Wikipedia:Reference desk/Archives/Science/2016 August 9

= August 9 =

Why are people able to swim 2.5% faster after the high-tech suit ban?
Didn't they say you'll almost never see a swimming world record at the Olympics again? Then some guy beats a high-tech suit record by 2 and a half percent without a suit. Sagittarian Milky Way (talk) 02:37, 9 August 2016 (UTC)


 * No sources unfortunately, I read/saw an interview with Dutch swimmers/coaches during/after the last Olympics explaining some advances in training. Years ago, trainers kept time with a chronometer and watched with their eyes how their charges swam. Now, it's possible with cameras both in and out of the water and the results of these are analysed, which could explain the new records even without a special suit. Rmvandijk (talk) 09:08, 9 August 2016 (UTC)
 * Another 0.1 second in time was gained by training with the sound of the starter signal, rather than a coach saying "go". Graeme Bartlett (talk) 22:00, 9 August 2016 (UTC)


 * VIDEO ANALYSIS FOR THE MASSES has a brief description of the video analysis used in top level competition and training. Our Stroke Analysis: The Two Best 1500m Swimmers In The World describes how the style and stroke rate is adapted to suit the height and build of the swimmer. Alansplodge (talk) 16:18, 10 August 2016 (UTC)

When will the most distant star/protogalaxy (UDFj-39546284) leave our observable universe?
Also, what is the comoving distance to the same object right now and is that outside of our observable universe right now?  Melmann (talk) 10:20, 9 August 2016 (UTC)


 * Good question. Nobody else has tackled this post so I will. Here goes.... Some might say that they will never leave the observable universe as galaxies leave a light-trail behind. But of course, as they move away, they move away faster and faster, so the Doppler effect will cause them to move more into the infra red end of the spectrum and maybe even lower, but we will (or rather any other life form that replaces us in the distant future) will be still be able to detect them. As to the 'comoving' bit. We may only be able to see these very distant objects  as they were billions of years ago, due to the speed of light taking that time to reach us and they have moved many light years on since, but then consider this. We can only see our own sun as it was 8½  minutes ago yet we still say we can observe it. Yet, that leads on to another question. Is the expansion of the universe infinite or finite. Ie, will expansion slow down and  eventually stop, only then for everything to go into reverse and end up at its original point of origin to  creating another big bang? Come on Stephen Hawking, you must read Wikipedia from time to time. What is the answer (if you don't answer, I might be inclined to hack your speech synthesizer and make you sound like Fenella Fielding). --Aspro (talk) 19:21, 9 August 2016 (UTC)
 * Assuming


 * its reported Redshift z= 11.9
 * the Lambda cold dark matter standard Big Bang cosmological model
 * this plot is correct


 * the Comoving distance from Earth of UDFj-39546284 is 34 Glyrs. AllBestFaith (talk) 23:11, 9 August 2016 (UTC)


 * The observable universe is usually defined to be everything with a worldline that intersects our past light cone at any point after the last scattering time (redshift z ≈ 1100, around 400,000 years after the big bang). By this definition, nothing can ever leave the observable universe. You can see in a diagram like this one:

/\   ← later / \      /    \     /  /\  \   ← now / /  \  \   /  /    \  \ ~  ← last scattering time
 * that anything that intersects the past light cone of Earth at an earlier time will intersect it at a later time.
 * "The observable universe right now" is the intersection of those same worldlines with space at the present cosmological time. So the things in the observable universe right now are the same as the things we can see (in principle) on our past light cone.
 * (Technically it is more complicated than this, because of peculiar velocities relative to the Hubble flow. If you define the "observable universe right now" by the Hubble flow, rather than the actual worldlines of galaxies, then some galaxies right at the edge of the past-light-cone observable universe aren't in the "observable universe right now", and vice versa. But I think that's an unimportant detail.) -- BenRG (talk) 00:25, 10 August 2016 (UTC)
 * What about going the other direction, though? When will it be the case that the future-facing light cones do not meet?
 * (And is there a name for that? I think I've also heard that described as the "observable universe", in the context of discussions of the Big Rip, which is sometimes described as the observable universe getting smaller.) --Trovatore (talk) 00:32, 10 August 2016 (UTC)
 * I don't think "observable universe" is used for future light cones. In standard (ΛCDM) cosmology, the past light cone of our infinite future is an event horizon beyond which we can never see anything. That horizon eventually shrinks to a constant apparent size (a sphere with a radius of about 16 billion light years). People often describe that as the observable universe, but that's inconsistent with the standard usage. (It could be a second standard usage, but there are a lot of incorrect statements about cosmology on the web, and even in published papers, some of which are documented in "Expanding Confusion". I don't think I've heard anyone use "observable universe" in this way who I trusted to know what they were talking about.)
 * That horizon shrinks to a point at the Big Rip. But if the universe ends at all, the past light cone of the end of any particle's existence is an event horizon for that particle, and shrinks to a point (as past light cones do), so the fact that it happens in the Big Rip doesn't distinguish it from any other end-of-the-universe scenario. -- BenRG (talk) 17:51, 10 August 2016 (UTC)
 * It would be good to have a convenient name for the portion of the universe inside that horizon. I gather that you personally may be in a position at some point to have some influence on that IRL.  Maybe you can work on it. 1/2 :-). --Trovatore (talk) 22:33, 10 August 2016 (UTC)

Okay, so, obviously things are more complicated than I assumed. Or maybe I just didn't explain myself well (probably likely). My understanding is that with the expansion of spacetime the relative speed between us (the observer) and UDFj-39546284 eventually becomes superluminal. At this point any light that leaves either end of this comoving distance (and heads towards the other side) can never reach the other side. At what point in the future will the relative speed between us and it caused by expansion become superluminal (thus severing the causality between any events here and there)?  Melmann (talk) 09:01, 10 August 2016 (UTC)
 * I think that's roughly what I was getting at with my question to Ben. But I'm not sure it really makes sense (or at least there may not be only a single interpretation) to talk about the "relative speed" between us and a distant galaxy, given that we don't share a common Lorentzian reference frame (happy to be corrected on that point though).  However, the formulation about the future-facing light cones not meeting seems unambiguous (though of course it would be vacuous if the universe were destined to collapse eventually in a Big Crunch, but that doesn't seem to be a common opinion these days). --Trovatore (talk) 09:15, 10 August 2016 (UTC)


 * Superluminal recession doesn't mean much. In an open universe without a cosmological constant (which never recollapses, and contains an infinite amount of matter), you eventually see light from arbitrarily distant objects at arbitrarily late times, even though Hubble's law still holds, and says that faraway matter has arbitrarily large recessional speeds. Fundamentally, the "recessional speed" is just not very meaningful. It's not the same as special-relativistic speed, which can't exceed c.
 * In standard (ΛCDM) cosmology, there is an event horizon which eventually behaves something like a black hole horizon turned inside out (see my answer above). The radius of that horizon at late times is is c/H∞, i.e. the distance at which Hubble's law says that the recession speed is c in the late-time limit. This annoying fact prevents people from understanding that this isn't true in general (it isn't even true in ΛCDM at earlier times).
 * At any rate, in ΛCDM, for any given object that we can see, there is a last cosmological time at which light it emits will reach us, namely the time at which it crosses that horizon. I haven't done the calculation, but I think that for an object with z=10, that time is already in the past. -- BenRG (talk) 18:12, 10 August 2016 (UTC)