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So I just learned that CMB redshift is 1100 regardless where we look (up down left right). According to Hubble's Law that makes it around 46 billion light years away, making it the farthest matter away from us. If CMB redshift is the same regardless which direction we look, then how could that be unless we're in the center of the universe?

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  • $\begingroup$ Everyone knows, that Unimatrix 01 is the center of the universe. Except you, it seems. $\endgroup$ – ott-- Apr 19 '16 at 20:33
  • $\begingroup$ Since the most up to date part of the universe you can see is literally at the tip of your nose, not only are we the center of the universe, but you personally are at its exact center. Within a Planck's Unit of Length, of course. $\endgroup$ – Howard Miller Apr 21 '16 at 15:13
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At the time when the universe was ~380,000 years old it was completely filled with a plasma and light. Light could only travel very short distances (I forget the specifics, but think a couple meters at the most) before being absorbed. So the entire universe was a fog, if you were there and there was an object 10 meters in front of you, you couldn't see it because all the light it emitted or reflected would be reabsorbed before it reaches your eyes. There are two things to keep in mind: 1) This fog disappeared across the entire universe simultaneously (plus or minus some time, but effectively simultaneously); at the same point in time after the Big Bang, 380,000 years after the Big Bang, the fog dissipated due to recombination. And 2) Light takes time to travel.

So let's say you are sitting in the fog-filled universe a few seconds before recombination happens. Light travels about 1 foot per nanosecond. So if the fog disappeared and there was a street lamp 10 feet in front of you, you wouldn't receive the first light from that lamp for 10 nano seconds. All of the light the lamp was emitting before recombination was just reabsorbed, but after recombination the first photons to leave the lamp no longer have anything to run into, so they continue on until hitting your eyes. A street lamp 20 feet away wouldn't be visible for 20 nanoseconds after recombination. The light of the plasma itself is effectively a street lamp at every location in space, so due to the 2 facts above, you would see successively farther street lamps in a shell centered on you.

After recombination happened and the entire universe became transparent. The fog is gone, but the farther you look the farther back in time you look, so you can only see back as far as when the fog existed, and the farthest you can see is the fog itself. After 13.8 billion years you can see all the way out to a shell of radius 13.8 billion light years around you (ignore expansion of the universe for now), and what you see is the light of the plasma (the fog) that was filling the universe 13.8 billion years ago, the CMB.

This scenario is the same if the universe is infinite in extent. We are not the center of the universe, we are simply the center of the observable universe, as is anyone else. Aliens looking towards Earth but are at the current location of edge of the CMB in some direction will not see Earth, they will see the last light of the CMB itself at our location when it was emitted 13.8 billion years ago; we are the edge of their observable universe.

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  • $\begingroup$ Thanks for the details. So if I understand you correctly, what we're seeing is not the entire universe, but the maximum distance due to plasma in the early universe. Now it makes me wonder if we are the center of all obserable galaxies, but that's another question, I guess. $\endgroup$ – Paul Apr 19 '16 at 19:12
  • $\begingroup$ Yes that is correct. Really there should be a 3rd point in my post: 3) The fog is extremely uniform everywhere. The plasma essentially made up the material that would later coalesce into galaxies. Yes, we are the center of all observable galaxies because the universe happens to be extremely uniform on large scales, so on large scales galaxies form at regular spatial intervals. $\endgroup$ – Quantic Apr 19 '16 at 19:29
  • $\begingroup$ Interesting. Something confuses me. The farthest oobseved galaxy is 13.4 billion years ago with a redshift of 11, but yet CMB, not much older, has a redshift of 1100. How could we see such an old galaxy far in the past without it being almost as far away as the CMB? $\endgroup$ – Paul Apr 19 '16 at 23:46
  • $\begingroup$ @Paul Well that's because I made a mistake, heh. Recombination happened about 400k years after the big bang, and the big bang was ~13.8 billion years ago, so recombination happened 13.799600 billion years ago, not 13.4 like I wrote (oops). The galaxy you mention was found near the end of the Dark Ages. I'm sure it pushes the specifics back a little bit, but it's still well after recombination and the Universe cooled enough and dark matter pulled in enough matter (in certain regions) to allow stars to form. Good picture here. $\endgroup$ – Quantic Apr 21 '16 at 14:29
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We're at the centre of the observable universe - but this is because everything (when observed over very long distances) is expanding away from everything else, so we are at the centre of what we observe.

The spherical shape of the observable universe is really an optical illusion, in conjunction with the large-scale expansion of the universe. We simply can't see anything that is apparently receding at (close to) light speed, so everything beyond a certain range is but off from our view.

The actual universe - well that's probably much bigger than the observable universe. What shape it is, and what conditions are like in other regions, we can only guess at right now.

(I have to lie down now, my head is beginning to spin.)

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  • $\begingroup$ The CMB being visible in every direction at the same distance is independent of the uniform expansion of the universe. Even in a completely static universe we would see the CMB in a shell around us at the same distance. The (uniform) expansion of the universe just affects the distance the shell is at, and the wavelength of the photons we receive. See my answer for more info. $\endgroup$ – Quantic Apr 19 '16 at 17:00
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In a way you could argue that we are the centre of the universe. From our stand point that is. As a matter of fact every other standpoint has the right to argue that they are the centre of the universe because the redshift is always the same regardless where in the universe you are.

So imagine observing the redshift of a different galaxy. Imagine there is a galaxy behind the first one at exactly twice the distance from earth. Now from earth this galaxy would appear to be more redshifted (I know the galaxy in the foreground obstructs the view but just in theory).

          (      far galaxy   (redder)
           )



           (     close galaxy (red)
            )




            O      Earth (observer)

If you lived in the closer galaxy, you would observe earth redshifted by the same amount as an observer on earth would see you redshifted. This seems logical. The interesting thing however is that the far away galaxy will not be redshifted the same amount as viewed from earth. In fact, the redshift of earth and the second galaxy will be exactly identical when viewed from the galaxy between them.

          (      far galaxy   (red)
           )



           (     close galaxy (observer)
            )




            O      Earth (red)

This is because all points in the universe are moving away from each other and not just all points moving away from one point.

You may have heard this analogy before but if you draw spots on a balloon and blow it up, they behave exactly like galaxies moving away from each other. No matter which point you observe all points seem to be moving away from it.

So while this may not be entirely accurate in cosmological terms you can imagine the galaxies to be the spots on the balloon and the balloon to be the fabric of space.

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  • $\begingroup$ The CMB being visible in every direction at the same distance is independent of the uniform expansion of the universe. Even in a completely static universe we would see the CMB in a shell around us at the same distance. The (uniform) expansion of the universe just affects the distance the shell is at, and the wavelength of the photons we receive. See my answer for more info. $\endgroup$ – Quantic Apr 19 '16 at 17:00
  • $\begingroup$ true you may be correct in that this analogy is somewhat flawed $\endgroup$ – Jaywalker Apr 19 '16 at 17:20

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