8 ms·
Possible interaction between baryons and dark-matter particles revealed
- noobermin 9y agoI 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 9y agoThere 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").
- deleted 9y ago[deleted]
- pmontra 9y agoAnother 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.
- sohkamyung 9y agoYes, 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-universes-first-stars-hints-at-odd-form-of-dark-matter/ https://arstechnica.com/science/2018/02/signal-of-the-univer...
- stared 9y agoOr in Scientific American: https://blogs.scientificamerican.com/observations/a-potentially-game-changing-message-from-the-dawn-of-time/ https://blogs.scientificamerican.com/observations/a-potentia...
- jen729w 9y agoLayman’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-in-wa-outback-receives-signals-from-180-million-years-ago-20180301-h0wtyj.html http://www.watoday.com.au/national/western-australia/antenna...
- platz 9y agohttps://www.reddit.com/r/Physics/comments/80ynfp/possible_interaction_between_baryons_and/duzmjzr/ https://www.reddit.com/r/Physics/comments/80ynfp/possible_in...
- xt00 9y agoI 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 9y agoPhysicists work closely with philosophers for a reason. It's philosophers who are masters of such thought experiments.
- codebolt 9y agoNatural philosophers are simply physicists who were too lazy to learn math.
- sova 9y agoOne could just as easily say that mathematicians are natural philosophers clouded by rigid symbols that have no inherent meaning on their own. For every zag, a zig.
- sova 9y agoSo many downvotes, but nobody wants to spar, what a pity.
- codebolt 9y agoAlright, I’ll bite. I challenge your assertion that math is without objective meaning. First I want to make an observation. Do you know the history of how the study of complex algebra/calculus came about? If so, I assume you will agree that it was initially a completely abstract thought experiment with no connection to anything in the “real” world. Given your assertion that math is without meaning, it would seem to me that mathematical ideas that originate purely out of human imagination would just be arbitrary semantics. Then how can it be that complex analysis only several decades after it was formulated turned out to be not only useful, but necessary to formulate the theory of quantum mechanics in a way that agrees with physical observation? I can name numerous other examples of the same phenomenon; namely that a purely abstract mathematical idea is long after its formulation shown to be profoundly reflected in physical reality. To me it seems obvious that the way these phenomena occur implies that part of the process by which humans use their imagination and reasoning to come up with abstract mathematical ideas, is more akin to using their intuition to map out objective ‘structures’ of logic (that are also reflected in the underlying structure of physical reality) than to simply play with semantics, as it seems you are asserting. Tl;dr Post modernist philosophers are fools and they should be ashamed. Qed
- cozzyd 9y agohttps://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 9y agoLast 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 9y ago> Not being able to read more than the abstrac https://sci-hub.tw/10.1038/nature25792 https://sci-hub.tw/10.1038/nature25792
- yCloser 9y agothere is no evidence that dark matter exists at all laying immaginary tiles upon immaginary tiles
- short_sells_poo 9y agoThere 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.
- raattgift 9y agoSean Carroll (Caltech) wrote up a quick reaction here http://www.preposterousuniverse.com/blog/2018/02/28/dark-matter-and-the-earliest-stars/ http://www.preposterousuniverse.com/blog/2018/02/28/dark-mat...
- chopin 9y agoNice 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 9y agoFrom 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
- gjem97 9y agoOk, 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 9y agoThe amount by which galaxies differ in their visible mass to their predicted mass varies on a per galaxy basis.
- schindlabua 9y agoYou 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 9y agoThere is evidence of the strongly interacting, superfluid dark matter every time a double slit experiment is performed, as it is the medium that waves.