Whenever cosmic background radiation is mentioned on the internet, it is accompanied by this image. What exactly am I looking at? Is this a map of the entire universe at some point in time? Is this radiation still everywhere or only did it only exist 13 billion years ago?
First of all, what you’re looking at there is the surface of a sphere. Mapping the sky is subject to all the same problems as mapping the Earth, in terms of converting a sphere to a flat image.
What you’re seeing there is the entire sky, as seen from our location, in microwaves. The reason such an image is of interest to cosmology is because the significant majority of what we see in it is of cosmological origin. The very early Universe was very hot, so much so that all of the matter in it (mostly hydrogen) was ionized. Ionized gases are opaque to light, so all of the light that was around in the early Universe would travel only a very short distance before being absorbed by some charged particle and re-emitted in a different direction. After a few hundred thousand years, though, the Universe cooled off enough that this plasma was able to combine into a gas of neutral hydrogen atoms. Neutral gases are far more transparent, so all of a sudden, all of this light that was bouncing around every which way was free to travel very long distances. Most of it will never again interact with any matter. Some small portion of it, though, happens to be heading for us, and some even happens to hit our instruments and be detected.
So when we’re looking at an image like that, what we’re seeing is a sphere, tens of billions of lightyears in radius, with us at the center. This sphere is not special by virtue of its place, but of its time: It’s the sphere that’s far enough away that it corresponds to the time when the Universe became transparent. This is, for practical purposes, the earliest time in the Universe that we can detect using light.
It isn’t everywhere (you aren’t emitting any, for example) but it is coming at us from every direction. Scientists predicted it long before it was found, and it was actually found kinda by mistake. As engineers tried to make better and better radio receivers they got to a point where they just couldn’t get rid of the noise. It was almost like radio noise was coming at them from every direction from the universe itself. What they had done was accidentally confirm the scientific theories.
If you could convert the microwave radio waves to visible light so that you could see them, and then flattened out the entire sky onto a flat piece of paper, you would end up with the image map that you linked to.
Another way to think about it:
Because the speed of light is finite, when we look at distant objects we’re looking back in time. If we look at a star that’s 10 light years away, we’re seeing it as it was 10 years ago.
As Chronos said, when the universe was young it was filled with hot ionized gas. This gas was opaque to light. But as it cooled, it became transparent.
So the most distant thing we can see in all directions is a wall of “hot fog”. The fog is now long gone. It dissipated billions of years ago. But we can still see it because the light that it emitted is just now arriving.
Well it would probably look much smoother (depending of course on how your eye/camera/detector is set up).
The difference in intensity that the light and dark areas in the image represent, is actually only something like 1 part in 100,000 IIRC.
Missed edit: my last sentence was a bit garbled. What I mean is, if the darkest areas of the image represent a microwave intensity of 1 unit, then the lightest areas represent an intensity of 1.00001 units (not sure what the units are).
Also, they subtracted out the biggest variation, that caused by the motion of the Earth relative to the average rest frame of the emitters that produced the cosmic background radiation. Without that subtraction, we’d see a hot spot in the direction that the Earth is moving and a cold spot on the other side. Once that is subtracted out, the remainder looks random; but careful statistical analysis shows angular correlations that give a wealth of information about cosmology.
While we’re on this question, can someone explain to me a point that I’m not understanding?
I don’t understand why we can see any of this early-universe radiation. My confusion is this: If this radiation originated relatively early in the formation of the universe, why isn’t it all, today, mostly to be found only around the outer edges of the expanding universe-sphere, heading outwards away from us (and away from everything else that was formed later than the CBR itself)?
That is, imagine that a bright star out there somewhere, 10 LY away, suddenly blinks. 10 years later, the light (or lack thereof) from that blink passes by Earth, and we see it. Now, it has passed us and we will never see it again.
Similarly with the CBR, why isn’t is all out “ahead” of the Milky Way galaxy, heading away from us, and why hasn’t it been so since long before the Milky Way even began to form? Why is there some of this CBR still “behind” us, heading towards us? And why do we see it coming towards us from all directions?
Alright. So, where is there a cool 3d-rendered image of this flattened sphere that I can rotate and look at and stuff?
Also, I think it’s neat that 6 light years away, someone is looking at us right now (or us 60,000 years ago) and possibly asking this question to their TM…
I love this science shit! 
Well, this may be a WAG (Considering I just used the term “science shit” a moment ago), but time is time. It’s constantly happening. We may have missed one wink from a star, but that doesn’t mean there’s not more. a lot more. Some bits have always moved by, but there’s always more (Until a light goes out forever).
Another WAG, but if we’re not on the outer edge (That issue itself (An actual edge of the universe) is discussed here, which we’re not, then of course that light is coming from all directions, no?
Well, the big bang basically happened everywhere, so the universe today is completely filled with its relic radiation, since for every spacetime point, there’s another point such that its radiation just now arrives. So the equivalent in your analogy would roughly be that there’s a star at every point in space, all of which suddenly blink. So at any given moment, we’re receiving microwave radiation from points that are further away.
I found this image which gives a cone-like look at it all…
…and I just found this animation
Half Man, Half Wit is correct but let me expand on his beginning. (physics joke)
Have you read the threads that cite physicists saying there is no center to the universe and no edge, just that every single point appears to itself to be the center of the universe and sees an isotropic universe on all sides? That’s really saying the same thing.
The small-sized universe that stopped being opaque 380,000 years after the big bang was all that is and will ever be. (Ignore all multiverses for this, since they’re irrelevant even if any flavor of them exists.) The radiation that flew around at that point didn’t go outward. There is no outside to the universe. By definition, all that radiation stayed inside and sprayed out in a 360º sphere from every single point constantly. The spot in space that we exist in was one of those points. Draw the lines to connect the dots and, again by definition, every one connects to every other one. We - the earth and the surrounding space - received radiation from every other point.
If nothing changed than the universe would still be a formless gas of radiation tsunami. But space itself was expanding at that time. The radiation had longer distances to travel before it hit another point and interacted. It was still spraying out in all directions but the receiving points didn’t see it until much later; call it a time lag.
Today, 13.8 billion years later, that time lag means we are just starting to see some of that radiation. It still comes from everywhere. The wavelengths have shifted into the microwave band but that’s just a detail. Every angle we can possibly point a telescope looks to the same background of spraying radiation. This is also true for every other point in the universe. The radiation hasn’t been directed outside, because there is no outside: every point still considers itself the center of a sphere.
The universe today is exactly the same universe that it was 13.8 billion years ago, just spread out and with the matter clumped. We’ll still on the inside. When we look out we see (within the limits of our viewing apparatus) all of it. There’s nowhere else for it to have gone.
To clarify this, we’ve always been seeing some cosmic background radiation or other. Some of it we’re just starting to see now, some of it we saw last year, some of it we saw ten billion years ago.
Not disagreeing but clarifying something that helped put a lot of things together for me here on the dope a year or two ago, and which I think can help with the question of why we still see cosmic background radiation everywhere today:
It turns out the consensus is that the universe was never smaller in the ordinary sense–just denser. It was always infinite, and there was always matter all over the place in it.
Based on the angular scale of fluctuations seen in the second picture in this article, there would be subtle differences between two locations 0.1 degrees apart, and almost no correlation for locations more than about 1 degree or so apart. Assuming the same period of variation radially as circumferentially*, the CMB picture would be subtly different 25 million years in the future. Only the largest structures would still be unchanged after 250 million years or so, when the surface of last scattering is another 250 MYrs farther away. The mottling would look completely different (although it would have the same general character).
I think Chronos needs to answer this, but from my understanding:
It’s true that if the universe is infinite today, it would always have been infinite but that is not at all a settled issue.
Even if the universe were always infinite, the matter we see today would have been in as small a place because the speed of light constrains what part of the universe we see.
For the purposes of seeing the background radiation, therefore, the issue of whether the universe is infinite or not is irrelevant. My personal opinion is that adding an infinite universe to the picture makes it harder to grasp, but this is very much a whatever works for you situation.
Whether or not the universe is infinite makes a difference if you are going start adding scalar functions to explain the differences between density and perturbations in density.
Is the universe all the stuff, or does all the stuff exist within something infinite we call the universe?
This is all correct.
