
First detection of the missing half of normal matter in our universe - lacksconfidence
https://www.newscientist.com/article/2149742-half-the-universes-missing-matter-has-just-been-finally-found/
======
iheartmemcache
Dang I read the first paragraph of the article and immediately went searching
for the real papers since I didn't expect any media outlet to include them at
the bottom, but here they are for anyone who made the same mistake I did!
[https://arxiv.org/abs/1709.05024](https://arxiv.org/abs/1709.05024)
[https://arxiv.org/abs/1709.10378](https://arxiv.org/abs/1709.10378)

Not a cosmologist but here's my go at the de Graff paper. (Let's get this out
of the way, the title is click-bait and the paper/researchers makes no such
claims as to anything near 50%. New Scientist is trolling for hits with the
word "half" or the journalist is fundamentally misunderstanding the work.) In
de Graff, et al, they claim 30% of "90% of the missing baryonic matter [that
composes the ~25% of our total universe observable from within our light
cone]" has been found in the CMB structured as filaments between galaxies.
They claim there's effectively a planar network layered on top of Minkowski
space composed this baryonic matter. The temperature was at this "Goldilocks"
midrange no one had previously analyzed (ranging from 10^5-10^7K). This wasn't
previously found because people were searching "only the lower and higher
temperature end of the warm-hot baryons, leaving the majority of the baryons
still unobserved(9)". [See "Warm-hot baryons comprise 5-10 percent of
filaments in the cosmic web.", _Nature_ , Eckert et al for more about baryons
of this composition.]

Additionally, these baryons have 10x the density of what we observe (so this
could potentially be evidence for the first stable baryonic matter composed of
second generation quarks, or more likely the binding energies are different
from our standard uud/udd nucleon quarks) permeating the universe, and where
the roads in the network meet ("dark matter haloes"), you have embedded
galaxies and galaxy clusters. They continue with their analytic methods of the
CMASS data, and claim within the framework 30% of the total baryonic content
(which, again, all analytical methods put this into no more than ~25%) is
composed of this form of this matter. I skimmed their methods and it seemed to
at least logically hold -- they are using the appropriate data (SDSS 12) and
didn't cherry-pick their galaxy pairs (so, no p-hacking here!).

~~~
DiabloD3
From what I can tell, this basically also proves that large scale plasma
exists between all bodies at any scale (planetary to systems to galaxies to
clusters), and universe sized Birkeland currents exist; which is something
cosmologists have been trying to prove/disprove for awhile.

So, not only did they find some of the missing matter, they found some of the
missing energy, too. This does, however, screw some of the more classical
cosmologists.

~~~
nolta
> So, not only did they find some of the missing matter, they found some of
> the missing energy, too.

Nope, this has nothing to do with dark energy.

> This does, however, screw some of the more classical cosmologists.

Not sure who you're referring to. These results are completely consistent with
the standard model of cosmology.

~~~
ab5tract
> Not sure who you're referring to. These results are completely consistent
> with the standard model of cosmology.

Not if what is being detected is simply mass associated with filamentary
currents of energy (with attendant magnetic fields) rather than particular
particles.

~~~
nolta
They're modelling filaments as cylindrical tubes of hot electrons connecting
pairs of galaxies. I don't know what you mean by "mass associated with
filamentary currents of energy", but while the electrons are hot (~million
Kelvin), they're non-relativistic and their kinetic energy is negligible.

------
lacksconfidence
[https://www.reddit.com/r/space/comments/75944s/half_the_univ...](https://www.reddit.com/r/space/comments/75944s/half_the_universes_missing_matter_has_just_been/do4mfjx/)

> The approximate distribution in the Universe is 5% regular matter, 25% Dark
> Matter, and 70% Dark Energy. Half of that 5% was missing, and now found.

> Regular matter makes stars and visible galaxies, so it is "bright". Dark
> Matter is so named because it does not make things we can see with
> telescopes directly - it is "dark". We can see the effects it makes with
> gravity, such as the rotation curves of galaxies, and gravitational lensing.
> So we know something is there, just not what it is made of. Dark Energy was
> invented to solve a couple of mysteries. One is the geometrical "flatness"
> of the Universe, and the other is the apparent acceleration of the
> Universe's expansion. Like Dark Matter, we don't yet know what it is. But
> something is causing the flatness and acceleration, so we gave it a name as
> a place-holder for theories.

> A similar situation happened a century ago, with the precession (shift) of
> Mercury's orbit with time. We thought it was caused by a planet inside of
> Mercury's orbit that we hadn't found yet. It was named Vulcan, after the
> Roman god of fire (not Spock's home planet). It turns out relativity was the
> right answer - the Suns gravity bends space near it, and causes the orbit to
> shift. Vulcan was just "a name we gave to whatever causes the observed
> effect".

> Dark Matter and Dark Energy could turn out to be something entirely
> different than types of matter and energy, but in the mean time it gives
> them names we can attach theories about them to.

~~~
everdev
Thank you! 2.5% of the universe's missing matter has been found.

~~~
phinnaeus
Not 2.5% of the missing matter. 2.5% of the matter.

~~~
astrobe_
I have a real trouble figuring out how they could say 2.5% was missing when
95% of all things in the universe is just a theory, according to those
figures.

------
mirimir
> At the largest size, Google image search tells me that it looks exactly like
> foam rubber. Foam rubber is created by combining a chemical agent that
> glomps together through the wonder of polymerization with another chemical
> agent that delivers lots of gas bubbles to make space between the polymers.
> Universes are created by rapidly expanding a superdense plasma that glomps
> together through the wonders of gravity, while lots of expanding vacuum
> makes space between the galaxy clusters.

[https://palmstroem.blogspot.de/2016/01/no-universe-is-not-
br...](https://palmstroem.blogspot.de/2016/01/no-universe-is-not-brain.html)

Edit: This explains, in a light-hearted way, why there are filaments between
galaxies and galaxy clusters.

~~~
dotancohen
Why should vacuum expand?

Is it due to gravity of the filament molecules pulling those molecules
together into larger 'strings' or 'planes' which leave 'holes'? Or some other
mechanism?

~~~
kijin
The universe expands. Things bound by gravity tend not to expand that much. As
a result, most of the space that expands is vacuum.

~~~
divs1210
'The universe expands' in what? I thought everything was the universe, as
meant by the term 'universal'.

~~~
logfromblammo
Imagine you have a meter-stick. It has uniform graduation markings to show you
centimeters and millimeters.

Now imagine that it is getting longer. No matter how hard you look at it, the
stick is still one meter long. That's what the markings on it say, anyway.
It's actually getting longer at the same rate as all the other meter-sticks.
That's the expansion of the universe.

Now imagine you balanced two marbles on the stick. They bow the stick ever so
slightly, and roll together at the 50 cm mark. They are not expanding--or at
least not expanding like the meter-stick expands. But they will continue to
roll together. They are detectable masses acting under the effect of gravity.
If you look closely, they appear to be shrinking. But it's actually the meter-
stick expanding.

When you expand this out to galaxy scale, two galaxies that are not moving
with respect to each other are getting further apart. The mass in the galaxies
have enough intra-galactic gravitational attraction to keep themselves from
flying apart along with the vacuum, but not enough the keep their neighbor
galaxies close. The inter-galactic gravitation doesn't pull hard enough to
overcome the expansion. So they surf away from each other, becoming further
apart.

At least, that's how I understand it. I could be wrong.

~~~
raattgift
> At least, that's how I understand it. I could be wrong.

General Relativity deals in spacetime, but it is often convenient to slice
spacetime into spacelike volumes in which every point in the volume ("the
spacelike hypersurface") is at the same timelike coordinate. One important
consideration is that no timelike axis is any more preferred by nature than
any other, and one can always find an observer who disagrees with any choice
of splitting one happens to make. In particular, two inertial observers
related by a Lorentz boost generally will not agree on what event (e.g. two
bits of matter colliding) is at what time coordinate, and thus this type of
3+1 slicing will result in different spacelike hypersurfaces for each such
observer.

The cosmological frame picks out a frame that a family of special observers
can agree on: these observers agree that at the largest scales, the universe
is homogeneous and isotropic, that they themselves are moving inertially, and
can agree on a function relating a time coordinate to the average density of
matter in a universe-sized spacelike hypersurface at that time coordinate.
Each such observer is assigned a spacelike location which persists into the
infinite future: the observers are all stationary at a constant set of three
spatial coordinates. The centres of mass of practically all galaxy clusters
are essentially this type of observer, so those remain at the same spatial
coordinates at all times too.

We then take this set of coordinates and apply it to a universe described by a
Robertson-Walker metric. Our universe _approximately_ obeys the Robertson-
Walker (R-W) metric outside of galaxy clusters; more on that in a moment. The
R-W metric relies on a 3+1 slicing of a homogeneous and isotropic universe,
and uses two coefficients r and k to determine respectively the radius and
shape of each spacelike 3-hypersurface. If we knock out a spacelike dimension,
we can think of an R-W universe as a stack of infinitesimally thin plates
where each plate at time t is related to its infinitesimally earlier
predecessor and infinitesimally later successor plate by a function giving its
radius r. (It is perfectly reasonable to rotate the axes so that you stack the
plates vertically from the floor upwards, where we substitute a height
coordinate for the timelike coordinate).

In an expanding-with-a-cosmological-constant R-W universe, r shrinks smoothly
towards the past and grows smoothly towards the future. The coefficient k
determines whether the 2-d planes give globally Euclidean geometry (k=0),
globally hyperbolic geometry or globally spherical geometry. Finally, if r >>>
the Hubble volume, there may be no practical way of recovering k != 0
observationally, or in other words the local geometry of a R-W slice of a
Hubble volume may be flat even if the global geometry is not.

On this R-W universe we apply the coordinate system above, but remember that
our observers stay at fixed spacelike coordinates. We need to notice here that
our coordinates do not determine distances by light-travel-time. That's fine,
we can use arbitrary measures of distance in General Relativity, and can
practically always find a consistent and useful transformation from a
description of physics in one system of coordinates to another. We just have
to be careful either to work only in generally covariant formulations, or to
recognize that using some systems of coordinates entice one into the use of
fictitious forces that disappear in other systems of coordinates. In this
case, an observer at the centre of mass of our galaxy in spherical coordinates
centred on her would naturally say that distant galaxy clusters on _this_
spacelike hypersurface now will be at a larger radial coordinate in future
spacelike hypersurfaces, in our cosmological coordinates she and the distant
galaxies are all working in coordinates _comoving_ with r, so their spatial
distance is constant at all times.

The metric expansion of space is just that: r increases.

> The inter-galactic gravitation doesn't pull hard enough to overcome the
> expansion

This gets trickier. In the Friedmann-Lemaître-Robertson-Walker model we treat
the sources of the matter tensor as a set of perfect fluids with some pressure
and density, and the fluids dilute away with expanding Robertson-Walker
universe. We ignore the local overdensities of matter ("galaxies" and "people"
and so on) and at the largest scales, that's reasonable.

However _inside_ galactic clusters and galaxies, in the standard model there
is no expansion at all; gravity doesn't work against it, it just isn't there
in the first place. From a technical perspective what we do is treat galaxies
as approximate sources of a Schwarzschild metric up to some boundary enclosing
the galaxy, and then we embed that into Robertson-Walker space. This is
certainly not faultless, but it matches observation extremely well. What does
_not_ match observation extremely well is naive quintessence models where the
metric expansion works within galaxies and is simply checked by the
gravitational interactions of the matter within them, but acts as a
cosmological constant outside galaxy clusters.

Likewise, comoving galaxy clusters are just drifting along inertially into the
future; even under a different system of coordinates there are no extra forces
working to separate them -- throwing away the 3+1 slicing with its preferred
timelike axis, in the spacetime view galaxy clusters' timelike worldlines
converge near the hot dense phase of the universe.

If you took your second paragraph's metre stick[1] and put it in space as a
comoving observer well outside galaxy clusters, it would still be a metre long
in the far future, whether measured locally or with a really really really
good telescope. "Rulers" aren't expanding in the metric expansion; instead the
cosmological coordinates on each spacelike hypervolume are adjusted, and in
general coordinates while useful are not themselves physical while an actual
metre stick is. Physical objects themselves do not change when we change
coordinates; distant galaxies can have no idea that you're putting
cosmological or spherical or cartesian or conformal coordinates on them, or
calculating their movements against those coordinates.

I'm afraid I don't understand the point in your second paragraph.

> two galaxies that are not moving with respect to each other

They aren't moving against comoving coordinates. But if you choose other
coordinates (e.g. spherical coordinates with the origin on the centre of one
of the galaxies) they can move against those. We can do various
transformations to convert the descriptions of the motions of these galaxies
(and any fictitious forces and relating to coordinate motion, and other
coordinate-dependent quantities) in one set of coordinates into another set of
coordinates. The trick is finding a set of coordinates in which one can
extract some intuitions about observables like the cosmological redshift, the
dark night sky (cf. Olbers's paradox), or the details of the cosmic microwave
background.

\- --

[1] in principle, and with some care, you could line up a hundred 1 cm objects
(e.g. ball bearings) and they would not separate from each other with the
expansion of the universe (one has to be careful about other things that may
cause them to move relative to one another, such as radionuclide decay within
the objects, interactions with cosmic rays or other particles, and so on; but
in standard General Relativity they should continue on their parallel timelike
worldlines indefinitely).

~~~
acqq
Can you please give any usable link for this:

> What does not match observation extremely well is naive quintessence models
> where the metric expansion works within galaxies and is simply checked by
> the gravitational interactions of the matter within them

Or write some additional details about it. I know that the "popular"
explanations claim that "everything" expands, and I understood from your reply
that what we see can be technically modeled in equations as if there's nothing
that expands inside of whole galaxies, but what is the _actual proof_ that
there are actually no expansion forces inside of the galaxies at all? Thanks
in advance.

~~~
raattgift
> "usable link"

How much technical detail do you want?

Starting with the "gimme hardcore!" end, I was thinking of how to construct an
argument using vierbiens and then how to boil it down to something accessible
(or at least representable on LaTeX-free HN), and then remembered that it had
already been done by Cooperstock et al.: [http://xxx.lanl.gov/abs/astro-
ph/9803097](http://xxx.lanl.gov/abs/astro-ph/9803097) The tl;dr is that _if_
the cosmological expansion induces strain on matter, the strain is too small
to be measurable.

Retreating from the hardest of answers, Peter Coles has an old moderate-detail
article on this at [https://telescoper.wordpress.com/2011/08/19/is-space-
expandi...](https://telescoper.wordpress.com/2011/08/19/is-space-expanding/)
and he refers to both Peacock's and Harrison's textbooks which give greater
detail (I recommend the latter if you can get your hands on it at a library).

His approach to the question you're asking ("roughly, does the solar system
expand with the universe?") is how I'd go about it too, following on from the
comment you replied to. My central point would be that in General Relativity
we use exact solutions of the Einstein Field Equations because they're well-
understood not because they're accurate. Natural systems don't source e.g the
exact Schwarzschild spacetime for several reasons including lack of perfect
spherical symmetry, lack of perfect vacuum to infinity outside the source, and
nonzero angular momentum. Yet we get good approximate results when we use
Schwarzschild to model the Earth or the Sun or the Milky Way, and usually the
bad results are fixable with linear corrections). But the _real_ picture is
that each of these bodies sources an unknown metric that is slightly different
from Schwarzschild, and additionally one has to stitch together two metrics
sourced by two bodies each sourcing (different, unknown) Schwarszschild-ish
metrics into an (unknown) expanding background.

Numerical relativity has opened up the study of approximate solutions which
give better results for real physical systems than the toolkit of known exact
solutions (plus linear in v/c corrections), so one could argue that the
central research programme in classical General Relativity is the study of the
mechanisms that generate the (true) metric.

All that said, we can be much more confident (because of analyses under e.g.
the parameterized post-Newtonian formalism and experimental data from
gravitational probes of many varieties) about the fit of exact solutions to
the bodies in our solar system than the fit of _any_ metric to the cosmos-in-
the-large. For the bodies in hydrostatic equilibrium that means Schwarzschild
to at least the first order in v/c [in linearized gravity]. If you accept that
Schwarzschild is an excellent substitute for the unknown real metric, then you
must have a very close fit to a static, asymptotically flat spacetime. Around
that you can establish a boundary condition. Outside the boundary is the
expanding spacetime, inside is asymptotically flat (i.e., not expanding).
Coles discusses some of this ( as does Hossenfelder at
[http://backreaction.blogspot.com/2017/08/you-dont-expand-
jus...](http://backreaction.blogspot.com/2017/08/you-dont-expand-just-because-
universe.html) ).

Alternatively, you can argue that the real metric sourced by e.g. the Earth
(or yourself at a distance where you subtend a very small angle on an
observer's sky, or one of your molecules) is less close to Schwarzschild. In
that case, Coles takes us back to Cooperstock via Ned Wright's Cosmology FAQ:
you will get bad results with poor choices of coordinates (so use e.g. Fermi
coordinates because then you know exactly where you have valid and comparable
results, and you are forced to consider whether and where geodesics drift
apart[1]).

