Re: Cosmology

["Folkerts, Timothy J" <FolkertsT@BARTONCCC.EDU>]

I don't know that I know enough about cosmology to comment on which cosmological models are in vogue, but I do have a quite different interpretation of this than Brian.  Let me present my model, and see if it flies.

One possible model of the universe would be a closed, spherical shape.  In principle, if you traveled far enough in one direction (and the expansion of the universe was slow enough), you could return to where you started.  As an analogy for this 3D universe, consider the surface of a 2D planet covered with water.  (if the universe is expanding, then we need an expanding planet.)

> > > If the universe were finite and smaller than the distance to the "surface
> > > of last scattering"
>
> A universe cannot be smaller than a specified distance within it
> measured comparably. 

Suppose you start a wave half way around the world on our 2D analogy planet.  If you wait, eventually the wave will reach you.  If you wait 3 times that long, the wave will travel past you, travel back past the staring point, and reach you again.  The "distance traveled" by the wave is larger than the "universe".


> > (the place in deep space from whence comes the 
> > cosmic microwaves and that constitutes the edge 
> > of the visible universe)
>
> The notion of a visible edge of the universe is I think,
> problematical.

The time before the CMB (cosmic microwave background) would be analogous to a time when the surface of the planet had lots of little islands.  Any waves created before that time would be scattered so you would have no idea where they originally came from or when they were created.  In effect, the wave were continuously recreated.  When the universe became "transparent" about 300,000 years after the Big Bang, it is like the islands suddenly disappeared.  The waves now traveled in predictable paths.  If the time since transparency is t, then the distance to the "visible edge" is vt (or ct for light). 

This "visible edge" is not any physical edge, but the surface of a sphere where the look-back time is equal to the time since transparency. 



> > multiple images should show up in the microwave background.

> ...the mechanism is given as gravitational lensing ...

That is one source of multiple images, but there are others.  Consider a source of waves on our waterworld.  Waves from the source could reach you in a straight line form the source, or they could reach you in the SAME direction by passing by you, travelling all the way around the world, and returning much later. It could also travel multiple times before reaching you.  What you would see is a source of waves, as source of much smaller waves, a source of much, much smaller waves, etc.  How many you see is limited by the age of the universe (and the lifetime of the source).  If waterworld is only an hour old, the waves could not have traveled all the way around, so you could only observe one wave source. 

So if the universe is old enough and small enough, you might see a galaxy "up close", and another, dimmer view of the same galaxy from when it was younger - the light having originally travelled past us eons ago and now returning after a lap around the universe.  

Of course, if the waterworld is really flat, then the wave will never return.  If the radius is expanding at a rate greater than the wave's speed, then the wave will never return.  


Tim F