Don't laugh. I'm good with understanding planets and stars and galaxies and the like, but you've got to remember that I'm still a kid and I haven't taken physics yet. Cosmology is hard for me to understand.
I was reading an article about COBE, and no matter how hard I tried, I couldn't understand what exactly the background radiation was, other than that it came from the Big Bang and showed hot and cold parts of the universe.

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There's nothing to laugh at. I have been reading about it for years and I don't always understand it either.

The Universe spent its first few hundred thousand years as a plasma: very hot, completely ionized, with photons interacting constantly with free electrons. At the end of this era, the Universe got cool enough for the first atoms to form and stay intact; the average photon energy was no longer high enough to ionize hydrogen. So the photons "decoupled" from matter, and were able to travel long distances without being absorbed. The temperature of the Universe at that time was around 3000K, so the photons that filled it would have had roughly the same spectrum as the filament in an incandescent lamp: lots of photons in the visible range.
Photons from the time of decoupling are still travelling through the empty spaces of the Universe, but they have been redshifted by the continuous expansion of the Universe in the billions of years since decoupling: redshifted about 1100-fold, so that they now have a spectrum which corresponds to a temperature of just 2.7K. That places their peak wavelengths in the microwave range: hence "cosmic microwave background".
When we detect photons in that spectrum, we know they've been travelling for 13 billion years, since they decoupled from matter. The actual distance they've travelled is complicated by the expansion of space while they've been travelling, so you'll see various numbers quoted for that value. We can't detect photons that come from farther away, because they would have been emitted during those first few hundred thousand years when the Universe was composed of plasma, and they would have been almost immediately absorbed by interactions with the sea of free electrons in the plasma. And we don't see cosmic microwave background photons from nearer at hand, because all the CMB photons were produced at the same time in the Universe's history, and the ones that were produced close to us have now passed us by and are on their way somewhere else; perhaps an observer billions of light years away is picking up CMB photons that were emitted in our vicinity.

Grant Hutchison

Nice explanation.

Great explanation. I almost understand it now.

I have seen figures of around 42 million light years radius for the size of the observable universe when the CMBR was emitted and so based on those figures would assume that the CMBR photons we detect today were originally emitted at that distance from this point in space, but took 13.7 billion years to reach us due to the expansion of the universe.

I know we cannot see further back in time than when the CMBR was emitted, but as time goes on, and the observable universe presumably increases in size as light has had more light to reach us, will we be seeing photons that were emitted at recombination at what was then a distance further than 42 million light years away?

Or does this issue rest on whether the observable universe constitutes only part of, or is the whole of, the universe?

I am puzzled whether there was any negative hydrogen ionization since this common to the Sun's photosphere, which is even hotter, or was the greater pressure at recombination enough to allow only positive ionization?

I think one problem people have in understanding the CMBR "sea" is finding a mental picture that is helpful to explain why only 13.7 billion year-old light exists for the CMBR. Maybe this analogy will help anyone here that has trouble with this. Imagine 5 bolts of lightning striking all at the same instant, and each radially spaced away from you by one mile, one after another. So the first one is one mile away and the next two miles, etc. Since sound travels about 1/5 mile per second, then the first thunder will come after 5 seconds, the next thunder will come 5 seconds after that and so on. The key here is that when the CMBR first activated (recombination) it all happened at the same instant, like our lightning. Thus, we will never see (like the hearing of thunder) the earlier CMBR go by, only that which is just now reaching us after 13.7 billion years.

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

George,

You'll want to edit your post to read "5 seconds" instead of "1/5 second".

-- Jeff, in Minneapolis

...except tomorrow we'll see the CMBR go by from 13.3 billion years + 1 day, right?

The radiation is being 'refreshed' every moment? As each moment goes by we see the radiation from the next 'moment' of space. The distance we see increases each moment.

Sorry if I've misunderstood the post - it just sounds a bit different to how I picture it.

... and after a re-read is it the case you're saying we won't ever see yesterdays CMBR again? In which case we're saying the same thing!

I think I'll have another coffee...

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If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

One other thing George, when I post I'm always a little unsure which time to quote. I see that you've quoted 13.3 billion ly for the CMBR (13.7 less the 380,000 years before decoupling, as the literature says). I've tended to just stick with 13.7, because it's one less number to remember! I actually think your way is better, so is there a consensus to quote 13.7 for the big bang and 13.3 for the CMBR?

