
Astronomers find 'missing matter', solving decades-long mystery of outer space - astdb
https://www.abc.net.au/news/2020-05-28/astronomers-find-universe-missing-matter/12291788
======
jbotz
This is about missing baryonic matter, not "dark matter". What's _not_ clear
from this article is why we knew (or thought) that there was baryonic matter
missing in the first place. Wikipedia has more info under "Missing baryon
problem"[1]... basically it's theorectical predictions of how much baryonic
matter there should be given Big Bang nucleosynthesis and the observed cosmic
microwave background. The article should have gone into that more, typical
sloppy science journalism.

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

------
Gys
> ...the team was able to figure out how — and where — the missing matter was.
> The results were published today in the top scientific journal Nature.

The article does not give many clues and refers to this publication:
[https://www.nature.com/articles/s41586-020-2300-2](https://www.nature.com/articles/s41586-020-2300-2)

~~~
boomboomsubban
Between galaxies, tiny amounts of space dust exists too small to detect with
our other tools.

------
Gatsky
This isn’t dark matter, it is the normal matter which was supposed to be there
but hadn’t been found before.

~~~
pseudosudoer
In a way it is dark matter, as dark matter was the name of the hidden variable
in the cosmos responsible for the rest of the gravitational interaction that
we see. It just so happens that it's not some new state of matter, but simply
difficult to spot regular matter.

~~~
jfengel
This isn't solving the galactic rotation curve. This is about intergalactic
matter. The problem of unknown sources of gravity within galaxies, for which
dark matter is the leading hypothesis, remains unproven.

~~~
pseudosudoer
Thanks for the clarification, I must've misunderstood the specification.

------
evolve2k
> The results were published today in the top scientific journal Nature... The
> team used the CSIRO's Australian Square Kilometre Array Pathfinder (ASKAP),
> — an array of 36 radio telescopes in the Mid West of Western Australia.

I’m disappointed to see that this important public research was gleefully
published into a closed private access journal.

Especially considering that science like this is likely all publically funded.

Where should have this been published if they were committed to Open Access?

~~~
raattgift
FWIW, as with practically any astrophysics paper published in practically any
journal, a preprint can be found by title or author search. In this case,
[https://arxiv.org/abs/2005.13161](https://arxiv.org/abs/2005.13161) (and note
"Comments: Published online in Nature 27 May, 2020").

Being published in Nature has some prestige still, even for astrophysics
collabs.; even if -- for the sake of argument -- one accepts there are
journals with better focus coupled with stronger editing of astrophysics
papers in this area, that might not be relevant to all of the sources of
funding for the collaboration.

------
musgravepeter
"The discovery still leaves much of the universe undetected. About 85 per cent
of its matter is thought to be "dark matter" — that which is undetectable
using ordinary methods."

If I understand the abstract, it reports finding "Ωb=0.051+0.021−0.025h−170
(95 per cent confidence; h70 = H0/(70 km s−1 Mpc−1) " which I _think_ means 5%
of the total (mass+dark matter+dark energy), since cosmology assumes Omega
total = 1.0

~~~
acqq
The relevant part of the abstract: "we _derive_ a cosmic baryon density of
Ωb=0.051" .. "This independent measurement is consistent with values derived
from the cosmic microwave background and from Big Bang nucleosynthesis."

So they derive the Ωb independently and get the value compatible to what we
already know to calculate using CMB and BBN.

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

"Calculated values : Baryon density parameter[b] Ωb 0.0486±0.0010[e]"

------
JoeAltmaier
If true, this revolutionizes cosmology to no small extent. What does Randall
Munroe (?) say? Bet against revolutionary theories - you'll win almost all the
time. And if you lose, you're not really disappointed.

~~~
klmadfejno
I'm pretty sure this is just people finding things that were expected to
exist. That's interesting but sort of the opposite of revolutionary. Like
filling gaps on the periodic table in their predicted form.

~~~
JoeAltmaier
As its intergalactic matter its surely not related to Dark Matter, which has
been generously noted in other comments. Dark Matter is responsible for
galaxies holding together, something this baryonic matter can not do.

