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

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The Cosmic Microwave Background (CMB) is a remnant from the early universe, appearing as a uniform radiation that fills space. After the universe became transparent about 380,000 years post-Big Bang, photons from this "last scattering surface" began to travel freely, and their density remains consistent throughout the universe. The misconception that the universe was small enough for CMB photons to have passed us by is clarified by understanding that the universe is either infinite or vast, allowing for a continuous influx of CMB photons. The discussion emphasizes that photons are indeed moving, and there are always more photons behind those currently observed, ensuring we continue to detect the CMB. The nature of the universe's expansion and the distribution of CMB photons support the idea that they will not run out, as they are constantly being emitted from distant regions.
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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?
 
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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?
 
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.
 
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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!
 
MikeyW said:
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"
 
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https://en.wikipedia.org/wiki/Recombination_(cosmology) Was a matter density right after the decoupling low enough to consider the vacuum as the actual vacuum, and not the medium through which the light propagates with the speed lower than ##({\epsilon_0\mu_0})^{-1/2}##? I'm asking this in context of the calculation of the observable universe radius, where the time integral of the inverse of the scale factor is multiplied by the constant speed of light ##c##.
The formal paper is here. The Rutgers University news has published a story about an image being closely examined at their New Brunswick campus. Here is an excerpt: Computer modeling of the gravitational lens by Keeton and Eid showed that the four visible foreground galaxies causing the gravitational bending couldn’t explain the details of the five-image pattern. Only with the addition of a large, invisible mass, in this case, a dark matter halo, could the model match the observations...
Hi, I’m pretty new to cosmology and I’m trying to get my head around the Big Bang and the potential infinite extent of the universe as a whole. There’s lots of misleading info out there but this forum and a few others have helped me and I just wanted to check I have the right idea. The Big Bang was the creation of space and time. At this instant t=0 space was infinite in size but the scale factor was zero. I’m picturing it (hopefully correctly) like an excel spreadsheet with infinite...

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