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JtS88 t1_j2k1squ wrote

Reply to comment by Aseyhe in Is any "movement" visible in the fluctuations of the CMB over time, or does it appear static? by JarasM

How simultaneously did the universe go from opaque to transparent? I'd imagine that the different CMB temperatures correspond to different densities and therefore different temperatures, so is "at the same time" just more or less the same time on a cosmic scale?

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Aseyhe t1_j2k4jp5 wrote

According to this paper, the last scattering surface has a comoving thickness of about 19 Mpc, which corresponds to a physical thickness of (19 Mpc)/1100 ~ 17 kpc or a duration of (17 kpc)/c ~ 56000 years.

Edit: The above concerns how thick the last scattering surface is at any given point on the sky (which is connected to how long recombination -- the process by which the universe became transparent -- took, as well as how opaque the universe was before recombination). I just realized that you are instead asking how the recombination time varied between different patches of the sky. Temperature variations in the CMB are around the 10^-4 level (one part in ten thousand), which implies that the recombination time varied to a similar degree. 10^(-4) of 370000 years is 37 years, so the spatial variation in the recombination time is of order tens of years.

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Magnergy t1_j2krhfu wrote

Tens of years... Wow, so short. Really puts the temperature uniformity in a new perspective for me.

Are this and this still about right for how we think it would have looked?

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Aseyhe t1_j2kswjm wrote

Sorry, I typed "reionization" but meant "recombination"... I've fixed that.

To clear things up:

  • Recombination is a process that occurred at a time of around 370000 years. At this time, the universe cooled enough that all of the free protons and electrons condensed into neutral hydrogen. Without all of the free electric charges, the universe became transparent. (The "re" in "recombination" is a complete misnomer.)

  • Reionization is a process that occurred at a time of around 200 million to 1 billion years. This is what those videos are showing. When the first galaxies formed, the light emitted by their stars and black hole accretion disks ionized essentially all of the neutral hydrogen in the universe. (The universe didn't become opaque again, though, just because the hydrogen was far too sparse by this time.)

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mpinnegar t1_j2lhckc wrote

One of the reasons we know the universe had to have been much smaller and closer together in the past is that to have that uniform temperature over such a large scale (the entire cmb) those parts needed to be close together at some point to "coordinate" on what temperature they should all be.

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mfb- t1_j2mffhy wrote

Tens of years shift in a process that took tens of thousands of years.

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ghandi3737 t1_j2km2ce wrote

Mpc? Kpc?

Not an abbreviation I'm familiar with.

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JtS88 t1_j2k3065 wrote

Right, found this myself after a quick Google search, correct me if I'm wrong. It's not the density or temperature of the plasma that really affect transparency, rather, it's the fact that this plasma is opaque only because there exist photons of high enough energy to rip apart nuclei and electrons again (the electrons can then scatter incoming photons galore). As the expansion of the universe redshifts the photons, at a certain point there will be no photons of sufficient energy to ionise atoms, hence simultaneity.

Follow up question: assuming we're talking about a Boltzmann distribution, at what timescales do we expect to go from e.g. 1% to 99% of photons falling below ionisation energy?

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