/De_Sitter_space](https://en.wikipedia.org/wiki/De_Sitter_space)

I'd also appreciate a link to the explanation of "What in GR we would consider
perturbations of dS and IR corrections."

~~~
raattgift
> I'd also appreciate a link to the explanation of "What in GR we would
> consider perturbations of dS and IR corrections."

Here's a quick overview of perturbation theory then an example of how one
might apply it in GR.

[https://www.wikiwand.com/en/Perturbation_theory](https://www.wikiwand.com/en/Perturbation_theory)

[http://theory.physics.helsinki.fi/~inf/Lectures/Lecture2.pdf](http://theory.physics.helsinki.fi/~inf/Lectures/Lecture2.pdf)

IR is "infrared", low-energy physics, as opposed to UV, "ultraviolet", high-
energy physics. In this context, UV means strong gravity where quantum effects
are expected to be important. Galaxy-galaxy gravitational lensing (which is
the subject of the Brouwer et al. paper that's the subject of the newscientist
article linked at the top) is purely a weak gravity problem[1], so is in the
IR limit of General Relativity. If you want different results from standard
General Relativity you can add corrections by hand in a variety of ways; in
the galaxy-galaxy lensing case they would be in the infrared.

[1] strong gravity is hidden behind the event horizons of the black holes of
the galaxies and, arguendo, if we could see the strong gravity near the black
hole centres, it would not be relevant by virtue of being drowned out by all
the stars, gas, dust and other matter.

ETA: This is nice and terse, too: [https://www.wikiwand.com/en/Non-
exact_solutions_in_general_r...](https://www.wikiwand.com/en/Non-
exact_solutions_in_general_relativity)

------
thejollysin
Physicist here. Please do not put too much stock in this paper.

Some of the folks on this paper are good scientists, but this appears to be
more about looking for funding than good science.

Their paper on the Arxiv is about building some math to make gravity look like
dark matter. But this has been done 1000 times before with no success. They
have no physical basis for their math; there is no reality behind it. I do not
expect this to get them the Nobel Prize.

------
empath75
'Kills off dark matter" is probably overstating the case, but I love this
theory because it ties together a lot of big ideas.

Verlinde's paper is here:
[https://arxiv.org/abs/1611.02269](https://arxiv.org/abs/1611.02269)

~~~
noobermin
What about the Bullet Cluster[0]? How does his idea address that?

His idea deserves some study but a title will "Kills off dark matter" is very
infuriating

[0]
[https://en.wikipedia.org/wiki/Bullet_Cluster](https://en.wikipedia.org/wiki/Bullet_Cluster)

~~~
greglindahl
If you read the Wikipedia article, it points out what MOND advocates say about
the Bullet Cluster: they try to handwave it away. It's likely that Verlinde's
theory needs a similar solution. Unfortunately for handwaving, over time, the
number of weird dark-matter-related observations is growing.

------
Jun8
You may find the top answer to this SE question helpful (asked 5 years ago,
though): [http://physics.stackexchange.com/questions/6561/are-
modified...](http://physics.stackexchange.com/questions/6561/are-modified-
theories-of-gravity-credible). Especially I think the following comment is
spot on:

"Galaxy rotation curves are not very good tests for GR itself, since there are
so many parameters about mass distribution in a galaxy which are simply not
nearly as precisely understood as GR itself"

~~~
Dylan16807
That says MOND is a non-theory, so now I'm even more confused since this
theory "builds on" MOND.

~~~
CoryG89
Now I'm confused. What is a non-theory? O.o

Theories by definition, may not be correct. It's still a theory.

How does one become a non-theorist? I suppose by non-theorizing.

~~~
Dylan16807
One way is to propose something that's mathematically equivalent to existing
theory. If you have no testable predictions then you're not really doing
science. You're doing philosophy.

I'm not sure if that's the objection here. They might also be calling it too
vague to be a theory, or something else.

