Possible interaction between baryons and dark-matter particles revealed

https://www.nature.com/articles/nature25791

by canjobear • 8 years ago

162 62 8 years ago

noobermin• 8 years ago

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• 8 years ago

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").

pmontra• 8 years ago

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

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.

sohkamyung• 8 years ago

Yes, it's just a hint. This [1] says the confidence level is at 3.8 sigma (5 sigma is the 'gold standard' for this kind of physics). Independent verification of the findings should settle the matter in the future.

[1] https://arstechnica.com/science/2018/02/signal-of-the-univer...

stared• 8 years ago

Or in Scientific American: https://blogs.scientificamerican.com/observations/a-potentia...

jen729w• 8 years ago

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...

platz• 8 years ago

https://www.reddit.com/r/Physics/comments/80ynfp/possible_in...

xt00• 8 years ago

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• 8 years ago

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

codebolt3• 8 years ago

Natural philosophers are simply physicists who were too lazy to learn math.

zeofig• 8 years ago

As a physicist, I can tell you that almost none of us work with philosophers at all.

gaius2• 8 years ago

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.

mannykannot2• 8 years ago

I recall someone (Eliezer Yudkowsky?) writing an essay with the thesis that a couple of millennia of epistemology did not foresee the epistemic issues raised by quantum mechanics.

amelius2• 8 years ago

I'd like to see a forum where people think this way.

elorant• 8 years ago

You're at it right now :)

lukev• 8 years ago

It's not crazy creativity, it's thinking of things in terms of math. When you start thinking about situations in mathematical terms, you can start playing with variables. When you translate that back into layman's terms, it seems non-intuitive. But there's nothing mystical about it; quite the opposite.

dsacco• 8 years ago

It's not crazy creativity, it's thinking of things in terms of math.

Thinking of things in terms of math in order to solve problems is intrinsically creative :). I'd agree there is nothing mystical about creativity or mathematics, though.

cozzyd3• 8 years ago

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• 8 years ago

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• 8 years ago

Not being able to read more than the abstrac

https://sci-hub.tw/10.1038/nature25792

Tepix• 8 years ago

That paper is online at https://astronomy.fas.harvard.edu/files/astronomy/files/bowm...

cozzyd3• 8 years ago

There are two simultaneous papers in Nature (which I linked to), one describing the experimental result from the EDGES, the other an explanation for the result.

yCloser• 8 years ago

there is no evidence that dark matter exists at all

laying immaginary tiles upon immaginary tiles

short_sells_poo• 8 years ago

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.

nbulka• 8 years ago

Or the models and maths fail at a certain threshold.

mmusson• 8 years ago

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.

raattgift7• 8 years ago

Sean Carroll (Caltech) wrote up a quick reaction here

http://www.preposterousuniverse.com/blog/2018/02/28/dark-mat...

chopin3• 8 years ago

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?

raattgift7• 8 years ago

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

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

gjem97• 8 years ago

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?

ouid• 8 years ago

The amount by which galaxies differ in their visible mass to their predicted mass varies on a per galaxy basis.

schindlabua• 8 years ago

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

mpc755• 8 years ago

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.

Possible interaction between baryons and dark-matter particles revealed

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