
It's time to take alternatives to dark matter seriously - starmftronajoll
https://aeon.co/essays/why-its-time-to-take-alternatives-to-dark-matter-seriously
======
knzhou
These articles always miss the mark because they get the history backwards.
Modified gravity theories were taken much more seriously in the 1970s and
earlier, where the evidence for dark matter was shakier. Cosmological and
astrophysical observations from the 1980s to 2000s have vastly strengthened
the case for dark matter, which is why it's the leading hypothesis now.

Popular articles won't tell you this, because _without exception_ they laser
focus on galaxy rotation curves, a piece of evidence that's nearly a century
old and by far the weakest one. But by neglecting to mention the actual
evidence we base our conclusions on, they (purposely or not) make us look like
fools.

Furthermore, dark matter hasn't been ruled out. One particular candidate of
what it could be (a WIMP) has been studied and mostly ruled out over the past
20 years, but by the nature of the business there are many possible
candidates. And of course we've shifted attention towards those other options,
which is exactly how science is supposed to work!

~~~
mehrdadn
Question: as a layman, knowing nothing about the quantitative nature of the
evidence, neutrinos seem like the perfect candidates for dark matter, since
we've already detected them and we _know_ interact so poorly with anything.
Why are they not the obvious candidate for dark matter? Is there any evidence
against them?

~~~
knzhou
That's a perfectly good question, and that's exactly why for a time in the 90s
they were one of the leading candidates for dark matter. Unfortunately it fell
apart when we got a better understanding of structure formation, which
determines the large-scale distribution of matter in the universe today. If
the 3 neutrinos have masses around where we think, then they'd be too light
and move too fast in the early universe, smoothing the distribution out and
thus suppressing small-scale clumping, contrary to observations. On the other
hand, if the 3 neutrinos were a lot heavier (possible we can only detect their
mass differences, not the absolute masses) they would screw up the CMB. The
qualitative case was great, but quantitatively it just didn't work out.

~~~
kerkeslager
I'm not an astrophysicist and well outside my depth here, but couldn't
observed clumping with low-mass neutrinos suppressing clumping be explained by
an older universe as well? Put another way: if clumping is suppressed, that
doesn't mean it doesn't happen, it just means it happens _slower_ than
initially thought, no?

~~~
knzhou
Also a good idea, but the details don't work out. Specifically, early on the
Fourier components of the matter density distribution grow independently.
Light neutrino DM would suppress the small scale clumping too much. Making the
whole process last longer would fix this, but it would also enhance large
scale clumping, contrary to observation.

It might be possible to make it all work out by adding a third effect (and
maybe somebody has already done that!), but there's also a danger of
constructing a Rube Goldberg machine -- if there are too many gadgets needed
to get the structure right it starts looking like epicycles. The great thing
about traditional dark matter is that it doesn't require any cosmological
complications, it just works.

~~~
kerkeslager
So lemme check my understanding here: are you saying that clumping has an
observed size distribution, and if dark matter were just neutrinos, there
isn't a timescale which would produce the clumping size distribution we see:
if the timescale were short then we would see less small clumping than we see,
if the timescale were longer then we would see more large clumping than we
see?

Neat.

------
exmadscientist
I used to work on dark matter experiments. (This was a few years ago now, and
wasn't for very long -- I was helping out colleagues -- but I don't think the
situation has changed much.)

I don't think anyone worth listening to considers dark matter to be a
Universal Truth or anything. It's a crappy theory that needs better
experimental evidence on the micro scale. It's just that all attempts to find
that evidence have turned up nothing. (Except DAMA/LIBRA, the bane of the
previously mentioned colleague's days and motivating factor for the experiment
in question... but nobody actually believes them, so let's value their reports
at "nothing".)

The biggest problem is that all alternatives to date are worse. They _all_
_have_ _problems_ , problems of equal or greater severity than dark matter.
That, more than any other reason, is the reason dark matter is still the
leading theory.

I do strongly believe we should work on the advancement of non-dark matter
theories. That's how you lap the current leader, after all. And neglecting
alternative theories was what got us into a couple decades of string theory
stagnation, a mistake we'd be wise not to repeat. If a new theory or new
evidence for an existing theory emerges, dark matter will lose mindshare
amazingly quickly.

There is, of course, a "nightmare scenario". The WIMP carries a weak charge,
by definition. It was dreamed up that way to make it detectable and have a
presence in the universe. (And there may have been a theoretical preference; I
wouldn't know, I've always been an experimentalist.) But there's no reason it
_must_ carry a charge, weak, strong, or electromagnetic. That would make it
really quite invisible to experiments. This may be the universe we live in.

~~~
sgt101
But doesn't it have to have some charge to interact with matter in some way so
as to achieve the observed effects that show that gravity from normal matter
is inadequate to explain how things move on a macro scale?

~~~
LoSboccacc
> have to have some charge

it can also be a new interaction driven by a completely different fundamental
force. it's extremely unlikely, but "have to" is quite a strong statement for
something we only know exists because observations deviate from our
expectations.

~~~
enkid
How do you even judge the likelihood of something like that?

~~~
LoSboccacc
same as everything else, the more changes it requires from the current model
the less likely it's the solution

~~~
enkid
That's not how I judge likelihood at all. If you were to say that there is
more mathematical wiggle room for other explanations, I would be more likely
to agree, but simply having to change models has no impact on likelihood. That
thinking would make Quantum Mechanics, Special Relativity, Natural Selection,
or Genetics unlikely. Your argument could even be used to explain why it's
more likely that the earth is flat because that's the intuitive model.

~~~
LoSboccacc
unlikely doesn't mean it's less good of a theory. it means the bar for
verification/falsification is so much higher.

------
kurthr
I just find this odd... we've seen Dark Matter fill in the gaps of standard
astronomy (general rel) models of galactic evolution, but it's been predicted
and seen in multiple places over the last 50 years and the measurements keep
getting stronger (cosmic background spatial distribution, galactic rotation,
bullet cluster). Each measurement rules out some model explanation, and we
search for different particles (WIMPs, MACHOs, neutrinos) that fit the models
that work.

We still have interesting testable explanations... but let's give up now, it's
boring? MOND again?

Sorry, axions are interesting and may explain other things (antimatter ratio
etc) and if that fails I think others will pop up, that don't require
reexplaining astronomy.

[https://phys.org/news/2020-06-case-axion-dark-gains-
traction...](https://phys.org/news/2020-06-case-axion-dark-gains-
traction.html)

~~~
AmericanChopper
The very nature of the study of physics has created a long history of
untestable theories to fill in the gaps. Because candidate theories that can
be tested either solve the problem or remove themselves from contention, so
until the problem is solved, the untestable ones are the only ones that last
any significant amount of time.

The theory of luminiferous ether had a large amount of circumstantial
experimental evidence for quite a long time, but all it really was, was a
placeholder waiting for an actual explanation. Theories like that will always
exist on the frontiers of science. It’s not really unique to physics either.
Miasma theory played a similar role in the history of biology for instance.

------
Nevermark
> But now it’s time to consider...

Almost invariably when an article title says "it is now time to..." there has
been no actual threshold that has been crossed making "now" special at all.

The growing types of evidence for a "dark matter" effect on large scales, vs.
the absence of any small scale evidence for it, has consistently begged for
new particle, gravitational, and other types of theory to be explored.

It is a strange situation to have something appear so consistent with one of
our pillars (general relativity), while completely absent from the other
(quantum mechanics).

I am crossing my fingers that any successful understanding of dark matter on
the small scale might shed light on how those two pillars could be combined
into a single theory. But any explanation will be most interesting!

~~~
QuesnayJr
It's pretty easy to get a dark matter candidate out of quantum mechanical
theories of elementary particles. The current leading candidate, axions, was
proposed all the way back in 1977 for reasons completely unrelated to dark
matter.

------
gorgoiler
If you enjoy science fiction, one of the better recent SCPs was about dark
matter:

[http://www.scp-wiki.net/scp-4170](http://www.scp-wiki.net/scp-4170)

SCP is a world-building collaborative creating-writing project, focused on
sci-fi cosmic horror. Each entry is standalone, though there is also some
canon which contributors are encouraged to follow.

SCP-4170 was generally received as a positive entry, though I read it as being
far more sinister.

 _Miriam: [Request to still be taken seriously]_

~~~
Baeocystin
That really was a good one. Thanks for linking it!

------
zamalek
MOND was a healthy idea to pursue, not because it was the correct alternative
(I really have my doubts that it was) but because it was _AN_ alternative.

Maybe some postgrad might have had less of the mainstream science groupthink
indoctrination because of it.

That being said, dark matter is a real phenomenon: it is a measurable quantity
of how wrong we are. Dark matter isn't the problem, WIMP is.

~~~
Beldin
We have a theory and observations. Observations don't match theory. So, at
least one of them is incomplete.

Dark matter puts the emphasis on observations(that is: assume the overall
theory is roughly correct): there are things out there that, if we would have
observed them, would make the theory fit.

Alternatives for gravity focus on the theory (accepting observations as
roughly complete): if we change the theory, it should fit the observations.

TL;DR we ought to have A=B, but there's a mismatch. The problem could be due
to either A or B. Or both.

Both should be worked on - at least until one can be fully ruled out.

------
wpasc
Doesn't the bullet cluster provide strong evidence for the existence of dark
matter? Shouldn't all alternatives need to explain the phenomenon seen there?

~~~
Fnords
It provides strong evidence of matter that we cannot detect. Milgrom's reply
is worth a read:
[https://web.archive.org/web/20160721044735/http://www.astro....](https://web.archive.org/web/20160721044735/http://www.astro.umd.edu/~ssm/mond/moti_bullet.html)

~~~
haolez
Thus... dark matter?

~~~
raverbashing
No. Because dark matter is understood mostly to be less interacting matter.
Don't think it's a ball of dark stuff, think something you couldn't even see

Whereas he's proposing regular (dark as in color) matter

~~~
balfirevic
> Whereas he's proposing regular (dark as in color) matter

Wouldn't that kind of matter be easily detectable because it would block EM
radiation, including visible light?

~~~
corty
Yes. "Dark" is misleading, it should be "sparsely-interacting" or something.
Well, MACHOs would have been dark in the traditional sense as well. Generally,
"we would have seen it" excludes a lot of dark matter candidates, thats what
actually makes it "dark"

------
mysterypie
> [Milgrom's alternative theory:] _But far in the outlying areas of the Milky
> Way, stars would feel a smaller gravitational force than previously thought_

Isn't it the opposite?

Dark matter is extra matter which implies extra gravitational force to hold
the galaxies together. If you want an alternative to dark matter, shouldn't
the alternative theory predict a _bigger_ gravitational force than previously
thought?

~~~
saberdancer
As far as I can understand, you are correct. The problem is that predicted
gravitational force at the edge of the galaxies is smaller than observed, so
any theory should find a solution for that problem. MOND in my opinion always
looked like trying to fit a solution to the observations which will lead to
overfit.

------
causality0
I would enjoy an explanation as to why the distribution of normal and dark
matter in galaxies is roughly the same. My ignorant intuition tells me that
since normal matter is subject to many more and different forces than dark
matter, one of them should be much more densely-orbiting the galactic core
than the other. But they aren't.

------
jfdi
Maybe dark matter is just a symptom of the universe’s connection to the
multiverse. Externalities in action.

~~~
amanaplanacanal
Gravitational effect from nearby parallel universes maybe.

------
tappio
I'm wondering always about the name "dark matter". It makes a good topic for
all kinds of pseudo-science and scifi. The name implies something mystical and
invites everyone to discuss a topic that no-one understands.

If the name was something else like "Rubin's matter" or "Unknown matter",
maybe it wouldn't invite so much unscientific discussion and speculation.

I don't feel that everyone has an opinion about higgs boson, but you can start
a discussion about dark matter with anyone. I'm not any kind of expert about
the topic either, but could certainly start speculating about it with other
laymen.

~~~
mr_toad
It’s literally dark.

------
ubittibu
> But far in the outlying areas of the Milky Way, stars would feel a smaller
> gravitational force than previously thought from the bulk of matter in the
> galaxy;

Shouldn’t it be: "a stronger gravitational force than previously thought"?

------
8bitsrule
A simple back-of-the-envelope estimate shows that it takes an -extremely-
small amount of very low-density 'invisible' matter - assuming that it's
spread out over the huge 3D 'spherical' volume of a galaxy - to make up the
missing mass. On the order of a few -atoms- per cc.

Given all of the unknowns about interstellar/ intergalactic matter, including
the density of EMF, it's not too hard to understand why the account sheet
won't balance. It's a hard and wonderful problem.

------
JoeOfTexas
What if dark matter is really just regular matter that is not visible through
our instruments?

Like large swaths of ice or dust in the voids, similar to our kuiper belt that
is difficult to analyze.

~~~
goodcanadian
There are some clever ways of detecting such things. The density of gas and
dust can be determined by measuring the effect on starlight passing through
it. Statistically, larger objects should occasionally pass in front of
background stars making them "blink off" briefly or in the case of black
holes, lens the light from the background star, making it briefly brighter.
Surveys have been done looking for such things. We can put upper limits on the
amount of "traditional" matter, and there simply isn't enough of it out there
to explain our observations.

------
choeger
What, exactly, is the prediction of dark matter, compared to MOND (or related
theories)?

As a layman it seems like:

1\. Dark matter is not universal (i.e. it is a "random" property of space) 2\.
Dark matter interacts with itself (and other matter) via gravity.

Is that all there is? I mean in such a case it seems to be no wonder that it
fits the observations better, as DM theory is by far less constrained then
MOND.

------
lytefm
While the stationary background universe model proposed by Peter Ostermann
doesn't rule out „dark matter”, it at least doesn't require „dark energy“ in
order to match with observational data.

[http://peter-ostermann.org/assets/sum_os14.pdf](http://peter-
ostermann.org/assets/sum_os14.pdf)

------
tylerjwilk00
Does anyone know if there are any effects from inflation [1] at galactic
boundaries?

[1]
[https://en.m.wikipedia.org/wiki/Expansion_of_the_universe](https://en.m.wikipedia.org/wiki/Expansion_of_the_universe)

------
Koshkin
MOND is appealing from a purely philosophical point of view: we have a
separate (“modified”) theory for when thing are very small, another one for
when things move very fast, so, the need for another modification for when
things are very large should not be seen as entirely unexpected, should it?

~~~
Misdicorl
We have none of these things. We have theories which say interesting things
about small and fast things. But the theories don't change when things are
small or fast.

MOND is excluded experimentally (violates observation of galaxies with and
without dark matter). It is unsound theoretically.

------
pontifier
I've often wondered if there is an equivalent of the magnetic field for
gravity. The magnetic field seems to be a consequence of resolving charge
interactions for relativistic observers, and the same could be true for
gravity.

------
phkahler
Can anyone Please point to the model used for the "expected" galactic rotation
curve? This whole dark matter thing started with observation not matching
prediction. I'd like to scrutinize the prediction.

~~~
ellis-bell
newton's theory of gravitation was the model used for predicting expected
galactic rotation curves

~~~
corty
Plus mass distribution of bright matter inferred from brightness curves.

It is a simple enough integral to do, grab any beginner's astronomy book.

------
LoveMortuus
Hope this isn't a stupid question, but does dark matter have different states
like solid, liquid, gas and plasma?

~~~
Koshkin
“Dark matter” is the name of an observable effect. We do not know if it’s
matter, even.

------
oldcopernicus
Is it possible that the effects of dark energy are in reality a previous
universe (like an outer sphere) that is pulling our universe matter?

~~~
fhars
You are reading too much Ian M. Banks. (Note: that’s fiction, not science.)

~~~
oldcopernicus
Never read Ian M. Banks. This is just a question. If not possible why?

------
nautilus12
Yes! I've been saying for years that dark matter is just a mathematical
convenience and that we don't have any real evidence for its existence. Our
model w repsect to it needs rethinking

------
leoxvi
Maybe physics needs a more holistic light theory based on stillness and the
two-way motion of each phenomena in the cosmos:
[https://wikischool.org/divided_light](https://wikischool.org/divided_light)

~~~
augustt
Haha what on earth is this. Every article starts with a bunch of quotes from
some crank with no scientific training. Reminds me of
[https://vixra.org/](https://vixra.org/).

