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Re: Cosmology



I am not the one to poke fun at cosmogonists who seek to compose
the irreconcilable: I simply recall that the squirming point of the
fin du 19ieme century was the ultra violet catastrophe, and the
comparable issue at the end of the 20th has been the mensuration
of time and space.

Einstein havered with a constant to render variable his cosmogony,
and more than one theorist is stringing together a pretty
necklace of dark masses, manifold dimensions and so forth.

There will, I dare say, first be:
a resolution of the red shift redder than the preferred Universal age:

then a reworking of the inflationary Universe: - if you're going to
believe in C, then act as though you believe in C, oh ye of little faith!

I could go on, but I won't.

Brian W


At 08:56 AM 6/18/2004, Tim F., you wrote:
//
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 were 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


Brian Whatcott Altus OK Eureka!