<p>In a way, yes. But I think you are confusing cause and effect. Angular diameter turnaround is not the reason, but another consequence of the way things are (or rather, were).</p>
<p>The answers to the question that PM 2Ring linked to already provide a lot of insight. I think the biggest misconception here is that if we drive angular diameter turnaround to the extreme, we will see something that is as big as the whole universe because it was the whole universe and filled everything over 13 billion years ago.</p>
<p>That is not the case. The CMB is background because it's the furthest (at the moment of writing, maybe more advanced neutrino detection may change that) we can look into the universe, which is also the furthest back in time we can look. I find this artist's rendition from <a href="https://commons.wikimedia.org/wiki/File:Observable_universe_logarithmic_illustration.png" rel="nofollow noreferrer">Wikipedia</a> by Pablo Carlos Budassi of our viewpoint very helpful:</p>
<p><a href="https://i.sstatic.net/y7k9z.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/y7k9z.jpg" alt="Observable universe, logarithmic" /></a></p>
<p>It is &quot;everywhere you look&quot;, but it's still 40 billion light years away (give or take, source: <a href="https://www.astronomy.com/science/ask-astro-how-far-away-is-the-cosmic-microwave-background/" rel="nofollow noreferrer">Ask Astro: How far away is the cosmic microwave background?</a>) due to the expansion of the universe in the last 13.8 billion years. So the signal you get when you measure microwave radiation in every direction is exactly that red-shifted light from way back. To be even more precise, the CMB did not start with the big bang and is not even 13.8 billion years old, rather about 13.5 billion. See <a href="https://astronomy.stackexchange.com/a/27476/51977">David Hammen's answer</a> for more details.</p>
<p>Here is a video by Nick Lucid that I found helpful in understanding angular diameter turnaround: <div class="youtube-embed"><div>
                <iframe width="640px" height="395px" src="https://www.youtube.com/embed/nSJtzn2H3Do?start=0"></iframe>
            </div></div></p>

CC BY-SA 4.0
2023-08-21T14:16:32.663Z
Oh, I see! Thanks for helping out and editing my answer.
YetiCGN
CC BY-SA 4.0
2023-08-21T15:13:57.090Z
I suspect there is a basic arithmetic error in stating the age of the CMB. It was formed 400 thousand years after the big bang.
ProfRob
CC BY-SA 4.0
2023-08-21T15:20:52.687Z
You're right. I just copied that from James K's answer in the linked question. But then again, "400 thousand years" is just as inaccurate as sources tell me it was 380 thousand years ago. ;)
13.4 billion years is much more accurate, given current data. But is it really that important? As Harald Lesch used to say "wow, 13.8 billion years to this date! Another anniversary!!" :-D
YetiCGN
CC BY-SA 4.0
2023-08-21T15:40:38.243Z
13.8 billion - 400 thousand = 13.8 billion, though. (Don't mix up thousand with million!)
Sten
CC BY-SA 4.0
2023-08-21T18:30:22.583Z
(epic facepalm emoji) Now I see! It's an error of units / magnitudes. That 13.5 billion number really got me on the wrong track. Add to the confusion that my native language uses long scale and that I didn't use WolframAlpha to check like I normally do.
Thanks!
YetiCGN
CC BY-SA 4.0
2023-08-21T12:00:51.143Z
2023-08-21T13:21:23.647Z
2023-08-21T13:21:23.647Z
YetiCGN
<p>the idea of objects nearer the start of the visible universe appearing larger to us now because when their light started out they were nearer to us leads to the idea that the &quot;largest&quot; such &quot;image&quot; would be a distribution of radiation found everywhere you looked, like the cosmic microwave background; is that a correct interpretation?</p>
<p>in this case i would think the &quot;object&quot; and its &quot;light&quot; would be the primordial soup, so it's not like it would mean there were two identifiable lobes of the big bang, two &quot;objects&quot; at the outset, and one of them happened to have the place we now inhabit on its energy surface and the other was &quot;down the block&quot; originally, but is the idea applicable to all the radiation we can measure from 'somewhere out there'?</p>

CC BY-SA 4.0
2023-08-20T20:57:59.030Z
Related: https://astronomy.stackexchange.com/q/27453/16685
PM 2Ring
CC BY-SA 4.0
2023-08-21T13:15:36.913Z
Using the scale of 0.061 kpc/arcsecond,  the milky way galaxy would be about ⅓ the size of the full moon, if it were (magically) transported to the time and place that the cmb appears to be  (That figure comes from ned weights calculator assume a flat universe)  That's quite surprising to me.
James K
<cosmic-microwave-background><angular-distance>
given angular diameter distance turnaround, is it then that the background microwave radiation is now background because it was 'close' initially?
Db D
# given angular diameter distance turnaround, is it then that the background microwave radiation is now background because it was 'close' initially?

