On Saturday, November 11, 2017 at 12:40:45 PM UTC-7, Bruno Marchal wrote: > > > On 11 Nov 2017, at 07:59, [email protected] <javascript:> wrote: > > > > 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. >> > > > Sometimes we discuss physics, but in general it is in or through relations > with fundamental problems, including the mind-body problem, notably. > > You might look at the Space-Times Series (it exists on YouTube, link > below). I just "understood" (as far as a mathematician can understand > physics of course) why the "tunneling from the vacuum" imposes a positive > cosmological constant to some of solution of Einstein's GR equation, or > others (probably the De Sitter one I guess). > > https://www.youtube.com/watch?v=X5rAGfjPSWE&t=712s > > (That is not the episode which makes me get what Brent said. >
I'll watch it. TY. I could be mistaken, but I thought the CC is (a possibly IMPOSED) positive for an expanding universe. Before it was discovered that the expansion is accelerating, cosmologists preferred a closed, finite geometry; presumably a huge expanding hypersphere. But Brent wrote that the open, flat, infinite model became preferable after that discovery, presumably because "tunneling from the vacuum" automatically IMPOSES a positive cosmological constant. Is it the automatic imposition of a positive CC that makes the open, flat, infinite model preferable? This is really a huge change in perspective (and FWIW, not one I am happy with). I wonder; does the imposition of a positive CC ALSO automatically imply open, flat, and infinite? TIA, AG > > > > 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 seem to take for granted that Reality is the or a Physical Reality. > But where that Physical reality comes from, and what is physical about it? > > Theoretical computer science, and its arithmetization, entails that if we > assume the local finiteness of the observers, the observable correctly > inferable of almost all finite observers emerges from a lattice of > computations restrained by self-correctness self-referential relations. > > God get just asleep and lost itself innumerably in a labyrinth of dreams. > technically, this should (and seems already) reduce the laws of the > observable to arithmetical self-reference (unfortunately only well known by > logicians (self-reference is handled in the arithmetical sense explained in > Gödel 1931 paper. > > The Universal machine of Turing has a theology (a science of its own > limitation toward truth if truth exists) which includes physics, so it is > testable/refutable/improvable. > > All mystics said this: the Universe/Truth is in your head. So I suggest we > compare the Universe which is in the head of the Universal (Turing) machine > and what we infer from observations. > > You are right, there is something irreducibly random, if only the first > person view of self-duplication i Arithmetic. But there are enumerable > pieces of computations with an extreme redundancy in elementary Arithmetic > (which is Turing complete). Physics is a first person plural view of > arithmetic, and there must be derivation of the role of each pieces of its > mathematics, like the group theory implied. Then the theology separates the > true from the rationally justifiable, etc. The big discovery is the > discovery of the universal machine (Emil Post, Alan Turing, Stephen Kleene, > ...). It lives already in the diophantine polynomial equations. > > Bruno > > > > > > > -- > 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] <javascript:>. > To post to this group, send email to [email protected] > <javascript:>. > Visit this group at https://groups.google.com/group/everything-list. > For more options, visit https://groups.google.com/d/optout. > > > http://iridia.ulb.ac.be/~marchal/ > > > > -- 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.