------
Florin_Andrei
> _Now, a team of astronomers studying the distribution of matter in more than
> 30,000 galaxies say their observations can be explained by an alternative
> theory that does away with dark matter. If this “modified gravity” is
> correct, it would up-end hundreds of years of fundamental physics._

No, it would reshape the "top" of these theories. The base would remain the
same.

------
brilliantcode
Can someone explain in layman's terms what really is dark matter and why
there's so much obsession around it? If we can't observe it and we are only
relying on inferences to explain it's effects, that seems like pretty weak set
of legs.

Obviously I'm missing the academic discipline to appreciate this topic but
nevertheless, I'm curious about our understanding of the very fabric and
mechanisms of our reality and the universe.

~~~
thejollysin
"Dark Matter" is a phrase we use to describe a hole in our understanding of
the universe. To put it simply, when we look out into space, at very large
scales like galaxies, everything is just too heavy. It _appears_ as if there
is a LOT more matter out there than we can see. Things are orbiting faster
than they should be, like galaxies are heavier than makes good sense..

Maybe there is some matter out there that is "dark", so we just can't see it.
That would be a solution. Or maybe the laws of gravity we wrote down are bad
at large scales. No one knows. Trust no one who tells you they know the
answer: if they did they would have a Nobel Prize.

~~~
misnome
Where "Dark" literally means: does not interact in any way with light

------
Animats
The title overstates the results of research work. From the article: "Now, a
team of astronomers studying the distribution of matter in more than 30,000
galaxies say their observations can be explained by an alternative theory that
does away with dark matter."

Suggest "Astronomers studying galaxy mass distribution say their observations
can be explained without dark matter."

------
FeatureRush
What would be the next test? Clusters or that dark matter dwarf galaxy causing
ripples in our Milky Way?

~~~
thejollysin
The "next test" would be to look at more galaxies. They did not actually try
to apply their theory too many real world observations. The goal of this paper
is to get funding to try and apply their new theory to a huge number of
galactic measurements. Until they do THAT, no one in the physics community is
going to listen to them.

------
andrewflnr

      “But then if you mathematically factor in the fact that
      Verlinde’s prediction doesn’t have any free parameters,
      whereas the dark matter prediction does, then you find
      Verlinde’s model is actually performing slightly better.”
    

What? How does this work?

~~~
fjarlq
From the discussion section of Verlinde's paper[1]:

> _We have shown that the emergent laws of gravity, when one takes into
> account the volume law contribution to the entropy, start to deviate from
> the familiar gravitational laws precisely in those situations where the
> observations tell us they do. We have only made use of the natural constants
> of nature, and provided reasonably straightforward arguments and
> calculations to derive the scales and the behavior of the observed
> phenomena._ [..]

> _In our view this undercuts the common assumption that the laws of gravity
> should stay as they are, and hence it removes the rationale of the dark
> matter hypothesis. Once there is a conceptual reason for a new phase of the
> gravitational force, which is governed by different laws, and this is
> combined with a confirmation of its quantitative behavior, the weight of the
> evidence tips in the other direction._

[1]: [https://arxiv.org/abs/1611.02269](https://arxiv.org/abs/1611.02269)

~~~
andrewflnr
That's just Occam's razor, though. The phrasing made it sound like they had a
quantitative argument for why fewer free parameters made it "perform" better.

------
daxfohl
The central equation in this paper is 4.40. It doesn't seem to account for
some galaxies recently discovered that are almost entirely dark matter.

------
jsilence
Totally OT: I read "Einsteins graffitti" and could watch my own brain trying
to make sense of the sentence. And fail.

------
josefdlange
Juuuust gonna leave this here...

[http://xkcd.com/1758/](http://xkcd.com/1758/)

~~~
deepnotderp
This is different because it's an experiment for this theory.

~~~
jessriedel
It's not different because this experiment can't discriminate between
Verlinde's ideas and long-known alternatives to dark matter.

[https://news.ycombinator.com/item?id=13197214](https://news.ycombinator.com/item?id=13197214)

