Answers to Approximately what percent of the sky has nothing in it? seem to suggest that with enough depth, we will finally hit something hence the sky is basically 100% full for any given resolution. While I agree with the reasoning there, I feel (maybe wrongly so) that something is missing in the analyses; As I understand, the amount of matter in the universe is finite and so we can't, even theoretically, actually good deeper ad infinitum. There is a limit to our depth.

To continue with this line of thought, we might also remember that the current understanding is that the universe is expanding, so is the distance between the galaxies and the stars and so most of the galaxies will become smaller (with maybe the exception of Andromeda). Why then can't we reach the position where most of the sky is empty? Or can we? If so when? Why, at this very moment, is the universe still baby that 100% of the sky is full - it is highly improbable since "most of the time", so it is sometimes said, the universe will be dark and cold.

Or as the universe gets old, so new matter is revealed to us?

We can rephrase this question from another angle: As we have depth-limit (since the universe in finite we think), there is a specific resolution in which the sky will be only 50% full even with maximal depth. What is this resolution?

  • $\begingroup$ Wrong way round. With resolution low enough we eventually hit something. $\endgroup$
    – ProfRob
    Commented Feb 3, 2023 at 7:06
  • $\begingroup$ @ProfRob, probably my wording (also using zoom-in) is not good; I mean that we could see even further objects. re-reading that linked question, it seems I should have used the term deep; and I should maybe specify a specific resolution in this question. Let's have it at only at 1 arcsec with maximal depth. $\endgroup$
    – d_e
    Commented Feb 3, 2023 at 8:09
  • $\begingroup$ In other words, The answers seems to suggest that the depth ultimately "wins" and it makes sense as we project 3d space of basically 2d manifold. So double the depth - we increase the observable matter by a factor of 8, but the angular size of the newly revealed objects will be only half size. So the sky will become fuller as we go deeper. My question is about the limit of the depth: eventually the matter is finite(?) and in time the depth ever rising without enough matter, it will sometime in the future "lose" this war. $\endgroup$
    – d_e
    Commented Feb 3, 2023 at 8:20
  • $\begingroup$ We increase the observable matter by more than that because we also start to see all the fainter, closer objects. The angular size also does not vary as you suggest. For a fixed physical size, the angular size increases for higher redshift objects! Anyway, it's a good question (although the limit is not really stars, it's galaxies). $\endgroup$
    – ProfRob
    Commented Feb 3, 2023 at 14:06
  • $\begingroup$ The Hubble Deep field would already look more than 50% full if observed at 1 arcsec resolution. The answer also depends on what wavelength is considered! $\endgroup$
    – ProfRob
    Commented Feb 3, 2023 at 14:07


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