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Colby
2011-Jun-15, 07:43 PM
I hear this all the time, but could someone clarify its meaning?

If, for example, we're looking at a proto-galaxy 13.2 billion years old, then presumably that proto-galaxy is no longer there and completely new galaxies have been formed by now. It's obvious that light from any more recent objects formed from the evolution of that galaxy cannot yet be seen because light from the more recent objects hasn't reached us yet.

But there also seems to be another use that relates to the fact that the known universe is larger than the observable universe. Light from the unobservable universe will eventually be observable. What confuses me here is that we can see the CMB, so it seems we could not ever see any farther than that (barring neutrino or gravity wave detectors, etc).

What am I missing? TIA

ngc3314
2011-Jun-15, 07:58 PM
What we see as the CMB is a shell of the early Universe centered on our position, which is expanding at the speed of light as the Universe ages. So what we see now as galaxies at high z were at one time at the right distance to be seen as the CMB.

antoniseb
2011-Jun-15, 09:04 PM
... which is expanding at the speed of light as the Universe ages. ... Faster, I think.

speedfreek
2011-Jun-15, 11:02 PM
The edge of our observable universe, known as the particle horizon, has an apparent recession speed of more than 3 times the speed of light. It would have to, to have reached a distance of 46 billion light-years in only 13.7 billion years.

Back to the OP. When we say the light has not reached here yet, you mention the CMBR, and quite rightly too. Well if you consider that the CMBR was emitted throughout the universe, then CMB photons must have been passing through here ever since, and will continue to do so in the future. The CMB photons that have yet to reach us were originally released at a greater distance than the ones we currently detect, but they haven't had time to reach us yet.

Colby
2011-Jun-16, 12:58 AM
The CMB photons that have yet to reach us were originally released at a greater distance than the ones we currently detect, but they haven't had time to reach us yet.
I understand this, and yet I haven't really figured out the implications or incorporated that into my thinking. This means the CMB photons we're receiving right now are the ones from 13.7 billion years ago simply because those are the ones which have exactly that amount of time to reach us since the BB (or close to it). The ones closer than that have passed us by. The ones farther than that haven't reached us yet. Right? (Seems obvious. Just checking.)
If the CMB were to disappear today (hypothetically), that would indicate we're seeing the edge of the known universe, since that would mean all the light from the CMB in all the universe had just passed by us. Right?
Since all galaxies are younger than 13.7 billion years old, and we can see the light from the CMB at 13.7 billion years ago just passing us now, then there is no light from early galaxies which hasn't passed us yet. IOW, what light hasn't reached us yet (besides the CMB)? (Besides the light from more evolved galaxies as I described in my first paragraph.) Any?
[edit: Having thought about this a bit longer, there's no reason why galaxies could not have existed farther away than light has traveled in 13.7 billion years. They wouldn't be older than 13.7 billion years, but if they were farther away, we still wouldn't be able to see them yet. Perhaps I understand now...Right?]

caveman1917
2011-Jun-16, 01:59 PM
I understand this, and yet I haven't really figured out the implications or incorporated that into my thinking. This means the CMB photons we're receiving right now are the ones from 13.7 billion years ago simply because those are the ones which have exactly that amount of time to reach us since the BB (or close to it). The ones closer than that have passed us by. The ones farther than that haven't reached us yet. Right? (Seems obvious. Just checking.)
If the CMB were to disappear today (hypothetically), that would indicate we're seeing the edge of the known universe, since that would mean all the light from the CMB in all the universe had just passed by us. Right?
Since all galaxies are younger than 13.7 billion years old, and we can see the light from the CMB at 13.7 billion years ago just passing us now, then there is no light from early galaxies which hasn't passed us yet. IOW, what light hasn't reached us yet (besides the CMB)? (Besides the light from more evolved galaxies as I described in my first paragraph.) Any?
[edit: Having thought about this a bit longer, there's no reason why galaxies could not have existed farther away than light has traveled in 13.7 billion years. They wouldn't be older than 13.7 billion years, but if they were farther away, we still wouldn't be able to see them yet. Perhaps I understand now...Right?]

All correct (the last one corrected by your edit).