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The way I have it is: the Observable Universe looks as follows. In some ball, all the galaxy clusters exist, then in a bigger concentric ball the dark ages exist (no galaxies), then on the surface of this larger ball at the edge of the observable Universe the Cosmic Microwave Background exists.

This leaves no room for an infinite or curled back on itself Universe, so it must be a wrong picture.

How do you picture it?

It would be advantageous to get a correct picture.

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  • $\begingroup$ Don't forget that as we look further in distance we're also looking further back in time. $\endgroup$
    – PM 2Ring
    Commented Jul 8, 2023 at 17:34

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In some ball, all the galaxy clusters exist

Not all. There are many galaxies and clusters that we expect to exist but whose light we simply have not received yet. (Due to dark energy, there should also be structures whose light we will never receive.)

then in a bigger concentric ball the dark ages exist (no galaxies),

We observe these regions at too early a time to have formed galaxies, but we expect them to form galaxies in their future.

then on the surface of this larger ball at the edge of the observable Universe the Cosmic Microwave Background exists.

The cosmic microwave background is everywhere. But I think that what you mean is the surface that the CMB light depicts. It's where the CMB light that we currently observe last scattered.

We can't see past the surface of last scattering with photons, but we still expect that more distant regions exist. They are simply distant enough that light that scatters around the time they become transparent does not have time to reach us. It still could in the future, though.

In particular, the last scattering surface moves outward over time. As time goes on, light that scattered around the time the Universe became transparent reaches us from farther distances.

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  • $\begingroup$ Are you implying that the galaxies outside of the Observable Universe are located beyond the CMB? i.e. beyond the surface where light was first set free? $\endgroup$
    – talanum1
    Commented Jul 13, 2023 at 15:48
  • $\begingroup$ @talanum1 Light was "set free" everywhere. The surface of last scattering is only where the light that is reaching us now last scattered. $\endgroup$
    – Sten
    Commented Jul 13, 2023 at 15:53
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The way I have it is: the Observable Universe looks as follows.

I think that's about right, with a very strong emphasis on the "looks as" part.

Note that the observable universe, as you are using the term, is nearly synonymous with the observed universe (with some exceptions made for better telescopes, and the cool new gravity-wave observations that are starting to be made).

So of course what we see in the far distance is also from far back in time.

This leaves no room for an infinite or curled back on itself Universe,

There is no room in the observable Universe for an infinite or curled-back-on-itself Universe, no. Or at least, there is no definitive evidence in the observed-so-far universe for these.

so it must be a wrong picture.

Cosmological models that posit the existence of an infinite or curled-back-on-itself universe cannot be confirmed or disproven by looking at the observable universe, because by definition the observable universe is finite and those models are positing structure that, without faster-than-light measurements, cannot be observed to be proven or disproven.

Just because we cannot and never can see it doesn't mean it isn't there, any more than it means it's not there. Unless someone can contrive a way to definitively measure what's beyond our limits of observation, we're pretty much doomed to never knowing the answer to this.

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