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If the universe is expanding, and perhaps even in an accelerated way, it would seem natural that as space expanded some of the distant matter inside the observable universe would leak out to be unobservable. If that continued indefinitely, the extreme would be that only our Solar system would remain, and thereafter the Sun, the planets and the Moon would disappear, perhaps leaving us all the last guy on the block.

My first question is whether this is how the future is viewed, or is there a balancing effect of some kind?

Also, we read that “In a closed Universe, a light beam will eventually return to its starting point”

(www.opencourse.info › astronomy › introduction › 35.universe_structure)

but what if the “circular” path of the ray takes it onside the edge of the observable universe – surely it wouldn’t come back again?

My second question along this line is: If the observable amount of matter shrunk inside our observable universe – as a result of the expansion -- would it not reduce the differences between the different cosmological alternative and make it more or less Euclidean whatever its earlier state?

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  • $\begingroup$ If the ray of light leaves the observable universe it could still return to us, just as a ship which disappears over the horizon can sail all the way around the world and return to us from the opposite direction. $\endgroup$ – sammy gerbil Feb 2 at 22:04
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    $\begingroup$ @sammygerbil “If the ray of light leaves the observable universe it could still return to us” - Not true. In the Friedmann cosmology, the particle horizon is absolute - nothing can cross it from outside. It expands faster than 3 times the speed of light. So light directed at us from there actually moves away from us at double the speed of light. $\endgroup$ – safesphere Feb 10 at 7:02
  • $\begingroup$ In a closed Universe, a light beam will eventually return to its starting point” - This depends on the cosmological model. In the currently accepted Friedmann model, this is not true (see above). The expansion is too fast for the light to ever return. $\endgroup$ – safesphere Feb 10 at 7:34
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If the universe is expanding, and perhaps even in an accelerated way, it would seem natural that as space expanded some of the distant matter inside the observable universe would leak out to be unobservable.

That is true with regards to the objects whose light which they emit today can be received by us in the future, and vice versa. The relevant horizon for that is the event horizon, which is currently at a distance of ≈16 billion light years.

For the objects whose light which they emitted in the past and can be received today it is the other way around, in the future we will see objects we can't see today, but the farther we will see, the closer to the big bang the time at which the light was emitted. The relevant horizon for that is the particle horizon, which is currently ≈3x farther than the event horizon.

So since the particle horizon grows, we will see farther than we see today, but we won't see the galaxies that are there today, but the gas that formed the galaxies later on. For the evolution of the event- and particle horizons in proper and comoving coordinates see here.

If that continued indefinitely, the extreme would be that only our Solar system would remain, and thereafter the Sun, the planets and the Moon would disappear, perhaps leaving us all the last guy on the block.

The cosmological constant is too small for that, galaxies will always stay bound, assuming the constant is really a constant, which we do in standard cosmology.

but what if the “circular” path of the ray takes it onside the edge of the observable universe – surely it wouldn’t come back again?

A closed universe has no edge, it is more like in the old computer games where you exit the screen on the right and enter it from the left, except there is no screen, or, as a 2D analogon, if you make a trip around the world and go back to where came from.

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  • $\begingroup$ Thanks for these clearly structured answers. I realize we may see light from distant (> 16 billion light years ) stars existent or in a formation process if they are nearer than 48 billion light years. My main question was not formulated clearly enough. I was mainly concerned with the disappearance of mass or energy through the outer horizon 3x16 billion light years as a result of the universe’s continued expansion. Is this true and how if the distant future of this process envisaged. You indicate that the cosmological constant is related to this. $\endgroup$ – Mikael Jensen Feb 6 at 12:08

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