Re: Whats wrong with the theory... The horizon problem!

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Date: 19 Mar 2005 20:23:13 -0800

bipin.gautam@gmail.com wrote:
> I'm confused about: http://www.astro.ucla.edu/~wright/nocenter.html
>
> if everything started with a point mass(big bang)... with no
obstacles
> to hinge to move the mass in all direction....... then everything
> should have a centre from where it was originated. It's like dropping
a
> STONE FROM A flying balloon in the middle of a lake....
> if there is no obstacle the disturbance travells in all direction
> equally...

The gist of it is that it's *NOT* like a wave expanding away from a
single point.

Have you ever played the old video game called "Asteroids"? The screen
is filled with lots of drifting asteroids and one small spaceship that
you have to pilot without crashing into one of the rocks. The thing
that's relevant about it is that, if an object on the screen drifts
off, for example, the right edge, it simultaneously drifts back
onscreen at the left edge. Same goes for objects drifting off any of
the other three edges; they immediately reappear on the exact opposite
side, traveling in the same direction. If the way is clear you can fly
the spaceship across the screen over & over without stopping.

In modern General Relativistic cosmology the space in the Universe can
be modeled as being like the screen in the asteroids game. If you
travel far enough you might conceivably find yourself right back where
you started, as if you'd just traveled around in a circle. But in
Big-Bang cosmology the screen of the video game itself started out
infinitesimally small at some time in the past, and has been expanding
ever since. In the current era space is large enough to acommodate the
presence of structures as large as the screen asteroids. It's also
apparent that, on a large enough scale, the number & sizes of the
asteroids is approximately the same withing any two arbitrarily chosen
regions of the screen. Thus, for any two observers who look in any
direction, the Universe looks essentially the same. There's no center
to all this in the easily visualisable sense. Any place is as good as
any other place. You can get from anywhere to anywhere without bumping
into an edge of the universe. It's a self-contained world.

Another possible General Relativistic world is simply one in which the
video game screen started out infinitely large at the inception of the
Big-Bang, and what's been expanding ever since has simply been the
space between every given two points in space. But in this scenario
it's still the same as the previous proposal of a self-contained space.
In each case, on average everything looks pretty much the same in any
direction from any location, and the space between every pair of
locations is expanding.

In General Relativity the difference between these potential universes
is largely a matter of the mass-density. A low enough mass density
allows the Universe to be infinitely large. If the mass-density is
large enough, the Universe necessarily becomes a finitely sized
self-contained space.

-Mark Martin

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