Wikipedia:Reference desk/Archives/Science/2023 September 5

= September 5 =

Cosmos
Regarding dark matter, Wikipedia, by Ethan Siegel, what we don't know #3., may have been around if there's more than one big bang and if not then the individual particles are made from new matter and continue being produced until all energy has been deleted throughout the cosmos. Jobie Steppe 2600:1702:2EB0:1E80:91AB:7FDA:9AFC:8588 (talk) 00:12, 5 September 2023 (UTC)


 * Is there a question hidden in here? --Lambiam 03:07, 5 September 2023 (UTC)


 * A so called "dark question", not visible but affecting the matter at hand? Rmvandijk (talk) 13:06, 5 September 2023 (UTC)


 * I detect echoes of the superceded Steady state theory. {The poster formerly known as 87.81.230.195} 51.194.81.165 (talk) 03:29, 5 September 2023 (UTC)

Is xenon-135's 3 megabarns of neutron capture still the strong force?
Even if every neutron that comes within 9,772 femtometers of the center had a 100% chance of absorption that's still a lot further than any strong or weak nuclear force is generally said to reach. Doesn't the nuclear binding force decay very rapidly beyond a handful of proton radii? Sagittarian Milky Way (talk) 02:18, 5 September 2023 (UTC)


 * For the perplexed, $$\sqrt{3\cdot 10^6\text{b}/\pi}=\sqrt{3\cdot 10^8\text{fm}^2/\pi}=9772\,\text{fm}.$$ --Lambiam 03:26, 5 September 2023 (UTC)
 * The wavelength of a slow neutron is in the order of 50,000 to 100,000 fm. --Lambiam 13:39, 5 September 2023 (UTC)
 * It's worth noting that while the cross section has units of area, it does NOT represent a physical area. So while the mental image of "higher chance of interaction = larger surface area" is simple, it's not always correct. There are other factors that can increase the chance of neutron capture, and those result in a bigger cross section, without changing the physical size of the nucleus.
 * In the case of xenon-135, it looks like there's still discussion of WHY the cross section is so high. Among other reasons, Xe-136 has 82 neutrons, which is a magic number, thus increasing the likelihood of capture. PianoDan (talk) 18:05, 5 September 2023 (UTC)
 * I knew it wasn't physical area but you can't make the cross-section bigger than the furthest possible capture I didn't realize those levels of fuzziness extended to such heavy particles. I suppose the thermal neutron is so slow compared to electron cloud electrons that its wavelength is still huge anyway. Sagittarian Milky Way (talk) 23:35, 5 September 2023 (UTC)

How did plants even go far as to evolve at an evolutionary pace comparable to such as certain extinct clades?
How did plants even go far as to evolve at an evolutionary pace comparable to such as certain extinct clades? Specifically, is there any basis to the theory that there was ever an alternative pathway through which the most diverse genus would have adapted in terms of different as well? Ассхол МакДелукс (talk) 03:52, 5 September 2023 (UTC)
 * I'm not sure there is any belief that plants evolved faster or slower than any other clades. Many plants are annuals, and even large trees can reproduce at a young age, especially if there has been an event that amounts to a selection pressure. Individual ramets or branches can sport at any time. So generation time shouldn't be a concern. Genome size (mostly from polyploidy) allows for "experimentation", so that's a wash too. Differences in mutation rates are a possibility, but those are more or less under the control of the organism, and selection is only purifying in important genes such as housekeeping genes.  Abductive  (reasoning) 04:29, 5 September 2023 (UTC)

Units
Why does Atlantic and Pacific hurricane articles mostly use imperial units and Fahrenheit first? Why they don't use Celsius first on sea surface temperatures? Could this be changed?--40bus (talk) 15:48, 5 September 2023 (UTC)


 * Manual of Style/Dates and numbers says:
 * The choice of primary units depends on the circumstances, and should respect the principle of "strong national ties", where applicable: In non-scientific articles with strong ties to the United States, the primary units are US customary.
 * But as weather is a scientific subject, then there seems to be a good case for changing the order. Before you do anything though, post your question at Wikipedia_talk:WikiProject Weather and see if there is a concensus. Alansplodge (talk) 16:19, 5 September 2023 (UTC)
 * WikiProject Tropical cyclones/Style does not seem to mention units and when to use metric vs. U.S. customary units. If you do want to know, you should ask directly at Wikipedia talk:WikiProject Tropical cyclones.  They are one of our most active Wikiprojects, and would know more about the style of their articles than any one here is likely to know.  -- Jayron 32 17:22, 5 September 2023 (UTC)
 * You might convince the americans there by saying climate change temperature differences will be smaller in Celsius 😀 NadVolum (talk) 17:29, 5 September 2023 (UTC)

Expanding universe questions
I have reread Expansion of the universe for the nth time after clumsily trying to explain it to my kid, and realized I had, and have, bigger gaps in my knowledge than I had thought, and my questions make me feel like a child myself.

