
Possible interaction between baryons and dark-matter particles revealed - canjobear
https://www.nature.com/articles/nature25791
======
cozzyd
[https://www.nature.com/articles/nature25792](https://www.nature.com/articles/nature25792)
is the more interesting article, I think, as it describes the experimental
measurement that forms the basis for the claim about early-universe gas
temperature.

Being the skeptic I am, I suspect that it's more likely that there is some
systematic error that is being underestimated or overlooked rather than new
physics. Fortunately the experiment is simple enough that it can be reproduced
elsewhere relatively easily.

Interestingly, the region around 75 MHz is between two ITU broadcasting
allocations. They claim to rule out the FM band as the source of the higher
spectral edge through several possible channels, but I wouldn't be surprised
if there was some way to get excess power in those bands in some way they
hadn't considered (although I guess any strange propagation mechanisms would
probably be time-dependent).

~~~
SiempreViernes
Last I heard, reionization was not understood in detail so I find it
surprising the author here is confident they found a dark matter signature.
Not being able to read more than the abstrac I can’t say for sure, but it also
looks like this is not an experimental paper but a theoritician trying to put
put a quick paper: no mention of the instrument in the abstract nor comparison
to existing exclusion limits on the dm today.

~~~
anon24476
> Not being able to read more than the abstrac

[https://sci-hub.tw/10.1038/nature25792](https://sci-
hub.tw/10.1038/nature25792)

~~~
mirimir
This is much friendlier than an insulting "Subscribe to Nature for full
access: $199" message.

------
raattgift
Sean Carroll (Caltech) wrote up a quick reaction here

[http://www.preposterousuniverse.com/blog/2018/02/28/dark-
mat...](http://www.preposterousuniverse.com/blog/2018/02/28/dark-matter-and-
the-earliest-stars/)

~~~
chopin
Nice write-up, thanks for sharing.

I am wondering, can't there be something like gravitational cooling? When
playing around with gravitation simulators one observes that sometimes high
velocity parts leave the bulk, which in my understanding leads to a cool down
of the pack (the escaping part carries away kinetic energy, after all). If I
let some "cool" matter interact with "hot" matter only gravitationally, I'd
expect that this leads to an equal "temperature" eventually. Is it, that due
to the very weak interaction, time scales are too long for this?

~~~
raattgift
From studies of the cosmic microwave background we can be pretty confident
that temperatures were very uniform at the time of recombination, and that the
cooling of the CMB (which was fixed at that time [1]) closely resembles the
adiabatic expansion of an ideal gas. So we're already seeing cooling-by-
separation. If we have a decent idea of the history of the expansion, then we
also have a decent idea of the temperature of the CMB at the surface of last
scattering. Since we look so intently for tiny overdensities or underdensities
to seed structure formation (collapse of matter into early galaxies), it's
hard to imagine where one could hide an enormous underdensity or overdensity
that could cool the CMB across an enormous region (we see the same temperature
along widely angular-separated lines of sight; this is the horizon problem
[2]) without leaving fingerprints.

The period between recombination and "first light" from the first stars
(triggering reionization) is not especially well understood, but the
expectation is that the expansion of the universe would be pretty smooth in
that epoch. So the neutral hydrogen around the time of first light being cold
enough to produce the unexpectedly strong 21cm absorption line is an oddity.

 _If_ evidence holds up suggesting that this gas is uniformly colder than
expected then we could take the position that our idea of the history of the
expansion is somehow wrong, or alternatively that some other process was at
work at the end of the dark ages. We have constraints on both; and in
particular "some other process" is tricky because the almost-exactly-black-
body spectrum of the CMB puts constraints on early cooling-by-clumping (where
cooling is up to "chemistry", where that's a(n exothermic, in this case)
reaction between particles of different species).

There could be "chemistry" in the dark matter sector that allows dark matter
to cool by extremely local clumping (for example, if in the "dark ages" before
recombination there are two oppositely-charged dark matter particles and at
recombination they combine into one neutral dark matter composite particle,
analogously to how electrons and protons recombined into neutral hydrogen),
but then you have to figure out how to have the dark matter be colder than the
hydrogen and how to thermalize the hydrogen to the dark matter. You also have
to avoid smearing out small matter density fluctuations.

If structure formation works the way we think, everything at recombination
seems too uniform and diffuse for local gravitational interactions to do the
job of transferring much momentum from the matter sector to the dark matter
sector in time for the 21 cm absorption line seen by EDGES. There are also
constraints on extra long-range interactions, so Carroll's raising the idea of
a zero-mass boson mediating the interaction is hard to imagine (a massive
mediator would be shorter-range, in general, and could carry more momentum
between matter and dark matter, but there are constraints on such interactions
from the much later universe -- for instance, we would expect missing momentum
in particle physics experiments, along the lines of the missing momentum that
led to the discovery of the neutrino).

The problem is that there are lots of strongly correlated observations and
pushing at one tends to cause problems with one or more of the others. But
that's what theorists in this area (model building) seem to like to do. :-)

\- --

[1] Peter Coles (cosmology, Cardiff) has a good, accessible write-up from some
years ago, adapted here:
[https://ned.ipac.caltech.edu/level5/Glossary/Essay_lss.html](https://ned.ipac.caltech.edu/level5/Glossary/Essay_lss.html)

[2]
[https://en.wikipedia.org/wiki/Horizon_problem?oldformat=true](https://en.wikipedia.org/wiki/Horizon_problem?oldformat=true)