(Asked by: username_0 on 2023-08-20)

the idea of objects nearer the start of the visible universe appearing larger to us now because when their light started out they were nearer to us leads to the idea that the "largest" such "image" would be a distribution of radiation found everywhere you looked, like the cosmic microwave background; is that a correct interpretation?

in this case i would think the "object" and its "light" would be the primordial soup, so it's not like it would mean there were two identifiable lobes of the big bang, two "objects" at the outset, and one of them happened to have the place we now inhabit on its energy surface and the other was "down the block" originally, but is the idea applicable to all the radiation we can measure from 'somewhere out there'?

**Tags:** cosmic-microwave-background, angular-distance

> **Comments:**
> **username_1**: Related: https://astronomy.stackexchange.com/q/27453/16685  
> **username_2**: Using the scale of 0.061 kpc/arcsecond,  the milky way galaxy would be about ⅓ the size of the full moon, if it were (magically) transported to the time and place that the cmb appears to be  (That figure comes from ned weights calculator assume a flat universe)  That's quite surprising to me.  

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## 1 answers

### Answer by username_3 on 2023-08-21. Score: 1

In a way, yes. But I think you are confusing cause and effect. Angular diameter turnaround is not the reason, but another consequence of the way things are (or rather, were).

The answers to the question that username_1 linked to already provide a lot of insight. I think the biggest misconception here is that if we drive angular diameter turnaround to the extreme, we will see something that is as big as the whole universe because it was the whole universe and filled everything over 13 billion years ago.

That is not the case. The CMB is background because it's the furthest (at the moment of writing, maybe more advanced neutrino detection may change that) we can look into the universe, which is also the furthest back in time we can look. I find this artist's rendition from [Wikipedia](https://commons.wikimedia.org/wiki/File:Observable_universe_logarithmic_illustration.png) by Pablo Carlos Budassi of our viewpoint very helpful:

[![Observable universe, logarithmic](https://i.sstatic.net/y7k9z.jpg)](https://i.sstatic.net/y7k9z.jpg)

It is "everywhere you look", but it's still 40 billion light years away (give or take, source: [Ask Astro: How far away is the cosmic microwave background?](https://www.astronomy.com/science/ask-astro-how-far-away-is-the-cosmic-microwave-background/)) due to the expansion of the universe in the last 13.8 billion years. So the signal you get when you measure microwave radiation in every direction is exactly that red-shifted light from way back. To be even more precise, the CMB did not start with the big bang and is not even 13.8 billion years old, rather about 13.5 billion. See [David Hammen's answer](https://astronomy.stackexchange.com/a/27476/51977) for more details.

Here is a video by Nick Lucid that I found helpful in understanding angular diameter turnaround:

> **Comments:**
> **username_3**: Oh, I see! Thanks for helping out and editing my answer.  
> **username_4**: I suspect there is a basic arithmetic error in stating the age of the CMB. It was formed 400 thousand years after the big bang.  
> **username_3**: You're right. I just copied that from username_2's answer in the linked question. But then again, "400 thousand years" is just as inaccurate as sources tell me it was 380 thousand years ago. ;) 13.4 billion years is much more accurate, given current data. But is it really that important? As Harald Lesch used to say "wow, 13.8 billion years to this date! Another anniversary!!" :-D  
> **username_5**: 13.8 billion - 400 thousand = 13.8 billion, though. (Don't mix up thousand with million!)  
> **username_3**: (epic facepalm emoji) Now I see! It's an error of units / magnitudes. That 13.5 billion number really got me on the wrong track. Add to the confusion that my native language uses long scale and that I didn't use WolframAlpha to check like I normally do. Thanks!  

---

