1

This is probably a very dumb question for this forum, if so I'm sorry; but from what I know it does not make sense that the CMB is still visible and measurable today.

From what I understand, the CMB dates back to a time which has been calculated at being 380,000 years after the big bang, time at which photons in the primordial "particle soup" were able to escape.

But then photons move, unrestricted, at the speed of light (save for space time curvatures and collisions) so I would expect that this batch of photons which were able to escape would be long gone by now... Yet COBE, WMAP and Planck can still trace the origins back to the CMB. And save for the precision of measurements, due to the progress of technology, they pretty much all give the same picture.

I am at a loss. Especially since between COBE and WMAP the proof that the expansion of the universe is accelerating over time has been established.

I must be missing something fundamental here; why is it that this "batch of photons" is still visible today? Or does that mean that the CMB still has a physical existence somehow?

fge
  • 119
  • Hint: where would the photons escape to? – Rococo Aug 20 '16 at 18:19
  • @Rococo argh... OK, I'm lost. There was no spacetime to escape to. But... Then what? I must be really stupid but I can't figure out what happened :( – fge Aug 20 '16 at 18:20
  • @fge You're not stupid, your intuition is just trained for cases in which we have a localized light source. Whereas in this case, the entire universe (which you can suppose is infinite in size) is the light source. – Rococo Aug 20 '16 at 18:26
  • Also: How can we detect cosmic background radiation? – John Rennie Aug 20 '16 at 18:31
  • @Rococo indeed I may have been confused; the source of this light is not localized; my question then is why such sources are not exhausted today and are still detectable. If you "detect" a photon, it is "destroyed", right? Or at least "transformed"; but it is no longer the original photon. So my question would then be how come there are still such photons from that time... I'll read the answers JohnRennie has linked to. I want to understand :( – fge Aug 20 '16 at 18:37
  • @JohnRennie will read; as mentioned right above I do want to understand – fge Aug 20 '16 at 18:37
  • @fge. Both links from Rennie have great answers. The most graphical and maybe easiest to understand intuitively is the one by Pela in the first link, but the answers by Anna and Rennie provide the physical rationale. You commented on Anna's answer then, just look at both. The redshift is about 1100 for the CMB, means the universe was about 1100 times smaller, and that CMB had to cover that distance to use from where it was emitted. Not too different from how we could detect light from a galaxy formed a billion years after the BB - it had a distance from us, and took 12.6 years to get to us. – Bob Bee Aug 20 '16 at 20:44

0 Answers0