~~~
chopin
Thanks for the elaborate answer. As far as I understand your fifth paragraph
refers directly to my question. How would uniformity rule out purely
gravitational interaction? As far as I see it gravitational forces should be
enough to get two reservoirs of different temperature onto equal temperature.
It's just the time scale which may be a problem. And if I start already with
two (almost) uniformly distributed reservoirs, long range interactions
shouldn't make a difference. Inflation should have flattened out everything
already anyways.

~~~
raattgift
I don't understand your idea. Gravitation squashes collisional matter
together, heating it. The hot matter radiates, allowing it to fall deeper into
dense structures. I don't see how this cools non-infallen hydrogen, rather
than lighting it up.

~~~
chopin
I'll try to elaborate: Let's say we have to reservoirs of baryonic matter, one
cool and one hot. If these interact through radiation (electromagnetic forces)
both reservoirs eventually get the same Boltzmann distribution and thus
temperature. Now we have the same situation but we switch one reservoir to
dark matter (the cooler one) and the force between the reservoirs is purely
gravitational. Beside the time scale, would that be any different? In a large
collection of masses, particles can exchange momentum and energy through
gravitation, afair. Therefore one could expect that the two reservoirs
equilibrate. I think what differs may be that gravitation is only attractive
and contraction may be faster than equilibration. But is this the case if the
initial temperature of the constituents is high enough, especially of the
baryonic stuff?

~~~
raattgift
Oh sorry I missed the "switch one reservoir to dark matter".

In that case, if the DM and matter can only interact gravitationally (and DM-
DM interactions are also gravitational), then you have a dance wherein the
collisional normal matter sheds momentum via radiation, letting the normal
matter fall into the DM eventually. The momentum exchanged between the DM and
the normal matter will be pretty small.

(This is similar to what people pursuing the cosmic web hypothesis try to
model, essentially, and is what's conjectured to be happening during the dark
ages).

ETA: the key thing here is that the collisionless cold dark matter will stay
cold, while the normal matter will get much hotter (and light up normal matter
further away still). Isn't this the nub of the EDGES result?

~~~
raattgift
I suppose you could attack your question by considering a DM galaxy and a
baryonic galaxy binary. On any given spacelike slice you can probably
represent the two as a barbell with the barycentre on the infinitesimally thin
bar, although there are lots of 3+1 formalism gotchas that would have to be
dealt with.

If you arrange the matter galaxy so that its internal structure is
sufficiently stable then the whole binary system would lose momentum-energy to
gravitational radiation. It won't be much, and that's a big "if" (and the "if"
also works against trying to see the matter galaxy cool, especially as the
energy loss would tend to contract the "bar").

~~~
raattgift
You might enjoy this crazy 1998 paper, which I just found, which adds a bit of
heft to the the "not much" guess above.

It seeks to examine gravitatonal waves shed by a 2-galaxy cluster.

"pessimistic point of view justified by the combination of great numerical
difficulties (three-dimensional calculations) together with very disappointing
small values expected for the gravitational-wave luminosity, the amplitude,
etc"

"In brief, our simulated cluster produces changes in the relative distance of
the order of 10-22—detectable with current technology—in a short period of 4
days."

(LIGO 2015 was about the same relative length change (h), but with a frequency
of 35 - 250 Hz. I'm not sure how catching part of a gravitational wave of ~
10^-17 Hz could be caught by a LIGO-like detector, since only higher
derivatives of h are useful for identifying gravitational radiation).

[http://iopscience.iop.org/article/10.1086/311446/fulltext/98...](http://iopscience.iop.org/article/10.1086/311446/fulltext/985163.text.html)

(but of course for cooling we're more interested in the luminosity, which is
small compared to that of an ordinary galaxy, but a bit more interesting
compared to the blackbody radiation of a galactic-mass diffuse cloud of atomic
hydrogen, although that's only considering the normal-matter member of the
binary, rather than the total energy leaving the region of spacetime the
binary is in.)

------
xt00
I get the sense the physicists are a different breed of thinker.. it’s like
normal people are like “nice I have a car that can get me to work and back..”
and physicists are like “imagine an atom was a car and you wanted to go to the
edge of the universe..” or like “nice a carrot grew in my garden!” And they
are like “how could you get a carrot to grow to the size of the earth..”..

I see a low res 2d picture and apparently they see cold particles that are
around 3x the mass of protons and the structure of the universe.. kinda nuts..