Finally, if you were asking about "naive quintessence models" and their
problems, ch 3.2 in Elise Jenning's _Simulations of Dark Energy Cosmologies_
(Springer, 2012) is a decent overview (it contains material from her Ph.D.
thesis; she is now at KICP/FNAL).

> "actual proof"

This would make an excellent postdoc research project !

Linked with [1] above, on proving the conjecture, the soft underbelly is the
behaviour of geodesics: in an expanding universe, comoving geodesics drift
apart. The geodesics in Schwarzschild spacetime do _not_ drift apart.
Geodesics in the solar system do not drift apart, and haven't over the course
of a few billion years. Geodesics at cosmological scales clearly do drift very
noticeably apart over the same period of time. Worse, evidence suggests that
the Hubble constant _isn 't constant_ in time, so where are the matching
perturbations in the orbits of various bodies in the solar system? However,
I'm not sure this is the right path to a definitive answer, since one is
likely to be able to claim that your atoms in general are not following
geodesics; their free-fall is interrupted by the surface of the Earth.

~~~
acqq
Many thanks, your answer covers everything I wanted to ask you. You recognized
that there were two directions in which I wanted to ask you (the second being
why you mentioned the "naive quintessence models") and you covered both.
Thanks for all the links.

(To anybody who's also interested, I've found the thesis by Elise Jennings
here: [http://etheses.dur.ac.uk/616/](http://etheses.dur.ac.uk/616/) )

------
skj
"Both teams took advantage of a phenomenon called the Sunyaev-Zel’dovich
effect that occurs when _light left over from the big bang_ passes through hot
gas."

That just seems unreal to me.

Edit: To clarify, I'm not accusing it of being made up. I just think it's
amazing.

~~~
vanderZwan
They can tell that the big bang is the light source because of Hubble's
Law[0]. In short, the universe itself (so _space itself_ ) expands[1], and it
does so fast enough to create a Doppler effect in light, causing redshift[2].

When we look further out into space, we effectively look "back" into time
because of the limit of the speed of light. The older the light, the more it
has redshifted because the more space has expanded while it was travelling. So
this redshift increases very predictably the further we look out into space
and back into time.

As a result, when observing space we can say when light is left-over from the
big bang, because we can tell the age of the light by how much its spectral
lines have redshifted.

[0]
[https://en.wikipedia.org/wiki/Hubble%27s_law](https://en.wikipedia.org/wiki/Hubble%27s_law)

[1]
[https://www.youtube.com/watch?v=_llA2q1rlSg](https://www.youtube.com/watch?v=_llA2q1rlSg)

[2]
[https://en.wikipedia.org/wiki/Redshift](https://en.wikipedia.org/wiki/Redshift)

------
andrewaylett
"made of particles called baryons rather than dark matter"

That's clumsily worded, as it makes it sound like it's still something exotic.
_We're_ made of baryons: this is just normal matter.

~~~
vanderZwan
As a former physics student, I see your point but disagree.

Apologies for the wall of text about just this one choice of wording, but
discussing why science communication is a _very difficult_ trade-off takes a
bit of space.

One reason to stick to "baryons" is that it is the term used by the
researchers in their papers. So it is more true to the research that is being
summarised.

The start of the article (unlike its title...) mentions that this research is
about normal matter quite clearly, so this should give enough context to avoid
confusion:

> _This is the first detection of the roughly half of the normal matter in our
> universe – protons, neutrons and electrons – unaccounted for by previous
> observations of stars, galaxies and other bright objects in space. You have
> probably heard about the hunt for dark matter, a mysterious substance
> thought to permeate the universe, the effects of which we can see through
> its gravitational pull. But our models of the universe also say there should
> be about twice as much ordinary matter out there, compared with what we have
> observed so far._

Baryons are subatomic particles[0][1], not atoms, which is what most people
think of when discussing "matter". If we say "normal matter", some people will
think we're talking about chunks of dirt floating between galaxies. I'm not
joking, have you ever been to a public physics lecture? And mistakes like this
are not the audience's fault - it's the expert's fault for assuming non-
experts know what they know.

For people with the right background (chemists, physicists, astronomers, etc)
it is clear this is "just normal matter". For everyone else - so most people -
_" normal matter"_ means matter in earth-typical circumstances of pressure and
temperature, in a solid/liquid/vapour phase.

The papers describe intergalactic plasma filaments with a temperature between
10⁴ to 10⁵ K. That is pretty exotic by normal-people standards.

And yes, fire is a plasma, but for most people fire is just very hot gas, and
the article even builds on that:

> _Because the gas is so tenuous and not quite hot enough for X-ray telescopes
> to pick up, nobody had been able to see it before._

So the other reason reason to stick to "baryons" is that this _is_ exotic
matter from the point of view of non-experts. When describing research you
have to start from _their_ model of the physical world, and build your way
back to the research.

I think the choice in wording appropriate here. The other paragraphs make it
clear this is about normal matter, while this sentence makes it clear it's not
normal matter in Earth-circumstances, which is how the average human thinks
about it if not corrected.

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

[1]
[https://simple.wikipedia.org/wiki/Baryon](https://simple.wikipedia.org/wiki/Baryon)
(the Simple English wiki is probably more useful to us non-cosmologists here)

~~~
andrewaylett
It seems I know just enough to be dangerous. Thank you for your well-written
explanation.

~~~
vanderZwan
You're very welcome, and don't be hard on yourself!

It is easy to underestimate how difficult it is to give good explanations to
non-experts, regardless of the topic at hand. I mean, just look at how how
much I wrote about the choice of _one word_.

When writing for non-experts, an expert must imagine what it is like to _not_
know the right answer. Our brains seem to be terrible at that: most people are
unable to "not see" the cow in that famous gestalt picture once spotted[0],
and I think the same thing applies to many kinds of thinking.

The only way out for an expert seems to be a lot of exposure to "dumb
questions" (there is no such thing, of course) from non-experts, to figure out
what logic the latter's wrong conclusions are. It doesn't help that there are
many more ways to come to perfectly logical but wrong conclusions based on
wrong premises, than there are ways to come to the right conclusion with the
right premises.

And _even_ once you are aware of these mismatches, you still have to explain
yourself in such a way that there is a path from the incorrect interpretation
to the more correct interpretation, without getting lost in trying to explain
_everything_.

So no wonder that even professional educators and science writers, who are
supposed to be skilled at this, tend to do a poor job!

[0] [http://icog.group.shef.ac.uk/what-can-you-see-some-
questions...](http://icog.group.shef.ac.uk/what-can-you-see-some-questions-
about-the-content-of-visual-experience/)

------
vorg
> Half the universe’s missing matter has just been finally found

The first paragraph clarifies that this headline means _" The missing
(regular) matter comprising half the (regular matter in the) Universe has been
finally found"_ rather than _" Half of the missing (regular) matter in the
Universe has just been found (and where could the other half be?)"_.

------
nielsbot
I am not an astrophysicist... but does this mean dark matter has finally been
"seen"? That's a big deal, right?

~~~
JohnBooty
__EDIT: My understanding is incorrect. __See the replies to the post for a
correction.

No. They're saying that 50% of the "dark matter" isn't dark, it's just regular
matter that we weren't able to see.

The density of this stuff (filaments of gas stretching between galaxies) is so
low that it is really, really, really hard to detect.

"Dark matter" theories posit that the missing matter isn't just regular matter
that's hard to see; they posit that this dark matter is a fundamentally
different... something.

I don't believe this announcement addresses some of the other oddities
(possibly) explained by dark matter, such as the rotation of galaxies:
[https://en.wikipedia.org/wiki/Galaxy_rotation_curve](https://en.wikipedia.org/wiki/Galaxy_rotation_curve)

~~~
justin_vanw
No, this has nothing to do with dark matter. Dark matter has to be in galaxies
because the reason we think it exists at all is because galaxies spin 'too
fast', so there is more mass in them and therefore more gravity than can be
accounted for. This article is talking about gas between galaxies.

~~~
k__
Isn't it more probable that the formulars to calculate the spin are wrong?

~~~
planteen
That's a theory called Modified Newton Dynamics (MOND).

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

~~~
wbhart
This theory (when appropriately hacked) describes rotation curves of galaxies
extremely well. Unfortunately it doesn't work at larger scales. There are many
nice videos on YouTube by Sean Carroll on MOND, which explain it's limits,
albeit for the fairly educated layperson.