1. Shape of the universe notes "One of the unanswered questions about the universe is whether it is infinite or finite in extent", but how does this square with "its volume increased by a factor of at least 10^78" in the inflationary epoch? If the universe's spatial extent is infinite now, when was it not infinite such that it had a volume?

2. Shape of the universe also says that if the universe's extent is finite, it doesn't have an edge, because, what would be beyond the edge? It also name-drops maths, but is vague. Is that all we've got to demonstrate that the "raisin loaf" model is incorrect?

3. Expansion of the universe says gravitationally bound objects don't expand. Is this just relative to each other? If we observe from Earth two supergalaxies that are gravitationally bound, at the maximum distance those structures can be bound, and one is farther from us than the other, would we observe the distant one receding from us faster than the nearer one? If so, how do I square that with gravitationally bound structures "not expanding"?

4. Am I correct in believing that a 1-meter rod measured by me a billion years ago is still 1 meter today, and not 1.07 meters (Hubble's law)?

Thank you in advance! Lavaship (talk) 22:22, 5 September 2023 (UTC)


 * 1. Picture a box in the Universe before inflation and watch what it does during inflation. You'll find that the volume of this box increases by 1078. And that is the same for all boxes in the Universe, even in an infinite one. (It would be nice to mark the box by something material. In the modern Universe one could use isolated galaxies or clusters of galaxies for that, but those weren't available during inflation).
 * 2. Not sure what you're asking here, but a finite Universe is an analogue of the surface of a sphere or the surface of a torus (just one dimension higher), neither of which have an edge. Or think of the game pacman where, when you leave the screen at the right you enter the screen from the left. Now imagine the room you're sitting in was like that: you leave through one wall and come in through the opposite wall.
 * 3. A cluster of galaxies is a gravitationally bound object; all galaxies in the cluster have the same cosmological redshift, i.e. recede at the same rate from us. (the measured redshifts differ because the galaxies are moving within the cluster and in addition to the cosmological redshift there is a component due to Doppler shift, but that's not immediately relevant to your question).
 * 4. You're absolutely right. A rod is a bound object, held together by electromagnetic forces, so it is not expanding. To measure the length of the rod I suggest using a laser measurement, not another rod ;-) --Wrongfilter (talk) 22:44, 5 September 2023 (UTC)


 * Thank you!


 * 1. I'm still having trouble, probably because the easiest, possibly lazy way to picture the Big Bang is a lot like an explosion. If the universe is infinite in its extent, does this mean upon the Big Bang occurring, suddenly an infinite universe exists, and expands like crazy for a while? Conversely, I had pictured all the universe's infinite matter and energy stuffed into a tiny volume, presumably measurable in its size, which then expanded somehow from a finite size to infinity (if, again, the universe's size is infinite). Lavaship (talk) 16:57, 6 September 2023 (UTC)


 * The Big Bang itself is difficult because we don't really know anything about it and we do not have the tools to make any very meaningful inferences about it. The earliest stage accessible to physics as we know it is a very dense, very hot state of the universe, finite or infinite. If it's finite early on it will stay finite forever after. Inflation did not start from the Big Bang but from an existing universe that just expanded very rapidly for a brief period of time. --Wrongfilter (talk) 17:54, 6 September 2023 (UTC)


 * Regarding #2) In three dimensions, a sphere is finite in surface area, and yet has no edge. It's perfectly geometrically consistent to take that type of structure up a dimension.  In 4-dimensional space-time, similar to a sphere being finite in 2D surface area, and yet edgeless, you can have a 4D hypersphere which is finite in 3D space, and yet edgeless.
 * Regarding #3) Metric expansion of space is only observable on the scale of galactic clusters. If a group of galaxies are close enough to each other to experience mutual gravitational attraction, the expansion of space happens through them, without affecting their distance.  You only see the effect of the metric expansion of space larger than galactic clusters.
 * Regarding #4) Absolutely; gravity (the weakest of the fundamental forces) is already strong enough to overcome the forces exerted by the metric expansion of space. Thus, any objects held together by stronger forces (like the electromagnetic force which holds together solid objects) is definitely strong enough to overcome the paltry, weak force of the metric expansion of space.  A solid rod will remain the same length forever.  -- Jayron 32 13:13, 6 September 2023 (UTC)  edit: Please ignore this answer.  See below.  -- Jayron 32 15:21, 6 September 2023 (UTC)
 * It would be nice of you to acknowledge my responses instead of largely repeating them. We could have a discussion on the points where we differ (I think the statement "space expands through them" is meaningless, although that might be a philosophical point. In a fully relativistic treatment, the expansion does not exert a force; you could describe it as such if you use a Minkowski background metric.) but I ain't got no time for that. --Wrongfilter (talk) 13:57, 6 September 2023 (UTC)
 * Sorry for being wrong. I will try to be better in the future.  -- Jayron 32 15:21, 6 September 2023 (UTC)
 * I didn't say you were wrong, I only mentioned "points where we differ". --Wrongfilter (talk) 17:54, 6 September 2023 (UTC)
 * No, you were correct to raise an objection. Clearly my links to other articles and additional information on matters such as 4D geometry and the nature of infinity were at best a distraction, and in most cases were legitimately incorrect.  I thank you for setting me straight, and I apologize again for taking away from your otherwise sufficient answer.  I will try to do better in the future.  -- Jayron 32 18:17, 6 September 2023 (UTC)
 * For what it's worth, I appreciated your now-struck-out link to the Hilbert's Hotel article, which was new to me and which helped. Lavaship (talk) 17:41, 8 September 2023 (UTC)