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If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

I didn't notice that George said 13.3 billion. That's an error. Your
rationalization of George's number is exactly the same mistake I once
made. In fact, 13.7 billion minus 380,000 is... 13.7 billion! (To three
significant digits of precision.) You (and I, when I made this error)
must have been thinking million, instead of billion. Subtracting
380,000 from 13.7 billion just doesn't have much impact.

-- Jeff, in Minneapolis

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"The other planets?
Well, they just happen to be there, but the point of rockets is to explore them!"
-- Kai Yeves

I think you've got it. All the light was emitted at approximately the
same time. The light we see today was emitted 13.7 billion years ago,
and traveled 13.7 billion light-years to reach us. The light we see
today was emitted from farther away than the light we saw yesterday,
and from closer than the light we will see tomorrow.

-- Jeff, in Minneapolis

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http://www.FreeMars.org/jeff/

"The other planets?
Well, they just happen to be there, but the point of rockets is to explore them!"
-- Kai Yeves

Oh crap! My bad, you're right of course. I'll go back to 13.7, thanks Jeff.

__________________
If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

My understanding is that at the time they were emitted, they were closer than 13.7 billion lightyears, but it has taken them that long to reach us because of the expansion of the Universe (the distance ahead of them expanded). And that their point of emission has by now been carried outwards to ~42 billion lightyears (the distance behind expanded as well).

There's an old puzzle involving a caterpillar crawling along an infinitely elastic rope. The rope starts a metre long, with the caterpillar at one end. After the caterpillar has crawled a centimetre, the rope stretches by a metre; for each subsequent centimetre the caterpillar crawls, the rope stretches by another metre. Will the caterpillar reach the end of the rope?
It will, because the puzzle reduces to asking if the sum 1/100 + 1/200 + 1/300 + 1/400 ... converges. It doesn't, so the caterpillar eventually arrives at the other end of an exceedingly long rope, after an exceedingly long time, having spent a great deal of its time in regions of the rope that were carrying it briskly away from its destination.
Likewise for CMB photons, and light from distant galaxies in general. (There's an animation and discussion here, showing how it works.)

Davis and Lineweaver's SciAm article, Misconceptions about the Big Bang, is useful for thinking about this, as is their more technical Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe.

Grant Hutchison

I saw all these numbers just days ago, but don't remember what they
were or where I saw them. Did speedfreek really mean 42 million, as he
said, or 42 billion, as Grant said? Were both figures right? Both wrong?
Find out in the next post! (Somebody do some research, quick!)

Grant, the fact that you italicised the word 'closer' makes me think you
misread speedfreek's '42 million' as '42 billion'. Both figures could be
right, because you are talking about different things, though of course
that is highly unlikely.

The animation you linked to includes the gravitational slowing of the
expansion. I made a variation on that animation which has constant
expansion, yet the caterpillar/photons still get to their destinations:
http://www.freemars.org/jeff2/expand2a.gif

The question was raised, though, as to what the grid lines represent
(in either animation). I really don't know.

I see that the page you linked to says "the very edge of the visible
universe is now about 47 billion light years from us". That would
appear to be your '42 billion' figure.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"The other planets?
Well, they just happen to be there, but the point of rockets is to explore them!"
-- Kai Yeves

I think speedfreek meant what he wrote, 42 million (small universe, long ago), and I misread that as being the forty-odd billion light years of the current comoving radius of the observable Universe.
Hence my odd emphasis on closer, when speedfreek had actually already said as much.

I've seen figures between the high 30s and mid-40s for the billions of light years represented by the current observable radius, so inappropriately adopted speedfreek's digits for my own use. Davis and Lineweaver, in the articles I cited, use 46 billion light years for the particle horizon in comoving coordinates.
(In a toy model of the expansion of the Universe, there's a factor of three linking age to comoving radius: hence ~14 billion years gives ~42 billion lightyears. Different models give numbers that differ somewhat from that simple multiple.)

Grant Hutchison

*grins* Am I allowed to say... The CMB is the afterglow of the Cosmic orgasm we call the Big Bang? Please?

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Jeff, you were editing this in while I was replying. We've come to the same conclusion.
My "42 billion" for the current comoving radius was in the right ballpark, but subliminally nudged away from Davis and Lineweaver's figure as I describe in my previous post. Speedfreek will need to let us know about his 42 million for the radius when the CMB decoupled.

Grant Hutchison

Speedfreek and I looked at this not long ago, so I hope he won't mind me chipping in:

Roughly 40 million light years was the radius of the surface of last scattering when it was emitted, the photons travelled 13.7 billion light years to reach earth, and the current co-moving radius is roughly 46 billion light years.

__________________
If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein