bstempleton@watmath.UUCP (Brad Templeton) (06/14/83)
Let's see, the universe is about 15B years old, and about 10B light years in radius. Thus nothing has gone around the whole thing by present standards, but no doubt something could have when it was smaller, in the case of collapse, when it gets smaller. Or would this collapse be invisible to those inside it? -- Brad Templeton - Waterloo, Ont. (519) 886-7304
gwyn%brl-vld@sri-unix.UUCP (06/15/83)
From: Doug Gwyn VLD/VMB <gwyn@brl-vld> One really has to watch out for mental traps when talking about the entire universe. One very important consideration is what a given amount of "distance" would ____mean in an "expanding" universe, or indeed what it means for the universe to "expand". If your means of measuring distance are affected by the expansion, it is possible (probable?) that such a phenomenon would be undetectable and therefore without objective significance.
FtG@rochester.UUCP (06/17/83)
I believe Sagan was referring to circumnavigating the KNOWN universe, which is a small fraction of the entire universe. The diameter of the known universe is taken to be a fixed constant and those objects currently within the diameter but receding away will eventually become "unknown". (The combination of inverse square and red-shift puts an upper limit on what we can "see", even with the very best [radio] telescope possible due to quantum mech. effects.) This diameter, in light years, depends on your choice of Hubble constant,.... etc. FtG @ rochester
rh@mit-eddi.UUCP (Randy Haskins) (06/26/83)
In "The Collapsing Universe," Asimov suggests that the current calculations about the volume and mass of the Universe put it within an order of magnitude of being a large black hole. He shows a good analogy of why the larger mass you have, the less the density needs to be for a black hole. Of course, this stuff only occurs at the edges. In the center, (where we presumably are) things carry on as normal. The book is good reading. -Randy
rlh@mit-eddi.UUCP (Roger L. Hale) (06/28/83)
From rh (Randy Haskins) Sun Jun 26 15:27:29 1983
... Of course, this stuff [coming to be a black hole
through accumulation of mass] only occurs at the edges.
In the center, (where we presumably are) things carry
on as normal.
NOOOO!!! Wrong! This person should be castigated!
There is no physical singularity anywhere in your standard
large enough gravitating mass, at the initial time. In a uniform
mass, things will collapse to a singularity first at the center;
the conditions defining the event horizon (the "edges") might
be observed by a synoptic observer in that light travelling outward
(the best possible escaping signal) here finally collapses into the
singularity and here finally recedes in a "hyperbolic" orbit,
most of its energy sapped and red-shifted as a distant observer
might see it.
Consider that in a 3-spherical universe, everywhere is equally
"central"; and with any positive density (I believe) everything
becomes singular after the same lapse of time.