On Friday, November 10, 2017 at 11:32:13 PM UTC-7, [email protected] wrote: > > > > On Friday, November 10, 2017 at 11:22:45 PM UTC-7, Brent wrote: >> >> >> >> On 11/10/2017 10:01 PM, [email protected] 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. >
I haven't read "Comprehensible Cosmos", but have done other reading. Vic had a way of glibly dismissing complex issues and it turned me off. Nevertheless, I basically respect his work. However, I really don't believe the Cosmos is Comprehensible. In fact, QM tells me the opposite. Ultimately, it's IRREDUCIBLY RANDOM as I think Bohr believed, and thus, IMO, INCOMPREHENSIBLE. God DOES play dice with the Universe. (Opinions can differ.) -- 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.
