Some years ago a lot was made of the fact that the COBE satellite confirmed the existence of ancient Big Bang-generated background radiation. This radiation is described as being uniform everywhere one looks (discounting for the purposes of this discussion the “wrinkles” which can be observed).
That’s by way of introduction.
Here’s my question: How come the radiation seems to come from everywhere? Since the Big Bang, the radiation has been speeding away (whatever that means) so how come we can see it at all? Any given photon detected by COBE as part of the background from the Big Bang had to come from somewhere. Where is that? And, since the Big Bang started from a single point, how come its remnants seem to come from everywhere?
Hope this makes enough sense to generate some responses.
IANAP(hysicist), but my understanding is this: If you imagine the inflation of the universe to be like the inflation of a balloon, you can see how the initial state (the Big Bang, the uninflated latex) will surround the interior on all sides. As you blow up the balloon, the outside of the balloon, if it could be measured in some way from inside the balloon, would appear to exist in all directions. In the same way, the remnants of the Big Bang (the background radiation) are seen to exist in all directions. As the universe expands, that “shell” of radiation moves away from everything else in all directions, so it appears to come from every side when it’s measured.
You’re thinking of the Big Bang as an explosion that happened at a specific point in space. This is a natural thing for you to do, given that every explosion you have experience with did work this way, but it’s not the right way to think about the Big Bang. Space itself was created in the Big Bang. Every point in the Universe is the point at which the Big Bang happened, since all of space was created in the Big Bang. That’s why the radiation comes from everywhere.
Again, the Big Bang didn’t happen at a specific point in our universe. It’s not like a supernova where there’s an explosion at one point in space and time and a shell of light expands from it, and once the shell of light has passed you you can’t see it any more.
I hope this makes some sense. Cosmology is weird and doesn’t mesh up well with human common sense.
The back ground radiation doesn’t come directly from The Big Bang, it comes from the point in time at which radiation became decoupled from matter, several hundred thousand years after the big bang.
As to why we see it in all directions, at the time of decoupling (say, 14 billion years ago) the Universe was filled at every point with this more or less constant radiation. At the current time, when we look out into space, that radiation which was formed at a points 14 billion light years away is now just reaching us. The end effect is we see a “shell” of points 14 billion light years away with more or less homogenous radiation coming out of them.