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I understand that we can never see much farther than the farthest galaxies we have observed. This is because, before the first galaxies formed, the universe was opaque--it was a soup of subatomic particles that scattered all light. But before the universe was opaque, the Big Bang happened, which is where the cosmic microwave background (CMB) comes from.

If the opaque early universe scattered all light, and the first few galaxies are as far back as we can see, why is the CMB observable? Where is it coming from?

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This is kind of a tangent, but it's also worth pointing out that the first galaxies formed long after recombination (when the universe became transparent to electromagnetic radiation). Recombination was at around 380,000 years; stars and galaxies didn't start forming until a few hundred million years later. See for example Timeline of the big bang on wikipedia. –  Jefromi Jun 2 '11 at 4:07
@Jefromi thanks for the link. I knew that stars and galaxies didn't just pop into existence but I only had a rough idea of the timescales so I may have oversimplified. –  Carson Myers Jun 2 '11 at 4:15

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The cosmic microwave background does not originate with the big bang itself. It originates roughly 380,000 years after the big bang, when the temperature dropped far enough to allow electrons and protons to form atoms. When it was released, the cosmic microwave background wasn't microwave at all- the photons had higher energies. Since that time, they have been redshifted due to the expansion of the universe, and are presently in the microwave band.

The universe is opaque from 380,000 years and earlier. The galaxies that we can see only formed after that time. Before that, all that is observable is the CMB.

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Oh! Fantastic, I had always thought the CMB was from the big bang itself. –  Carson Myers Jun 2 '11 at 0:59
@Carson: Well, it certainly wouldn't be there if the universe hadn't started out quite energetically. Thinking of it as an afterglow is a decent analogy. –  Jefromi Jun 2 '11 at 3:31
That's not explaining why we still can see it. If the CMB was emitted in a universe of age 380ky, then that universe cannot have been very large, say, at most twice that size in diameter (760kly). According to GR each observer can be taken as a point of reference and for each is valid that he should have seen the last of that CMB at most 760ky after that event. Where's my mistake? –  Alfe Dec 5 '13 at 11:09
@Alfe: There are many things that we don't know for certain, however the universe's size does not correspond directly to its age for various reasons (for example, inflation). In addition, the universe does not have a "center" from which the CMBR began propagating - it began propagating from roughly every point in the universe as the temperatures cooled. So we will never "see the last of" the CMBR, we will merely see it cool and become more sparse. –  voithos Dec 5 '13 at 21:26

The CMB we see is the state of the universe when it became transparent, 380000 years after the Big Bang. Our lines of sight away from us can't see any farther than this. Think of the Sun, which is a big ball of gas, but appears to have a surface because that is where the accumulated gas along the line of sight to the center has become opaque.

The universe became transparent after 380000 years because the cooling caused by its expansion allowed ionized gas to recombine into neutral hydrogen and helium, which is transparent to visible light. At this time, gravity could begin to work on the slight density variations causing the denser parts to slowly become more and more dense until nuclear reactions would start at the densest core, marking the beginning of star formation. But because the universe was still hot and the initial density variations very slight, it still took millions of years for the first stars to form.

While in the neutral state, the universe was transparent to visible light but opaque in the far ultraviolet due to absorption from the electrons around the neutral hydrogen atoms. When we try to see back to this time with Hubble and other optical telescopes, the very high redshift has moved the far ultraviolet through the visible spectrum and into the infrared, making most of the neutral period invisible to us. The James Webb Space Telescope (JWST), will have detectors that work far enough into the infrared that we can see what happens during the neutral time. After hundreds of millions of years, the ultraviolet light from hot stars reionized the universe making it once more transparent to the ultraviolet (and now transparent in the visible as well owing to the low density after hundreds of millions years of expansion).

The idea that we see the actual moments of the Big Bang when we look at the CMB arises because the density variations could not change during the first 380000 years, so those we see in the CMB were there from the first instants of the Big Bang and represent quantum-mechanical fluctuations present at the very start.

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