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 accross 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?

While the ballon analogy is good with regard to the geometry
of the cosmic expansion, it's not terribly good for the
physics. It suggests that there is some sort of expanding
entity that the material bits of the universe are attached
to and that isn't so. Basically the Big Bang was an
explosion and in an explosion all the bits are rushing away
from all the others, and the relative speed between any two
bits depends on how far apart they are. However, the
explosion analogy suffers because that suggests there is an
edge to the exploding material and that isn't so either.

For your particular problem here consider that when you look
at something what you see is not what the thing looks like
now, but how it looked when the light you are seeing left
it. So when you look into the night sky you see things not
so much a long way away, but rather a long time ago. And a
very long time ago the entire universe was filled with a
glowing plasma. So it doesn't matter which direction you
look, when you look past the more recent objects you see
that plasma. The light from that plasma is the microwave
background.

K Harwood.