With that understanding, it increases confidence that the existing model of
Big Bang processes? So no, not revolutionary, more like confirmatory.

------
at_a_remove
I feel like I should say this in every post about this kind of topic. "Dark
matter" was a term used by astronomers to refer to anything that was not
shining in a star. Astronomers work with photons in all manner of ways. If it
emits photons in a star, then you might call it "light matter."

In the original definition, if it was a bit of dust between stars or even
galaxies? It's dark matter. Asteroid ejected from a solar system? Dark matter.
Itty-bitty planet you cannot resolve with a telescope because it is so close
to the parent star and so tiny? It's dark matter. Pinpoint black hole? Dark
matter. An electron anti-neutrino sliding unnoticed through galaxies and the
hearts of stars for the last few billion years? Dark matter. If it isn't
emitting photons due to recent, local fusion and you have had to infer its
existence somehow? It's dark matter.

The term itself has evolved and now _often_ has an implicit "non-baryonic" in
the mix, but don't always count on that.

~~~
raattgift
> I feel like I should say this in every post about this kind of topic.

Please don't. Just link to the actual paper which makes it clear.

For example: Preprint version :
[https://arxiv.org/abs/2005.13161](https://arxiv.org/abs/2005.13161) (n.b.
"Comments: Published online in Nature 27 May, 2020)

"census of baryons" (title) and "More than three quarters of the baryonic
content of the Universe resides in a highly diffuse state that is difficult to
observe, with only a small fraction directly observed in galaxies and galaxy
clusters." (abstract's first sentence) gets the point across.

I don't know where "dark matter" got to you in the context of the linked
article, but it certainly wasn't from the paper or its preprint. It wasn't
even from the article linked at the top, which carefully and correctly says:
'Astronomy is full of missing stuff. Most of the universe is understood to be
"dark matter" and "dark energy", which nobody has ever directly seen. But even
more of a mystery for astronomers was that they couldn't find about half the
ordinary matter in the universe.' Further down it says '"dark matter" [...] is
undetectable using ordinary methods', which is a bit less careful, but
reasonably correct.

> In the original definition

I doubt there is anyone who can actually submit to arXiv:astro-ph who would
fail to distinguish baryonic matter from dark matter in the way you do.

> don't always count on it

Ok, there may be some on vixra.

Lastly:

> electron anti-neutrino

We should distinguish here by saying that electron anti-neutrinos are not
_cold_ dark matter, in the \Lambda-CDM sense, but otherwise are a very-low-
mass form of weakly-interacting massive particle (WIMP). Electron neutrinos
and their antis are so light that they are almost always ultra-relativistic
(viz.
[https://physics.stackexchange.com/questions/71874/ultrarelat...](https://physics.stackexchange.com/questions/71874/ultrarelativitistic-
particle-what-kind-of-a-particle-is-this) and
[https://phys.libretexts.org/Bookshelves/Relativity/Book%3A_S...](https://phys.libretexts.org/Bookshelves/Relativity/Book%3A_Special_Relativity_\(Crowell\)/04%3A_Dynamics/4.01%3A_Ultrarelativistic_particles)
both of which will take you to deeper treatments), importantly including in
the cosmological frame.

 _Cold_ dark matter is non-relativistic in the cosmological frame.

 _Hot_ dark matter like neutrinos (described 1934, detected 1956) was easily
excluded as a significant part of the explanation for the observations
detailed at Rubin, Ford & Thonnard (1980)
[https://ui.adsabs.harvard.edu/abs/1980ApJ...238..471R/abstra...](https://ui.adsabs.harvard.edu/abs/1980ApJ...238..471R/abstract)
: neutrinos shine out of galaxies and don't linger at the edges of them, or
deep in galaxy clusters, very much like electromagnetic radiation shines away
rather than lingering.

~~~
at_a_remove
I am speaking not of the paper or of any paper in specific but of people's
usual reactions to the term.

