6 ms·
Astronomers solve the missing baryon problem
- tux1968 6y agoDecently 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
- dang 6y agoThere was also a smallish thread a few weeks ago: https://news.ycombinator.com/item?id=23335794 https://news.ycombinator.com/item?id=23335794
- supernova87a 6y agoThis 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 6y agoVeritasium 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 6y agoI 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 6y agoDerek did his PhD on the public education of science. He’s been honing his craft for a /long/ time & it really shows.
- mehrdadn 6y agoVeritasium videos are pretty darn high quality. Fun to watch and learn from, sometimes mindblowingly. Definitely recommend subscribing. One of my favorites is Derek's video on compliant mechanisms: https://www.youtube.com/watch?v=97t7Xj_iBv0 https://www.youtube.com/watch?v=97t7Xj_iBv0
- joe-collins 6y agoDecently covered in a single paragraph by the top comment. Videos are excellent for content that should be shown. For descriptive content like this, a video is a strictly inferior mode of transmission.
- simonh 6y agoI thought the visual explanation of the Lyman-alpha forest, showing how the distribution of shift in the absorption lines give us a precise map of neutral matter density along the track of Quasar transmissions, was particularly good.
- Ancalagon 6y agoThat website is giving me some scammy looking pop ups on mobile.
- turndown 6y agoAgreed, I gave up on reading the article it was so bad.
- tux1968 6y agoSorry, I should have turned off uBlock and checked it before submitting.
- pdm55 6y agotry https://theconversation.com/half-the-matter-in-the-universe-was-missing-we-found-it-hiding-in-the-cosmos-138569 https://theconversation.com/half-the-matter-in-the-universe-...
- thehappypm 6y ago"The “wind resistance” effect on radio waves is incredibly small, but space is big." This reads like something from XKCD!
- varjag 6y agoHalf 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 6y agoThe 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 6y ago> 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.
- simonh 6y agoPlasma can behave differently from regular matter in this respect.
- piannucci 6y agoBlackbody 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.
- idlewords 6y agoThey are always in the last place you look.
- deleted 6y ago[deleted]
- Robotbeat 6y agoAbout 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 6y agoI 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 6y agoAmerican 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.
- solstice 6y agoSo that means we'll need a sort of magnetic snow-plow-like device for intergalactic travel to shovel all that hot plasma out of the way. Maybe it's easier to convert a whole planet into a starship to take advantage of its magnetic field... /s
- kristianp 6y agoNow for the small matter of the 95% of unknown Dark Matter.
- not_the_nsa 6y agoA discovery by the ASKAP (I work across the road from their data centre, the Pawsey SC Centre), code on GH, data in CKAN. Nice.