
Astronomers solve the missing baryon problem - tux1968
https://astronomy.com/news/2020/06/half-the-matter-in-the-cosmos-was-missing-but-astronomers-found-it
======
Robotbeat
About half of the 5% of regular (ie baryonic) Matter was missing from usual
astronomical observations. Some hypothesized it took the form of warm-hot
intergalactic gas. This could in principle be detected by looking at the
dispersion (ie slightly varying transit speeds of waves) through this gas of
fast radio burst from distant galaxies, and we just found like five of these
FRBs that showed dispersions that matched the predicted amount of baryonic
warm-hot gas. Problem solved.

~~~
CarbyAu
I like concise prose. It saves time and effort.

Similarly, I am frustrated that videos come up as top hits for simple things.
While a picture can say a thousand words, the point is communication, not
verbosity.

~~~
m12k
American publishers, YouTube and newspapers all suffer from the same
affliction of rewarding their content creators for how long they can keep
their audience engaged, nudging the content creators toward bury the lede.

------
tux1968
Decently covered in 10 minutes or so in video form:
[https://youtu.be/Kp_kqamkYpw](https://youtu.be/Kp_kqamkYpw)

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

~~~
supernova87a
This is a pretty good video -- not overhyped and not filled with frenetic
video clips mashed together from across the internet, and also not overly
focused on a Youtuber's overly-bubbly personality seeking self-adulation and
clicks...

~~~
seusscat
Veritasium is a great channel for such content. He has a degree in online
pedagogy, IIRC. So he's particularly good at teaching complex concepts through
such a medium. Having a degree in physics helps him too.

~~~
supernova87a
I just watched a bunch of his videos -- how interesting! Thanks for the
recommendation.

I wonder what his special techniques are to keep viewer engagement and help
explanation, and whether he's consciously choosing to do them.

For example, I notice that unlike most Youtubers who'll record the narrated
portion purely from their desk (showing their face) perfectly still and nicely
lit, he will even handhold his camera and move around a room. Or when
interviewing someone, hold a camera himself showing the interviewed person,
and yet have _another_ still camera filming him and his other camera and the
interviewee. Or standing in front of a wall projector screen video and talking
(despite the projected image being mediocre quality) when most people would
insert that as a original video source clip and narrate over it.

And yet while this might usually be distracting, for some reason with the
quality of the narration, this makes for a nice break from usual. Or makes it
seem more "authentic" and engaging.

Interesting!

~~~
pja
Derek did his PhD on the public education of science. He’s been honing his
craft for a /long/ time & it really shows.

------
Ancalagon
That website is giving me some scammy looking pop ups on mobile.

~~~
turndown
Agreed, I gave up on reading the article it was so bad.

~~~
tux1968
Sorry, I should have turned off uBlock and checked it before submitting.

------
varjag
Half of baryon mass as million K intergalactic plasma is a lot of matter. Any
ideas how can it exist that hot and plentiful and avoid detection?

It has to be rather dense to be that hot for so long, no? Yet it's not some
sort of stars?

~~~
isaacg
The WHIM is actually extremely non-dense: 1-10 particles per cubic meter. It
turns out that matter can be extremely hot without being dense at all, as long
as there's nothing to cool the matter off, as is the case in the space between
galaxies.

Matter is easy to see when it emits light (like stars) or absorbs light (like
uncharged matter). But charged plasma does neither, so it's hard to detect.
Fortunately, the WHIM has a prism-like effect, where it smears out light
travelling through it, so we can detect it.

~~~
a1369209993
> Matter is easy to see when it emits light (like stars)

That's the part I don't get; if the intergalactic medium is that hot,
shouldn't it be emitting huge amounts of blackbody radiation? Supposedly
ionization interferes with that, but even black holes emit blackbody radiation
(hawking radiation), so that seems like a insuffient explaination.

~~~
piannucci
Blackbody radiation describes an electromagnetic field in thermal equilibrium.
Typically, we associate this radiation with a particular source, or boundary;
but the blackbody spectrum itself is a Maxwell fact, not a matter fact. Real
systems have many degrees of freedom, some matter, some Maxwell. In this
particular example, the Maxwell degrees of freedom have a temperature of 3K,
while the matter degrees of freedom have a much higher temperature.

Clearly, then, the matter is not in equilibrium with the electromagnetic
field. How can this be?

Equilibration is a process that proceeds through a large number of
interactions that each exchange energy between two thermal systems (or
subsystems). If energy transfer is prevented, for instance by the conservation
of mass and momentum, or because no post- states of an appropriate energy are
available for one subsystem to evolve into, then the systems may remain far
from equilibrium relative to each other.

I’d have to do some homework to verify this, but I think a correct statement
is that free massive charged particles don’t gain or lose energy in net from
passing radiation; and conversely, they cannot radiate. This is guaranteed by
Lorentz symmetry, btw: if you examine a million-kelvin electron in a frame of
reference moving alongside it, you’d agree there’s no way it can be radiating:
there’s a frame in which it is stationary, and stationary charges don’t
radiate. If it isn’t radiating in one inertial frame, it isn’t radiating in
any inertial frame.

Only during collisions between charged particles is there a possibility for
the matter and Maxwell degrees of freedom to exchange energy: and these are
very rare in tenuous plasma. (We would call these “three-body interactions”
because the photon counts as one.)

[edit to add] Now one might object that charged particles are always
interacting, because their fields fall off like 1/r^2; they don’t just end.
Indeed! But one of the effects of being in a plasma is that charges are
“screened”. You can think of that as meaning the net effect of many-body
interactions can be described as altering the “effective” field of each
particle to fall off much faster, like e^-r.

~~~
TeMPOraL
> _If it isn’t radiating in one inertial frame, it isn’t radiating in any
> inertial frame._

Maybe this is the answer to a question I've been harboring in the back of my
head since high school: if moving charges radiate, they should lose energy,
and thus eventually disappear. Almost nothing ever is stationary, so how come
all the charges haven't radiated away?

A quick googling now (I don't know why I never bothered to do that before)
pointed me to [https://physics.stackexchange.com/questions/65339/how-and-
wh...](https://physics.stackexchange.com/questions/65339/how-and-why-do-
accelerating-charges-radiate-electromagnetic-radiation), which would suggest
that it's not movement but _acceleration_ that makes charges radiate. But
again, almost nothing ever is stationary, nor is it ever in truly uniform
motion, so how come all the charges haven't radiated away?

~~~
throwaway2048
The gas is so not-dense that it's particles effectively never collide, so its
acceleration never really changes, so it never radiates.

~~~
a1369209993
For some reason this (along with piannucci's explaination) really helped it
click for me - while the gas has a very high blackbody spectrum temperature,
emitting hard radiation when it's particles collide at high speed, it
_effectively_ has very little _surface area_ from which to emit that blackbody
radiation.

------
not_the_nsa
A discovery by the ASKAP (I work across the road from their data centre, the
Pawsey SC Centre), code on GH, data in CKAN. Nice.

------
idlewords
They are always in the last place you look.

------
thehappypm
"The “wind resistance” effect on radio waves is incredibly small, but space is
big."

This reads like something from XKCD!

------
kristianp
Now for the small matter of the 95% of unknown Dark Matter.

