Why does the CMB appear to us the way it does?

[GreatBigBore]

Seems like the universe was opaque for 380k years, and then suddenly nuclei capture electrons and there's an almighty flash, in all directions. By this time the universe is what, a million light-years in diameter, or less? Seems like the light from the last scattering surface would have gone by us already. Why can we still see it as though the surface were sitting there like a star 13.4B years ago? Is there a "Cosmic Microwave Background for Dummies" book or link?

[Calimero]

Where did you come up with one million light years in diametar? Spatial size of universe is not known for any point in time. Take a look at this thread, it may help: http://www.physicsforums.com/showthread.php?t=398487

[GreatBigBore]

About the million light-years thing, please note my final question and infer from that how much I probably know about cosmology. My main point was just a guess that the universe was small enough that light from the CMB would have crossed all the way from one end of the universe to the other by now, given almost 14B years.

The Wright model I found is confusing to me because the photons don't move. I've never heard anyone say that the CMB photons don't move. So I'm still trying to figure out why we still see photons from the CMB, if it was just a single flash. Why wouldn't all of the photons from that flash have long since gone past us?

[Calimero]

Nop, watch carefully, photons are moving. Note that that simulation corresponds to a closed universe.

It is wrong to picture Big Bang as explosion in pre-existing space, from which stuff races outward. Whatever was size of universe at that time, it was uniformly filled with hot and dense material. Imagine such universe evolving further. At some point it becomes transparent, and photons can freely move. Now, wherever photons go, there will be some galaxies, and maybe someone to talk about them. Since our universe is homogeneous and isotropic, and was even more homogeneous and isotropic (at smaller scale) at times of Big Bang, density of CMB photons is same everywhere. Photons we are today registering as CMB, originated far, far away.

[MikeyW]

Your idea of a particularly small universe is not correct; for the time being, try to think of the universe as either infinite or very large to the point where the observable universe which we CAN see is very small in comparison. This should remove the problem of "why are the photons not running out if they are still coming towards us?" - because there are always photons behind the ones we see right now, waiting to be seen in the near future!

[Calimero]

Your idea of a particularly small universe is not correct; for the time being, try to think of the universe as either infinite or very large to the point where the observable universe which we CAN see is very small in comparison. This should remove the problem of "why are the photons not running out if they are still coming towards us?" - because there are always photons behind the ones we see right now, waiting to be seen in the near future!

Size is not that important (most common lie you will here from women :wink:). If you follow cosmological principle (assuming that it was true from Big Bang), you can't run out of CMB photons (neglecting things like cosmic horizon, to keep it simpler).

To GBB: watch Ballon model here: http://www.astro.ucla.edu/~wright/Balloon2.html