On Sunday, January 26, 2020 at 6:39:00 AM UTC-7, John Clark wrote: > > On Sat, Jan 25, 2020 at 10:27 PM Alan Grayson <[email protected] > <javascript:>> wrote: > > *> JC wants to know how to distinguish a finite spherical universe from an >> infinite flat universe* > > > No, JC doesn't care if the universe is spherical or flat, he just wants to > know if he keeps going will he always keep getting further from his > starting point, or will he start to return. >
If the universe is expanding, even lower than c, you'll never return to starting point. AG or will he eventually hit some sort of wall. > No walls, ever. AG > > > *he thinks if he can't directly MEASURE non-observational regions, >> which both have, he can't distinguish the cases. * > > > Correct. > > >> *> The difference is this: every observer in a spherical universe can >> calculate its radius if he knows the rate of expansion and how long it has >> persisted for,* > > > Incorrect. He can't determine how big the universe is, from the date of > the expansion and its rate of acceleration he can only calculate how far > into the universe he can see. > *If it's a hyper-sphere, its curvature is constant. So if you can measure OR calculate its curvature, by simple trigonometry you can calculate its total finite volume from the curvature, including the non observable region. According to the article on posted on a related thread, the authors claim that the CMB data suggests the universe is curved. I anticipate that at some time in the future. we'll be able to measure the curvature from the CMB. AG * You're error is you ignored a fact that is fundamental and very important, > the speed of causality is not infinite. > *I have not. AG* > That's why regardless of if the universe is infinite or finite I doubt > there is a cosmologist alive who thinks that what we'll someday be able to > see with even tomorrows planet sized telescopes is all of the universe that > there is. > *For an expanding universe, we'll never be able to observe the entire universe, whether hyper-spherical or flat. But for a hyper-spherical universe we don't have to. All we need is to determine its curvature, from which we can easily calculate its total finite volume, including the non-observable region. AG* > There will always be part of the universe we will be unable to see even in > principle because parts of it are moving away from us faster than the speed > of causality. > *Agreed. That's why I objected recently to Brent's claim that light from a galaxy in our non-observable region can cross our event horizon. Those photons are not traveling fast enough to overcome the rate of expansion of space. AG* > We have a good lower limit of how big that unobservable part is, it's > larger than zero, but its upper limit is pure speculation and always will > be. > *Doesn't matter if our universe is a hyper-sphere and we want to determine its finite volume, provided we can determine its curvature. AG* > > John K Clark > -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/599eea82-eba6-4c15-9a36-d775881b2331%40googlegroups.com.