~~~
eddd
Physicists work closely with philosophers for a reason. It's philosophers who
are masters of such thought experiments.

~~~
gaius
Nietzsche solved philosophy; everything since has been quibbling over
semantics, there is no original thought anymore. Whereas physics’ Nietzsche
was Newton, and there have been continuous advances since his time.

~~~
dotancohen
> Nietzsche solved philosophy

Maybe in the sense the Von Braun solved rocketry. I agree that everything that
has come since have been refinements to Nietzsche's developments (dare I say
models) but those refinements have brought about the real-world application of
philosophy more than Nietzsche ever did.

See Mao, or Popper, or even though I can't stand the guy Chomsky.

~~~
gaius
That’s a fair assessment

------
platz
[https://www.reddit.com/r/Physics/comments/80ynfp/possible_in...](https://www.reddit.com/r/Physics/comments/80ynfp/possible_interaction_between_baryons_and/duzmjzr/)

------
noobermin
I get that this is in nature, but isn't fitting to the data with some
phenomenological model what every (:?astro)?particle physicist does? As an
outsider, I can't tell whether this is significant phenomenological
explanation or not.

~~~
_rpd
There are multiple competing models of the early universe. We now have
increased confidence in those models in accordance with this new observation,
and decreased confidence in those models that are at odds with the
observation.

In particular, this new observation constrains the nature of dark matter
(lower particle mass than expected and "highly non-relativistic").

------
gjem97
Ok, so my understanding is that the term "dark matter" refers to a difference
between the theoretical and observed exapansion speeds (accelerations?) of the
universe. Can someone summarize for me why this is a useful concept, and not
just simplify it to "we have the theory of universe expansion wrong"? What is
the evidence for there actually being something like matter involved?

~~~
schindlabua
You are thinking of dark energy, which is a completely different beast (also a
lot more vague).

Dark energy is the "why is the universe expanding so fast"-stuff and dark
matter is the "why are galaxies so heavy"-stuff of the universe.

Galaxies appear to be heavier than what you get by adding up all the visible
mass (you can tell by how much a galaxy bends light coming from behind it), so
it makes sense to talk about matter. It's dark because it doesn't seem to
interact electromagnetically with ordinary matter.

Looking at just all the gravity (dark or not) in the universe you'd expect all
the matter in the universe to slowly start clumping together, or at least slow
down the expansion of the universe, but we find that things are flying apart,
and increasingly quickly so. So there's some force at play, and that's dark
energy.

edit: letters

------
pmontra
Another article on about the same subject from the non paywalled section of
Nature

[https://www.nature.com/articles/d41586-018-02616-8](https://www.nature.com/articles/d41586-018-02616-8)

It's more about the light from the first stars but it explains why the signal
hints to light and cool dark matter.

~~~
jen729w
Layman’s article from the local paper. We’re proud of Swinburne Uni here in
Melbourne, they have a great astro dept.

[http://www.watoday.com.au/national/western-
australia/antenna...](http://www.watoday.com.au/national/western-
australia/antenna-in-wa-outback-receives-signals-from-180-million-years-
ago-20180301-h0wtyj.html)

------
mpc755
There is evidence of the strongly interacting, superfluid dark matter every
time a double slit experiment is performed, as it is the medium that waves.

------
yCloser
there is no evidence that dark matter exists at all

laying immaginary tiles upon immaginary tiles

~~~
short_sells_poo
There is ample evidence that dark matter exists, as in, there is something
there that we can indirectly observe. We have fairly good ideas about what it
isn't, we just don't know what it is for certain.

~~~
mmusson
Dark matter (LCDM) solves some problems and creates others, just like modified
gravity theories solve some problems and create others. Hopefully more
experimental data will lead to an answer.

