Nate Smith wrote:

Roger wrote:

I was hoping someone could help me with the following
question. I am trying to understand something of the size
of the universe, and I came across a quote that said
"the universe is expanding in all directions, and that
the 'Cosmic Microwave Background' (the remaining heat
from the Big Bang, is found at a distance of 15 billion
light years from us in all directions."

Tow questions result from this:
- does this not suggest the universe is ball-shaped, and
- that the earth is pretty much at the centre of the
universe, near where the Big Bang occurred?

Any input you can give me (in laymen's terms, please)
would be welcome!

Rogier


imagine the surface of an inflating balloon.
there are a number of dots inked on it. these
dots are attached and not part of the expanding
rubber surface, so they do not grow in size as
the balloon inflates. one of these dots is where
we are. from our vantage point it looks like all
the dots are receding from us. the dots further
away are receding faster because there is more
expanding balloon between us and them.

in the rubber space between the dots there is a
residual glow from the big bang, your cosmic
background. the bang corresponds to when the
balloon was empty. the cosmic background reveals
when the balloon was empty.

notice that our dot is not at the center of the
balloon's surface, nor is any other dot. infact,
also notice, although i did not specify it, that
the balloon is not necessarily spherical. it might
be close to exact to satisfy certain cosmological
theories. now consider that the balloon's surface
is a two-dimensional surface.

in the real universe, the corresponding surface
manifests itself, as far as we can be practical about
it, as a 3-dimensional surface in a higher order
setting. we are as hampered in comprehending the
actual picture as 2-d creatures on the balloon would
be trying to imagine the center of the balloon or
being "inside" or "outside" of the balloon.

The balloon analogy is good for the geometry but bad for the
physics. It implies that there is something pushing the
bits apart, which is not the case. The Big Bang was
actually an explosion and, in any explosion, all the bits
are rushing apart with a speed that is proportional to how
far apart they are. However, this implies that there is an
edge to the expanding matter, and that's not the case
either, the expanding matter occupies the entire universe.

For this particular problem recall that when you look at
something you see it, not as it is, but as it was when the
light you see left it some time ago. That's about one
nanosecond per foot. When you look out into space you see
objects as they were further and further into the past the
further away they are. If you look past all the more recent
objects you see the universe as it was at the time when the
entire universe was a glowing plasma; well, strictly
speaking you see it as it was at the time it stopped being
a glowing plasma and became transparent. It's the light
from that plasma, seriously redshifted, that is the
microwave background. And it's constant (more or less) all
round us because whatever direction we look, if there's
nothing in the way, we see that far back in time.

The `more or less' arises partly because we are whirling
around our galaxy which produces a slight asymmetry in the
direction we are moving and partly because the entire
universe didn't become transparent all at the same instant,
bits came first and bits came later, which produces tiny
ripples in the cosmic background. By looking at these
ripples we can deduce things about the universe before it
became transparent.

Keith Harwood.