How can radiation from the Big Bang come from all directions?

[bapowell]

By now, the walls are (apparently) 13.7 billion light-years away from us.

They are closer to 46 billion light years away.

 

[nikkkom]

They are closer to 46 billion light years away.

I meant that CMB light we detect today traveled 13.7 billion years. I don't want to get into the discussion of many more different ways to express the distance to the surface of last scattering.

 

[bapowell]

I meant that CMB light we detect today traveled 13.7 billion years. I don't want to get into the discussion of many more different ways to express the distance to the surface of last scattering.

That's fine. Just pointing out that what you wrote is incorrect, lest it lead to confusion. Because of the expansion, the distance traveled by CMB photons is longer than just time multiplied by the speed of light.

 

[Ophiolite]

Surely that point is covered by nikkom's paranthetic use of the word "apparently"? I actually found your clarification muddied the issue for me, since the use of "apparently" seems to make his statement correct. Now I don't understand why you would consider it wrong. In what way does the CMB not appear to be 13.7 billion years away?

 

[Jorrie]

In what way does the CMB not appear to be 13.7 billion years away?

As bapowell said, you cannot multiply light travel time by c and hope to get any cosmological distance that makes sense. Some popular writings try to call it "light travel distance", but even that's wrong. The 13.7 billion years is just light travel time and that's that.

When the light left the region of the observed CMB, it was only 42 million light years (proper distance) from the region where we find ourselves in. By now that distance has expanded to over 45 billion light years. Yes, there are various other ways to express cosmological distances, but none of them gives you 13.7 billion ly for the CMB.

 

[Ophiolite]

I appreciate your effort to help me understand. However, what you have said seems to align with my previous understanding. But - because he has included the words "apparently" - it also, to me, accords with what nikkom said. You and bapowell appear to be correcting something that nikkom has not actually said - but would have said if he had not included the word apparently.

That said, I do not wish to derail the thread any further with a discussion that revolves around my reading comprehension.

 

[bapowell]

I appreciate your effort to help me understand. However, what you have said seems to align with my previous understanding. But - because he has included the words "apparently" - it also, to me, accords with what nikkom said. You and bapowell appear to be correcting something that nikkom has not actually said - but would have said if he had not included the word apparently.

That said, I do not wish to derail the thread any further with a discussion that revolves around my reading comprehension.

Ophiolite, have a look at the section entitled "The edge of the observable universe" here: https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/. In particular, Figure 5 illustrates why the expansion of the universe increases the travel distance of light as compared to a static universe.

 

[Ophiolite]

Ophiolite, have a look at the section entitled "The edge of the observable universe" here: https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/. In particular, Figure 5 illustrates why the expansion of the universe increases the travel distance of light as compared to a static universe.

I really am reluctant to be taking this off-topic, however... I think I understand the article you have linked to. I believe it reflects the understanding I have had of the subject for three decades or so.

But, you are now causing me to thoroughly doubt any part of that understanding. Perhaps the answer to this question will bring the matter to a close. In what way do the most distant galaxies not appear to be 13 billion light years away? I understand they are much further than that; I think I understand why they are much further than that; I do not understand why you assert they do not appear to be some 13 billion light years away.

Thank you for your efforts.

 

[phinds]

I really am reluctant to be taking this off-topic, however... I think I understand the article you have linked to. I believe it reflects the understanding I have had of the subject for three decades or so.

But, you are now causing me to thoroughly doubt any part of that understanding. Perhaps the answer to this question will bring the matter to a close. In what way do the most distant galaxies not appear to be 13 billion light years away? I understand they are much further than that; I think I understand why they are much further than that; I do not understand why you assert they do not appear to be some 13 billion light years away.

Thank you for your efforts.

What do you mean by "appear". A light beam hits a detector. It appears as a microwave. In what way does that lead you to conclude that it originated 13billion years ago? More to the point, in what way does that lead you to conclude that it started out 13 billion light years away?

My point is that it doesn't appear as anything other than a microwave that could have been sent from down the block. To reach any conclusion about when and where it started out, you have to use knowledge of cosmology and do calculations.