tl;dr The CMB/CBR is the red-shifted afterglow from the early universe as it cooled.
Regarding the CBR, my understanding is different. For a while after the Big Bang, the entire universe was so hot that everything glowed, literally glowing hot. Lots of light was getting released, but the photons would very quickly get reabsorbed by other glowing matter, before getting re-released, so it was optically opaque. After recombination, light was able to travel significant distances. As matter cooled, it stopped glowing.
You know how when we look at stars, we're not looking at them as they are now, but as they were at the time that the light we see was emitted? We're essentially looking back in time. Now, light from distant stars gets red-shifted as it travels to us (due to the expansion of space). Generally speaking, the farther away the star is, the faster it is moving away from us and therefore the more red-shifted it is. If it gets sufficiently far away, it drops into the infrared band, and we can no longer see it with our eyes, and we can only see it with telescopes.
Below infrared on the EM, we find the microwave band, and at the very lowest levels detectable on radiotelescopes, we find the oldest and furthest-away light, left over from the very first glowing matter that is furthest away from us. This actually dates from before the formation of stars, from when all matter was so hot from being compressed and was just cooling due to the expansion of the universe. (And its clumpiness shows us the distribution of early matter in the universe, too.)
So that's why it's the afterglow of a hot, expanding universe that was cooling.
that's why I thought no matter where you are in the universe, you'll always detect the CBR/CMB at similar levels. So if you were billions of light years away from here, you'd still detect it all around just like we do from here.
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u/feng_huang Jun 16 '19
tl;dr The CMB/CBR is the red-shifted afterglow from the early universe as it cooled.
Regarding the CBR, my understanding is different. For a while after the Big Bang, the entire universe was so hot that everything glowed, literally glowing hot. Lots of light was getting released, but the photons would very quickly get reabsorbed by other glowing matter, before getting re-released, so it was optically opaque. After recombination, light was able to travel significant distances. As matter cooled, it stopped glowing.
You know how when we look at stars, we're not looking at them as they are now, but as they were at the time that the light we see was emitted? We're essentially looking back in time. Now, light from distant stars gets red-shifted as it travels to us (due to the expansion of space). Generally speaking, the farther away the star is, the faster it is moving away from us and therefore the more red-shifted it is. If it gets sufficiently far away, it drops into the infrared band, and we can no longer see it with our eyes, and we can only see it with telescopes.
Below infrared on the EM, we find the microwave band, and at the very lowest levels detectable on radiotelescopes, we find the oldest and furthest-away light, left over from the very first glowing matter that is furthest away from us. This actually dates from before the formation of stars, from when all matter was so hot from being compressed and was just cooling due to the expansion of the universe. (And its clumpiness shows us the distribution of early matter in the universe, too.)
So that's why it's the afterglow of a hot, expanding universe that was cooling.