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This question already has an answer here:

I m having difficulty understanding what is after the "Future Horizon" as defined per wikipedia

https://en.wikipedia.org/wiki/List_of_cosmological_horizons#Future_horizon

I read the similar "If the universe is expanding, what is it expanding into?" question and answers. I might have misunderstood the answer but I fail to understand what is meant by the answer "the universe isn't expanding into anything".

What exactly happens when the universe extends. Does extending mean fill vaccuum with something? Does this vacuum outside of our future horizon extend infinitely? I thought that infinity was not possible in physics.

I searched for answers here but most answers were using analogies of a circle in a 2D world or a sphere in a 3d world but I m not satified by these answers as in each case there is always a higher dimension which "contains" the universe. The question is then what is at the boundary of this higher dimension.

Maybe there is something I don't understand. Could someone come up with a clear explanation?

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marked as duplicate by sammy gerbil, Jon Custer, Chris, Mitchell, Kyle Kanos Jan 27 '18 at 13:51

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ See physics.stackexchange.com/q/67412/109928 $\endgroup$ – Stéphane Rollandin Jan 25 '18 at 8:49
  • $\begingroup$ Don't believe everything you read. Our universe is not "the exponentially expanding de Sitter space". $\endgroup$ – safesphere Jan 25 '18 at 8:55
  • $\begingroup$ I sympathise. The same question has been asked multiple times, with the same answers being given. I don't find them satisfactory either. I suggest that you ask in the hbar chatroom. Refer to a particular answer. If you get a satisfactory response from someone who has already posted an answer, encourage him/her to update that answer. $\endgroup$ – sammy gerbil Jan 28 '18 at 23:16
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Think about it this way. We know the universe is expanding because almost all galaxies in the sky are receding from us. This rate of recession depends on the distance - the further away a galaxy, the faster it recedes (this is Hubble's Law).

We also know that the rate of expansion is increasing. This means that, in the future, these faraway galaxies will be receding from us even faster than they currently are. At some point, these galaxies are going to be receding from us faster than the speed of light. If that's the case, we can no longer see them. We're able to see distant galaxies because light has had time to travel from those galaxies to Earth; if the galaxies are moving away faster than the speed of light, we cannot see them.

Our "horizon" in this context are the things that can affect us, and vice versa. This is crucially related to the speed of light, since anything that can affect us can travel at most at the speed of light. If you're only interested in the next minute, then the Sun is outside your horizon, since the Sun is 8 light minutes away. But if you care about the next year, then you must consider the Sun and the rest of the Solar System (but not the nearest star, which is further than 1 light year away).

The future horizon is a mesh of these two concepts. Once the distant galaxies are moving away from us faster than the speed of light, they're beyond our horizon since they'll never be able to affect us again (unless the rate of expansion slows down).

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This is obviously a highly speculative question; but hopefully I can bring some clarity; more in the sense of stimulating thought in the right direction. If you think about what we know so far in the most general sense; that is, basically 5% of our universe is observable, leaving 95% so far that which we cant observe; of which our best guesses is that the rest of the 95% percent consists of dark matter and energy; however, even though there is certainly evidence for such, the degree of plausibility is not that high; meaning, these are just the best names we've come up with so far. However, considering the fact we've given them these names, seems to imply more physical substance we are assuming it to be. I would propose to consider integrating into your framework of thought the possibility of non-physical phenomena, if you haven't already. The ontological structure of the universe must be generated and sustained by that which it is not; for its variations of expression consist of that which is most fundamentally —information. This of necessity implies an intelligent, eternal, non-spatial entity, which is the sustaining and generating source. All these attributes must consist of the essence of this source, since being other than that which it sustains, such an entity must, of necessity, subsist with a non-relation to time, and any spatial dimensions. Therefore, the future horizon of the expanding universe becomes irrelevant, in the sense that, relative to its generating source, there is no traversing of relative inertial frames, since by virtue of its non-relation to time, the eternal presence of such a distinct entity, being non-spatial, is therefore simultaneously existent within and without all inertial frames. So the universe then, may not necessarily extend infinitely in the sense of distance; yet, it will be sustained along an infinite continuum, in that, its essence is eternally preserved by its eternal generating source.

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    $\begingroup$ I think this is unclear... $\endgroup$ – QuIcKmAtHs Jan 25 '18 at 7:31
  • $\begingroup$ How so? Anything specifically? $\endgroup$ – animum scrutari multipliciter Jan 25 '18 at 7:53
  • $\begingroup$ It is surprising to see such an answer on the site limited to consensus approved opinions. A good question, an insightful reply, but both will likely be downvoted, closed, or deleted. $\endgroup$ – safesphere Jan 25 '18 at 8:38
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    $\begingroup$ -1 Not clear what you are saying. Your answer appears to be : "God". This reads more like a mystical philosophical opinion rather than an explanation using physics. $\endgroup$ – sammy gerbil Jan 25 '18 at 12:09
  • $\begingroup$ What created the "intelligent, eternal, non-spatial entity, which is the sustaining and generating source" ? Where does it reside? $\endgroup$ – Fred Jan 26 '18 at 8:00

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