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I understand the big bang to have (at least mathematically) started our universe at a virtually precise moment and within a virtually infinitesimal volume. Yet some of the light from that moment/volume reached us in the distant past, some is reaching us now, and more will reach us in the distant future (expanding particle horizon).

That seems strange to me.

I am thinking that maybe the big bang really is sort of the limiting case of the event horizon approaching $0$ distance from us. That is, if time began at the big bang ($t=0$ at the moment of the big bang), then at any $t>0$, there is a finite event horizon (albeit very tiny for $t \approx 0$) and, if object A is closer to it than object B, then object A's light will reach us later than object B (and object A will reach the particle horizon later than object B). But, at $t=0$, the actual big bang, I think all this breaks down. However, perhaps we can talk about how and when we receive light emitted as $ t \to 0$, and interpret light "emitted at the big bang" in that way?

Or maybe there is some kind of quantum uncertainty regarding the moment of the big bang, so that the event horizon was never (with no uncertainty) exactly $0$ distance from us? Then, at $t=0$, we have a non-trivially uncertain event horizon.

In any case, talking about "light emitted at (or during, if you prefer) the big bang" sounds problematic.

David
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  • From the link: "So to conclude: the Big Bang is the zero time limit of the FLRW metric, and it's a time when the spacing between every point in the universe becomes zero and the density goes to infinity. It should be clear that we can't associate the Big Bang with a single spatial point because the distance between all points was zero so the Big Bang happened at all points in space." Seems like it agrees the big bang is a limiting case. Is that what you mean by it answers me? Also, I read we see no light from before 100K years after BB, so there would be a finite distance event horizon, then. – David Sep 24 '19 at 20:45
  • @David Instead of "we actually see no light from before 100K years ago", you should have said " we actually see no light from 100K years after the Big Bang". – D. Halsey Sep 24 '19 at 20:50

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Here's a spacetime diagram to illustrate what's going on:

          The future -->                 E
                                        /E\
                                       / E \     E = Earth
         The present -->              /  E  \
                                     /  /E\  \    / \ = light
                                    /  / E \  \
            The past -->           /  /  E  \  \    ~~~ = glowing plasma
                                  /  /  /E\  \  \
                                 /  /  /   \  \  \
                                /  /  /     \  \  \
                               /  /  /       \  \  \
CMBR emission time -->  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Light travels diagonally at the speed of one ASCII character per ASCII line. At every time in Earth's history, there has been somewhere in the universe from which CMBR light traveling at the speed of light since it was emitted was just reaching Earth at that time.

The universe was completely filled with glowing plasma. If you trace the path of a photon, or any object, back in time from Earth's current location, or any location, you will eventually hit the plasma, because it was everywhere.

benrg
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  • Thank you! I think I agree: At the time of the CMBR (which was the beginning of radiation, as I understand it), the universe had a finite size and so an event horizon that was non-zero distance from us. The parts of the plasma that were nearest (but still inside) the event horizon at that time emitted radiation that will reach us later than radiation from the parts of the plasma that were closer to us at that time. Hence, the big bang CMBR, which I think was all emitted at more or less the same time, arrives to us at different times. Does that follow your line of reasoning? – David Sep 25 '19 at 03:51