The CMBR is, in essence, the echo of The Big Bang…or more accurately, the flash of light emitted when the very young universe stopped being opaque and instead transparent to photons.
Since, due to farther objects moving faster away from us, the frequency/wavelength is red-shifted.
What was the radiation’s true frequency/wavelength prior to redshift?
Also, can this background be thought of as an “edge” to the universe?
The farther out you look, the farther back in time that object is seen, so is the CMBR something that is at a distance <, =, or > the Hubble Radius…the distance where objects recede away from us at lightspeed?
Or…is the CMBR not something at the edge of space, but something filling the entirety of the volume of the universe uniformly (more or less)?
I can’t answer those questions, exactly… But I can make things even more complicated by mentioning “Expansion.” Per the “Expansionary Phase” enhancement of the Big Bang theory, space may have spread out VERY fast (much faster than the speed of light) for a brief period, early in the new universe’s history. The cool thing about this is that it explains a lot of things very nicely – why the CBR is so smooth and even; why we seem to live in a universe dominated by matter instead of anti-matter; a few other things.
In this idea, we can’t see anywhere close to the true cosmic horizon. We can only see a tiny, limited bit of space. Other chunks of space, still in our own universe, are forever isolated from us, having expanded away so far, light from those stars simply won’t ever reach us. It’s even vaguely possible that some of the physical constants of the universe are slightly different in those regions than they are here – speculative, but worth considering.
Anyway, with or without the Expansionary Phase, the CBR isn’t the “edge of the universe,” but is a kind of noise which totally pervades the universe. It isn’t microwaves coming from somewhere else; it’s microwaves coming from everywhere. The CBR microwaves are coming from here too. i.e., someone very far away from us would detect those microwaves coming from our direction.
Dunno if I’ve helped or not…but it’s a lovely, lovely subject. I wish Isaac Asimov were still alive and writing science essays; that man could make anything clear!
Note; it is true that the CMBR comes from everywhere, including here; but it does not come from everywhen, but rather it was emitted at a specific time, 13.7 billion years ago. This event was the point at which radiation was first able to freely travel through the universe; the Unverse became transparent to light (before that time it was too dense for radiation to travel freely). At that time the temperature was about 3000K, the temperature of a red dwarf star or an incandescent lightbulb. (Most of this information is included in that wikipedia page if you look closely).
Note as well that the place which emitted that radiation was (at that time) relatively close to our location (or rather the place where we would be, 13.7 billion years later); the photons were emitted at a distance of only 42 million light years from us, but they have taken 13.7 billion years to get here, simply because the intervening space has expanded in the meantime. Imagine an ant crawling across an expanding rubber sheet at a constant speed; all the while the distance is getting larger, but the ant keeps on crawling until it gets to its destination.
To confuse matters still further, we could perhaps consider the status of the location from which those photons were originally emitted; when the universe became transparent, that location was 42 million light years away. Because of expansion that location is now about 46 billion light years away, and receding much faster than light.
It didn’t, and doesn’t now, have a single frequency. It was and is a blackbody spectrum. Indeed the spectral distribution is the best fit to a theoretically perfect blackbody radiator ever measured. A black body spectra does have a peak, but it isn’t sharp. There is lots of energy at wavelengths quite some distance away (just as the sun has no easily definable colour.) So the question is better rephrased to be “what is/was the temperature?” Right now it is 2.7255 K. The universe became transparent at about 3000K.