Let's say that the number of large galaxies in the observable universe is $n$ (approximated to 350 billion).

If the universe is homogenous and isotropic, what are the estimations for the total number of large galaxies in it?

$5n$, $10n$, $50n$?

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    $\begingroup$ possible duplicate of How large is the universe? $\endgroup$ – user4552 Nov 10 '14 at 15:44
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    $\begingroup$ @BenCrowell How many galaxies is related to how large the universe is, but not entirely. If we knew how large the universe was, we would still not know how many galaxies there are because of the many different models and theories. No matter the size, we could be the only pocket with galaxies or they could all be evenly spaced. So the answers for that question do not provide adequate answers here $\endgroup$ – Jim Nov 10 '14 at 15:47
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    $\begingroup$ @Jim, this question specifies "homogeneous and isotropic", which makes it identical to: "How large is the universe, measured as a multiple of the observable universe?" $\endgroup$ – Foo Bar Nov 10 '14 at 20:23
  • $\begingroup$ @FooBar True, but a thorough answer here would include reasons why the number of galaxies cannot be estimated in general, not just in the specified case. Whereas in the linked question, that would be arguably not relevant $\endgroup$ – Jim Nov 10 '14 at 21:04

By definition, anything outside of the observable universe is unobservable. This has the annoying effect (eye twitch) of making it so we have practically no idea what the universe is actually like outside of what we can observe. We can assume that it is homogeneous and isotropic and that there are other large galaxies out there, but there is a non-zero probability that we live in a privileged area and outside of what we can observe there is nothing. There is also a chance that the universe is infinite and homogeneous. This is why Dirk said between zero and infinity, we can't observe anything about it and therefore we cannot know how many galaxies there are.

I do desperately wish there was an accurate number I could give you, but there are no estimations for the number of galaxies outside of our observable limits.

  • $\begingroup$ If they use the Hubble constant to calculate the rate of expansion of the Universe, can they make an estimation of the total size of the Universe? $\endgroup$ – Unixy Nov 10 '14 at 15:42
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    $\begingroup$ No because we don't know what size it was to begin with. If it was infinite initially it's infinite now. We can use it to estimate the size of the observable universe, but not the size of the entire universe $\endgroup$ – Jim Nov 10 '14 at 15:44
  • $\begingroup$ I don't think you really mean to say "absolutely no idea what the universe is actually like outside of what we can observe". Some pretty basic assumptions about the lawfulness of the universe (assumptions that never seem to be contradicted by any observation) would have to break for the limits of the observable universe to exactly coincide with an abrupt change in what "things are like". $\endgroup$ – orome Nov 10 '14 at 20:50
  • $\begingroup$ @raxacoricofallapatorius would "barring the technicalities that bring about some of the more obvious properties we could expect, we figuratively (not literally of course, because of those technicalities) have absolutely no idea...." sound better to you? Or is that too much of a mouthful? But beyond wording, observations not contradicting assumptions isn't a valid stance if we live in a privileged observational area, which is always a non-zero probability until we observe everywhere $\endgroup$ – Jim Nov 10 '14 at 21:00
  • $\begingroup$ That there is a non-zero probability that we live in a privileged observational area is true. That we have "absolutely no idea what the universe is actually like outside of what we can observe" is false, regardless of how it is qualified. $\endgroup$ – orome Nov 10 '14 at 21:30

Somewhere between zero and infinity, if one believes the eternal inflation scenario. BTW, Max Tegmark covers some of this here

Eternal inflation posits that in the false vacuum from which our own universe inflated there may be any number of others doing the same, beyond our event horizon, all with various combinations of starting conditions and fundamental constants. It is one of the multiverse scenarios that Tegmark covers. As an aside, a multiverse can also arise from a single cyclic universe if it cycles through all possible states sequentially.

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    $\begingroup$ To answer "somewhere between zero and infinity" to a question that asks for the size of a quantity is, without good justification, the worst way to answer it. $\endgroup$ – ACuriousMind Nov 10 '14 at 19:35
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    $\begingroup$ It is, however, the literal truth $\endgroup$ – user56903 Nov 11 '14 at 7:40

You assume homogeneous and isotropic structure. That limits the possibilities as follows: It is just as likely that there are observers at the 'edge' of our Observable Universe as there are here. Assume two exist opposite each other. They both see the same Universe we do. Repeat the argument with them. Now consider they could be in any direction. So, instead of the Observable Universe being ~93E9 ly in diameter, this new construct has a diameter proportional to the number of these observers/edges. In principle there are three possibilities: 1. The Universe is flat. 2. The Universe has spherical curvature 3. The Universe has hyperbolic curvature. All of these are consistent with a isotropic, homogeneous Universe. The Universe is flat to a very good approximation, so that if it is curved, then the curvature is not noticable with our current methods. This implies that at a minimum the Universe is several hundred times larger than the Observable Universe. So given your assumptions say that the diameter is between 1E+13 ly and ∞. My take, not that you asked, is that asking to quantify what is by definition unknowable makes as much sense as trying to figure out how many Angels can dance on the head of a pin: meaningless.


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