 * There are (deep breath) an infinite number of infinities. Further, those different infinities have different sizes; one infinity can be smaller that than another. I know, sounds wild but it's completely true. . So there's actually nothing weird at all about saying, the universe (maybe) is infinite in volume, and, this volume grew enormously. Yeah doesn't that stretch yer noggin a little? Reality is under no obligation to obey our human intuition, which is designed for surviving as subtropical/tropical plains apes.
 * A surface can be finite in extent, but have no "edge" or boundary. One of these is the thing we are both resting on right now: the surface of Earth. The Earth's surface is finite in area, but there is no edge of the Earth; it's the surface of a(n) (approximate) sphere. If you pick a direction and travel that way in a "straight line"—a great circle route, the route aircraft and ships attempt to stick to on long distance routes because this is the shortest distance between two points on the surface of a sphere, the curved surface equivalent of the notion of a straight line on a flat plane—eventually you come right back to the place you set out from. Really think about that for a moment. (Also no Columbus didn't prove this that is an 18th-century myth)
 * Yep, gravitationally-bound structures don't expand due to the expansion of the universe. Because, gravity binds them and holds them together, and completely overpowers this expansion. The expansion is observable in the huge intergalactic voids devoid of nearly all matter. Being almost entirely empty space, nothing there to resist the expansion. Not much matter to speak of, so not much gravity either.
 * So the further back in time we look—every telescope is a time machine, because light has only a finite speed and thus takes time to traverse space, so when we look across great distances we're looking back in time—the faster that galaxies are moving away from us, relative to ones closer in distance and thus time. Or restated, if space  wasn't  continuing to expand, all galaxies ought to be moving away from us at the same rate.
 * We really have no clue at present what exactly happened at the Big Bang itself. People have all kinds of hypotheses, but, they're just that, guesses basically, until we have ways to test and attempt to falsify them, because that's how we Do Science. All we can confidently say At The Very Beginning is, the universe was in a very (very) hot, very (very) dense state, and then expanded to a cooler, less dense state, thus eventually producing the state we observe it in today.
 * Once we reach a universe a mere seconds (or, 0. 000 000 000 000 000 000 001 s, please double-check me there) young, we start to understand things better, because the energies reach ones low enough that we can reach them in our biggest baddest particle accelerators, where we can make testable predictions using our physics theories and math and analyzing those predictions.
 * Keeping cultivated a childlike curiosity about the world, is a good thing! Stay hungry. Stay foolish. --47.155.41.104 (talk) 16:59, 8 September 2023 (UTC)
 * "a great circle route, the route aircraft and ships attempt to stick to on long distance routes" attempt is the keyword here. Defending upon a number of factors such as the jet stream and number of planes aircraft transatlantic routes vary day-by-day.  Prevailing winds, meteorological conditions and ocean currents affect shipping to varying extents.  so do landmasses and reefs, but that's blindingly obvious! Martin of Sheffield (talk) 17:20, 8 September 2023 (UTC)
 * Greatcircle Jetstream routes.svg Right. Some routes do tend to stay pretty close. US West Coast to Japan westbound (and I assume Canada too but not sure about Mexico) tends to stick close to one because the prevailing winds are against it so there's usually no point in doing otherwise. Confuses people who don't understand the whole map projection thing because it looks like they're going way out of the way. Before the modern era of ultra long-haul aircraft they'd stop in Anchorage to refuel. More: Longest flights --47.155.41.104 (talk) 21:55, 8 September 2023 (UTC)
 * At least, they do not need to factor in the expansion of the universe. This may become an issue in determining the optimal flight path in any future intergalactic exploration ventures. --Lambiam 08:05, 9 September 2023 (UTC)
 * Galactic Overlord Xenu will no doubt be concerned. 47.155.41.104 (talk) 15:00, 10 September 2023 (UTC)