How we are still able to detect CMB photons?

[CuriousNilave]

This might be a silly question. But recently I got to know about CMB(cosmic microwave background) and this question is nagging me. During CMB , the universe was pretty small compared to now, so our current point of existence in space would have been very near the CMB, 13.7 bill yrs ago. So the photons from the CMB would have passed our point very early itself. Why is it passing our point now? Did the space between our point and CMB points increase faster than speed of light and because of this we are able to see CMB photons now?

 

[Orodruin]

The CMB was released everywhere in the Universe. What we see now is simply microwaves from the part of the Universe that is just far away enough for the light from the CMB to take us 13.7 billion years to reach us. At the time of release, this was much closer to us than 13.7 billion light years, but it has taken so long because while the light has been progressing, the distance has grown due to expansion. The distance to those parts of the Universe today is significantly larger than 13.7 billion light years, which makes sense as part of the journey happened when the distance was smaller.

 

[CuriousNilave]

Thanks Orodruin. So did the expansion of space happened faster than speed of light during CMB period?

 

[Orodruin]

So did the expansion of space happened faster than speed of light during CMB period?

First of all, expansion happens at a rate, not at a velocity. This does mean that some distances will grow faster than the speed of light but this is not due to anything actually moving in relation to something else. Even today distances will grow faster than the speed of light if you go far away enough.

Second, I suggest you read this FAQ: https://www.physicsforums.com/threads/at-what-velocity-does-the-universe-expand-can-it-be-faster-than-light.508610/

 

[CuriousNilave]

Thanks a lot.. This is very helpful!