I was reading an article on Olbers' Paradox (why the universe isn't as bright as the sun) and the more I read on it, the more the same question came to mind...

We know the observable universe is approximately 93G LY across and we know the age of the universe is 13.8 B years. We also know that there are galaxies moving away from us faster than the speed of light due to ever-increasing expansion.

If we cannot see these galaxies due to the redshift of their light... how is it that we can see the CMB which is behind them (to our perspective)? Shouldn't it be moving even faster away from us?

If we can see it, why can we not see the galaxies furthest from us?

  • 4
    $\begingroup$ The CMB is not at a particular distance, it is everywhere in the universe. $\endgroup$ – Slereah Mar 21 at 12:48
  • $\begingroup$ We also know that there are galaxies moving away from us faster than the speed of light due to ever-increasing expansion. Not true. See physics.stackexchange.com/questions/13388/… . $\endgroup$ – Ben Crowell May 28 at 16:25
  • $\begingroup$ Just to elaborate on @BenCrowell's objection: There are indeed galaxies receding at $v>c$ (and we easily observe them), but the reason is just expansion, not the "ever-increasing" expansion. $\endgroup$ – pela May 28 at 18:32
  • $\begingroup$ @pela - Correct me if I am wrong, but haven't astronomers proved that the speed of the universe's expansion is increasing? $\endgroup$ – Rick May 28 at 19:02
  • $\begingroup$ @Rick Yes, that's true (though I don't like the word "prove" in physics). What I meant to say was the increasing expansion rate is not the reason that some galaxies recede at v>c. Even in a hypothetical universe where the expansion rate were constant or even decreasing, as long as it is expanding, you'd have galaxies receding at v>c. The reason is that (assuming expansion is the same everywhere) recession velocity is proportional to distance, so for large enough distances, namely $d>c/H_0$, you'll have v>c. In our universe that distance happens to be ~14.4 Glyr (at redshift z ~ 1.47). $\endgroup$ – pela May 28 at 20:22

If we can see a particular object at some moment, then we will always be able to see that object in the future, no matter how quickly the universe expands. It's analogous to how we can never see anything completely fall into a black hole. In both cases, the image you get just gets more and more redshifted over time. This is the point made in a comment: we could certainly see the CMB at some point, since it occupies the entire universe, so we will always be able to see it. It just continues to get more and more redshifted; it's already been redshifted by a factor of $1000$.

Here's another more direct argument. There are galaxies we can never see, because they form and begin emitting light when already outside of our horizon. But the CMB photons we are currently seeing have an enormous head start because they have been traveling towards us at the speed of light for billions of years, and furthermore doing so in an era when the universe was expanding relatively slowly. So it is certainly possible to be able to see these photons, but not ever see the photons from a newer galaxy that forms closer to us than the relevant CMB photons did.


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