Why does the CMB appear to us the way it does?

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In summary, the Cosmic Microwave Background (CMB) appears to us the way it does due to its origin during the Big Bang and the subsequent expansion of the universe. This radiation is the oldest light in the universe, and its characteristics, such as its uniformity and specific temperature, provide evidence for the Big Bang theory. The CMB also helps us understand the large-scale structure of the universe and the cosmic microwave background anisotropy gives insight into the distribution of matter and energy in the early universe. Overall, the CMB is a crucial component in our understanding of the universe and its origins.
  • #1
GreatBigBore
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Seems like the universe was opaque for 380k years, and then suddenly nuclei capture electrons and there's an almighty flash, in all directions. By this time the universe is what, a million light-years in diameter, or less? Seems like the light from the last scattering surface would have gone by us already. Why can we still see it as though the surface were sitting there like a star 13.4B years ago? Is there a "Cosmic Microwave Background for Dummies" book or link?
 
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  • #3
About the million light-years thing, please note my final question and infer from that how much I probably know about cosmology. My main point was just a guess that the universe was small enough that light from the CMB would have crossed all the way from one end of the universe to the other by now, given almost 14B years.

The Wright model I found is confusing to me because the photons don't move. I've never heard anyone say that the CMB photons don't move. So I'm still trying to figure out why we still see photons from the CMB, if it was just a single flash. Why wouldn't all of the photons from that flash have long since gone past us?
 
  • #4
Nop, watch carefully, photons are moving. Note that that simulation corresponds to a closed universe.

It is wrong to picture Big Bang as explosion in pre-existing space, from which stuff races outward. Whatever was size of universe at that time, it was uniformly filled with hot and dense material. Imagine such universe evolving further. At some point it becomes transparent, and photons can freely move. Now, wherever photons go, there will be some galaxies, and maybe someone to talk about them. Since our universe is homogeneous and isotropic, and was even more homogeneous and isotropic (at smaller scale) at times of Big Bang, density of CMB photons is same everywhere. Photons we are today registering as CMB, originated far, far away.
 
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  • #5
Your idea of a particularly small universe is not correct; for the time being, try to think of the universe as either infinite or very large to the point where the observable universe which we CAN see is very small in comparison. This should remove the problem of "why are the photons not running out if they are still coming towards us?" - because there are always photons behind the ones we see right now, waiting to be seen in the near future!
 
  • #6
MikeyW said:
Your idea of a particularly small universe is not correct; for the time being, try to think of the universe as either infinite or very large to the point where the observable universe which we CAN see is very small in comparison. This should remove the problem of "why are the photons not running out if they are still coming towards us?" - because there are always photons behind the ones we see right now, waiting to be seen in the near future!

Size is not that important (most common lie you will here from women :wink:). If you follow cosmological principle (assuming that it was true from Big Bang), you can't run out of CMB photons (neglecting things like cosmic horizon, to keep it simpler).

To GBB: watch Ballon model here: http://www.astro.ucla.edu/~wright/Balloon2.html"
 
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1. Why is the CMB uniform in all directions?

The CMB (cosmic microwave background) appears uniform in all directions because it is the result of the Big Bang, which was a rapid and uniform expansion of the universe. This initial expansion created a uniform distribution of energy that later cooled into the CMB.

2. What causes the fluctuations in the CMB?

The fluctuations in the CMB are caused by tiny variations in the density of matter in the early universe. These variations were amplified by the expansion of the universe and eventually became the large-scale structures we see today, such as galaxies and galaxy clusters.

3. Why is the CMB radiation in the microwave range?

The CMB radiation is in the microwave range because it has been stretched and cooled by the expansion of the universe over billions of years. This stretching has shifted the radiation from its original high-energy state to the lower-energy microwave range.

4. How does the CMB help us understand the origin of the universe?

The CMB provides evidence for the Big Bang theory, which is currently the most widely accepted explanation for the origin of the universe. The uniformity and fluctuations in the CMB support the idea of a rapid expansion from a hot and dense state, as predicted by the Big Bang theory.

5. Can we see the CMB with our naked eye?

No, we cannot see the CMB with our naked eye because it is in the microwave range, which is outside the range of visible light. However, specialized instruments and telescopes can detect and map the CMB radiation, allowing us to study its properties and gain insights into the early universe.

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