On Friday, November 10, 2017 at 11:22:45 PM UTC-7, Brent wrote: > > > > On 11/10/2017 10:01 PM, [email protected] <javascript:> wrote: > > > > On Friday, November 10, 2017 at 2:16:04 PM UTC-7, Brent wrote: >> >> >> >> On 11/10/2017 1:01 PM, [email protected] wrote: >> >> >> >> On Friday, November 10, 2017 at 12:19:05 PM UTC-7, Brent wrote: >>> >>> >>> >>> On 11/10/2017 4:06 AM, Alan Grayson wrote: >>> >>> >>> >>> On Thu, Nov 9, 2017 at 10:32 PM, Brent Meeker <[email protected]> >>> wrote: >>> >>>> >>>> >>>> On 11/9/2017 9:15 PM, Alan Grayson wrote: >>>> >>>> >>>> >>>> On Thu, Nov 9, 2017 at 10:05 PM, Brent Meeker <[email protected]> >>>> wrote: >>>> >>>>> >>>>> >>>>> On 11/9/2017 8:55 PM, [email protected] wrote: >>>>> >>>>> >>>>> >>>>> On Thursday, November 9, 2017 at 8:00:45 PM UTC-7, Brent wrote: >>>>>> >>>>>> >>>>>> >>>>>> On 11/9/2017 6:23 PM, Alan Grayson wrote: >>>>>> >>>>>> The difference between spatially flat and asymptotically flat for a >>>>>> huge universe would be virtually impossible to distinguish by measuring >>>>>> the >>>>>> sum of angles in a triangle. Moreover, I don't see how spatially flat >>>>>> can >>>>>> have nothing to do with extent, since in applying Euclidean geometry we >>>>>> surely seem to be dealing with an infinitely extended plane. TIA. >>>>>> >>>>>> >>>>>> Not necessarily. You could have periodic boundary conditions. But >>>>>> most cosmologists do assume the universe is infinite in spatial extent. >>>>>> Of >>>>>> course the flatness isn't measured by triangulation. It's measured by >>>>>> comparing the spatial spectrum of the CMB variations to model >>>>>> predictions >>>>>> with different mass densities. >>>>>> https://arxiv.org/pdf/astro-ph/0004404 >>>>>> >>>>>> Brent >>>>>> >>>>> >>>>> However flatness is measured, the criterion still seems Euclidean and >>>>> hence infinite in extent if one believes the triangle measured has >>>>> combined >>>>> angles of 180 degrees. And I don't see how this is distinguishable from >>>>> asymptotically flat for a huge but finite universe. >>>>> >>>>> >>>>> It's not. >>>>> >>>> >>>> That's my point. No way of distinguishing flat from asymptotically flat >>>> for a huge universe, so the assumption of infinite spatial extent by >>>> cosmologists seems unwarranted. But as you note below, the universe could >>>> have begun with infinite spatial extent. But ours didn't AFAIK. It began >>>> as >>>> astronomically tiny and expanded via inflation. >>>> >>>> >>>> But you don't know that. According to Einstein's equations the visible >>>> part of the universe started at *zero* size. Of course no one takes >>>> that entirely seriously since at very small distances quantum mechanics >>>> must invalidate Einstein's equations. >>>> >>>> Brent >>>> >>> >>> If you're invoking QM, aren't you conceding it started out very small, >>> if not exactly zero size? So it seems more plausible to assume it started >>> out very small, surely not infinite. But according to your previous >>> statements and those that I have read by cosmologists, the assumption of >>> infinite spatial extent is generally accepted and IMO unwarranted. >>> >>> >>> If it's flat or has negative curvature then the equations imply it's >>> infinite or perhaps periodic (no matter what the scale factor is). If the >>> curvature is positive then it's finite and closed and the scale factor can >>> be taken to be the radius, so it indeed starts small in the absolute >>> sense. Atkatz and Pagels showed that only FRW universes that are closed >>> (positive curvature) or De Sitter (flat with a positive cosmological >>> constant) can "tunnel out of nothing". >>> >>> >>> http://www.quantum-gravitation.de/media/99f63994b9064eb6ffff8004fffffff2.pdf >>> >>> So most cosmologists liked the closed universe model, until it was found >>> that expansion is accelerating. So now more of them look to some >>> modification of the De Sitter space universe. >>> >>> Brent >>> >> >> Modification of De Sitter will be flat and therefore open. I find the >> open universe model in contradiction to the finite age of the universe. Is >> this unreasonable? >> >> >> Well, if you have an infinite universe, and toward the past it is scaled >> by a factor a, and a->0 does the universes size go to zero? >> >> Why is the closed universe model less favored when it was discovered that >> expansion is accelerating? >> >> >> Because the De Sitter universe that can "tunnel from the vacuum" >> automatically has a positive cosmological constant. >> >> Brent >> > > Unfortunately, my understanding of the scale factor and cosmological > constant as they relate to the various geometries is insufficient to > appreciate your comments. Maybe you could restate your above comments with > that in mind. TIA. > > > Didn't you read Vic's "Comprehensible Cosmos"? Why are you over here on > the everythinglist asking physics questions anyway, Alan? You should try > the stackechange or quora. > > Brent >
I wasn't aware of those sites. Since "everything" includes physics, I thought this group would be appropriate. And I see some topics here include physics. I don't see the harm. -- 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 post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