------
lifeisstillgood
Can I just check - my understanding is that we can only see 5% of the expected
mass in the universe - so we have just found another 5%? meaning dark matter
needs to account for 90%?

plus, how awesomely beautiful is the idea of tendrils of has connecting the
galaxies through space.

~~~
slimshady94
Tendrils between galaxies is mind-blowing. Aren't galaxies very very far
apart? And there is basically nothing in between? How are subatomic particles
- baryons - able to survive the constant expansion? Do the tendrils keep
getting thinner? Can they be a medium to transmit intergalactic information
someday?

~~~
netzone
Also, to expand on that (I have no knowledge, just making assumptions), the
space between galaxies would need to either create new matter, or the matter
would get infinitely spread out over time?

So, if the stretching thing is true, would there be areas with absolutely no
matter or anything at all?

------
wheresmyusern
im confused about baryons. they say that it is a particle (presumably like an
electron or photon or other particle) but then they go on to say that its a
gas, not a particle. very confusing.

~~~
Meegul
A baryon is a subatomic particle that is composed of three quarks, such as a
proton or a neutron. An electron would be categorized as a lepton, rather than
a baryon as it is not made of multiple quarks and is rather an elementary
particle, for reference.

Here's a good reference for subatomic elementary particles, where you can see
some familiar names like electron and photon:
[https://en.wikipedia.org/wiki/Baryon#/media/File:Standard_Mo...](https://en.wikipedia.org/wiki/Baryon#/media/File:Standard_Model_of_Elementary_Particles.svg)

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nur0n
The best visualization I could find is on pg 14 of the pdf on this page:
[https://arxiv.org/abs/1709.10378v1](https://arxiv.org/abs/1709.10378v1)

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ZenoArrow
Is it just me, or does anyone else get put off by hyperbolic scientific news
coverage? Even if the result is interesting, the framing of one result as
representative of all unknown normal matter makes me not want to bother
reading the article, it's hard enough splitting out facts from bullshit when
you have a decent grounding in a subject, it's even harder when your knowledge
is almost nonexistent.

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macca321
Does this mean the universe is more weird or less weird than we thought?

~~~
ecshafer
Less weird. A large amount of the missing matter, but not all lot it, has been
observed where it was theorized. The issue was we've not had reliable ways to
measure that specific matter in the area it is

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kpil
Stacking a hundred thousand scans in order to find something made me think of
the dead salmon study that won the igNobel prize.

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greggarious
Well, I wouldn't exactly say I've been _missing_ it OP.

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epigramx
The title is effectively click-bait because of:
[https://www.reddit.com/r/space/comments/75944s/half_the_univ...](https://www.reddit.com/r/space/comments/75944s/half_the_universes_missing_matter_has_just_been/do4mfjx/)

~~~
kwelstr
We should assume by default that every headline is clickbait now,
unfortunately this is the business model of the internet. It's not good for
accuracy and most articles are just a headline and some inane filler up words.
I hope we'll smart up and find a better model for news soon. :/

~~~
24gttghh
We could pay for it like we did for the hundreds of years prior to the
internet, but they'll still make-up eye-grabbing headlines.

~~~
xenophonf
We're still paying for it. It's just that the currency's changed.

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dushyantk
Duped by
[https://news.ycombinator.com/item?id=15439321](https://news.ycombinator.com/item?id=15439321)

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chewbacha
Ok, it seems like a lot of people here think dark matter is a “thing” but it’s
not. Dark matter is simply unaccounted for mass in the universe. When we
discover mass that we didn’t know about before, the amount of “dark” matter
decreases. The word “dark” here merely means “known unknown” because we
observe the effect but not the cause. This discovery has unveiled some of the
dark matter as baryon particles in a plasma. So some of that dark matter is
now known.

~~~
BeetleB
>This discovery has unveiled some of the dark matter as baryon particles in a
plasma. So some of that dark matter is now known.

The article clearly points out that this is not the case. The amount of dark
matter is still the same.

>Dark matter is simply unaccounted for mass in the universe.

The article clearly points out that this is not the case.

~~~
chewbacha
You are correct, i misread one of the first paragraphs.

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hoodoof
They probably looked under the cushions on the couch.

Anything you lose is always there.

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Robotbeat
Missing /regular/ matter. Doesn't count dark matter. Shame on New Scientist
for incorrect headline.

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numbsafari
So on the eve of the seventh day, god swept half the matter under a rug and
then went out to play.

I guess we really are created in his image...

