remmeler said:
Sorry to press, but I don't exactly follow your answer. If the leading edge of the background radiation (not the edge of the universe) is , let's say 13 billion light years away. That is a finite distance. If I can see that finite distance in any direction, then it stands to reason, in my mind, that, unless the background radiation is something else, you would have to not be able to see the leading edge of the background radiation in at least one direction unless you were in the unlikely spot as being at the center of where the big bang happened.
The trouble is with your mental picture, the U as material expanding out into empty surrounding space.
One way to get a different image is to google "wright balloon model" and watch it. It is a 2D toy model of a 3D expanding space so imagine that all existence is concentrated on that 2D surface.
You will see small white flat galaxies scattered on the surface, getting farther apart but always staying at the same approximate (lat and long.) location on the balloon surface.
You will see small colored wrigglers traveling across the surface---these are photons of light.
Imagine being a flat 2D creature living in one of the galaxies. The amount of light you get is the same, on average, from every direction. (All directions are in the balloon surface, there is no surrounding space "outside" or "off" the surface. There is no center of expansion that you, as a 2D creature in that U can point your 2D finger at.)
Imagine that it is the same with us, except we are 3D creatures in a 3D space. The space has no edge, no boundary. There is nothing "outside". There is no center of expansion that we can point our fingers at. There is simply expansion.
And so the ancient light comes to us with approximately the same intensity from all directions.=============================
Notice that the photon "wrigglers" of light start out being shortwavelength and blue, and as the 2D sphere surface they live in gets larger they themselves get stretched out and get longerwavelength. The artist/animator has emphasized this by having them change color as they get longer, from blue to green to yellow to orange to red.
This is a "false color" way of calling the viewer's attention to the change. A real CMB photon would typically start out, say, reddish orange (a 3000 Kelvin glow) and gradually fade into an infrared color that our eyes can't see, and gradually get longerwave and longerwave until it was in the microwave-oven millimeter radar part of the spectrum, so no visible color. The animation symbolizes that by showing the photons having false-color red.
You can get a lot of insight if you watch that simple 2-minute movie carefully.
http://www.astro.ucla.edu/~wright/Balloon2.html
It is always avaliable if you just google "wright balloon model".
