nelson@cvl.UUCP (Randal Nelson) (10/14/86)
From one of Ethan Vishniac's postings:
> The flatness problem makes everyone uncomfortable...
The mention of hyperspherical models made me wonder how we know that
the universe really is as extensive as the distance computed from
highly red-shifted objects suggests.
In particular, how do we know that the univers is not (relatively)
small with a topology that permits closed geodesics.
In this case, highly red-shifted light has just looped the loop
a couple of times, which would allow the entire universe to be in
communication.
This would have interesting consequences. For instance, certain
object would represent earlier stages of our own galaxy if we knew
where to look.
I have never heard this notion discussed.
Does anyone know what (if anything) eliminates such a model??
Please reply to the net.
Randal Nelsonethan@utastro.UUCP (Ethan Vishniac) (10/15/86)
In article <1733@cvl.UUCP>, nelson@cvl.UUCP (Randal Nelson) writes: > > The mention of hyperspherical models made me wonder how we know that > the universe really is as extensive as the distance computed from > highly red-shifted objects suggests. > In particular, how do we know that the univers is not (relatively) > small with a topology that permits closed geodesics. > In this case, highly red-shifted light has just looped the loop > a couple of times, which would allow the entire universe to be in > communication. > This would have interesting consequences. For instance, certain > object would represent earlier stages of our own galaxy if we knew > where to look. > I have never heard this notion discussed. > Does anyone know what (if anything) eliminates such a model?? One of the simple tricks that could allow this is just to divide the usual model up into cubes and then require that all the cubes be identified with each other. This makes me slightly uncomfortable, but I can`t think of any reason why it can be ruled out. Then the only limitation on the smallest size that could be tolerated comes from local observations of the distribution of clusters and galaxies. I *think* these require a "cell" size of at least a few hundred megaparsecs. I remember John Ellis talked about this a few years ago at a seminar here. He may have published the idea in Nature, but I really can't remember the reference. -- "More Astronomy Ethan Vishniac Less Sodomy" {charm,ut-sally,ut-ngp,noao}!utastro!ethan - from a poster seen ethan@astro.AS.UTEXAS.EDU at an airport Department of Astronomy University of Texas
litow@uwmeecs.UUCP (Dr. B. Litow) (10/15/86)
> From one of Ethan Vishniac's postings: > > > The flatness problem makes everyone uncomfortable... > > The mention of hyperspherical models made me wonder how we know that > the universe really is as extensive as the distance computed from > highly red-shifted objects suggests. > In particular, how do we know that the univers is not (relatively) > small with a topology that permits closed geodesics. > In this case, highly red-shifted light has just looped the loop > a couple of times, which would allow the entire universe to be in > communication. > This would have interesting consequences. For instance, certain > object would represent earlier stages of our own galaxy if we knew > where to look. > I have never heard this notion discussed. > Does anyone know what (if anything) eliminates such a model?? > Please reply to the net. > > > Randal Nelson I don't know much about this but a starting point for references might be "The Large Scale Structure of Spacetime" by Hawking & Ellis *** REPLACE THIS LINE WITH YOUR MESSAGE ***
ethan@utastro.UUCP (Ethan Vishniac) (10/15/86)
In my previous article I referred to John Ellis when I meant
George Ellis. People should strive for more distinctive names :-).
Also, the repeating universe idea does not explain the large scale
statistical homogeneity of the universe (the flatness problem)
unless the cycle distance is *very* short. How short depends on
what you think a "natural" initial state might be. If one assumes
total chaos then a few billion Planck lengths might be OK.
(About 10^-34 cm). If one assumes more irregular, but not
totally chaotic conditions then perhaps sub-galactic scales could
be used. Obviously since we know the cycle distance must be greater
than a few hundred mpc this isn't a promising way to solve the flatness
problem. Which doesn't mean it couldn't be true on some large scale.
--
"More Astronomy Ethan Vishniac
Less Sodomy" {charm,ut-sally,ut-ngp,noao}!utastro!ethan
- from a poster seen ethan@astro.AS.UTEXAS.EDU
at an airport Department of Astronomy
University of Texasbsw@cbosgd.ATT.COM (Ben Walls) (10/16/86)
In my High School Elementary Analysis with Trig Book, there were these "Great names in math" type of things. One was about a man, whose name I forget, who created a universe that is spherical. The center is hot, and it cools towards the edges. So the closer you get to the edge, the slower you go, and the smaller you get. You can never reach the edge. My question is who was this man, and does anyone have any more info about this universe, or any other nifty theories by him? Ben Walls bsw@cbosgd.UUCP -- Ben Walls ...!cbosgd!bsw or bsw@cbosgd.uucp