# Infinite universe? **URL:** \ **Category:** Great Debates\ **Created:** [January 3, 2026, 5:54am UTC](https://boards.straightdope.com/t/infinite-universe/1026295 "2026-01-03T05:54:35Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![EllisDee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ellisdee/32/4531_2.png) [@EllisDee](https://boards.straightdope.com/u/EllisDee)\ **Post date:** [January 3, 2026, 12:20pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/21 "2026-01-03T12:20:52Z")
> [@Der\_Trihs](#): > > That’s not so, they’d be able to look in one direction and see a much younger universe than in the other direction. Or just be incinerated by it. Why would it be younger? It could be emptier, sure, but younger? If spacetime expanding faster than c would incinerate us, why haven’t we been incinerated by the observable universe expanding faster than c? ---
**Author:** ![Johanna](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johanna/32/8318_2.png) [@Johanna](https://boards.straightdope.com/u/Johanna)\ **Post date:** [January 3, 2026, 12:27pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/22 "2026-01-03T12:27:51Z")
Yoko: The universe is approximately infinite. John: How can it be approximately infinite? It’s either infinite or it isn’t. Yoko: Well, it depends on which part of the universe you’re in. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [January 3, 2026, 12:28pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/23 "2026-01-03T12:28:16Z")
> [@EllisDee](#): > > Why would it be younger? It could be emptier, sure, but younger? > > If spacetime expanding faster than c would incinerate us, why haven’t we been incinerated by the observable universe expanding faster than c? Because no specific point is doing so, assuming it even still is and didn’t do most of its expanding in the [inflationary era](https://en.wikipedia.org/wiki/Cosmic_inflation). And it would be younger because that’s literally what is being described; more universe coming into being. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [January 3, 2026, 12:35pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/24 "2026-01-03T12:35:01Z")
> [@EllisDee](#): > > I don’t understand: Why _must_ a flat “sheet of paper” universe be infinite? Well, I’m not sure it’s accurate to say that it _must_ be infinite. However, according to our best current theories, universes with this property _are_ infinite—unless you do something like introducing a boundary ‘by hand’ (more specifically, I’d guess there are no solutions to the Einstein equations that are flat, topologically trivial, geodesically complete, and have finite spatial volume, since all solutions obeying the first three should be isometric in their spatial part to R3, thus infinite; but I’m just a quantum mechanic and not a cosmologist, so I’m happy to be corrected by someone with more relevant expertise (@Chronos?)). And anything with a boundary of course invites the question: what’s on the other side? ---
**Author:** ![EllisDee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ellisdee/32/4531_2.png) [@EllisDee](https://boards.straightdope.com/u/EllisDee)\ **Post date:** [January 3, 2026, 12:49pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/25 "2026-01-03T12:49:18Z")
> [@Der\_Trihs](#): > > assuming it even still is and didn’t do most of its expanding in the [inflationary era](https://en.wikipedia.org/wiki/Cosmic_inflation). I thought everything in the observable universe is accelerating away from everything else still today, and the explanation for that acceleration is that space-time itself is expanding, propelled by dark energy. Corrections welcome. > [@Der\_Trihs](#): > > And it would be younger because that’s literally what is being described; more universe coming into being. It’s not more universe, it’s the same old universe expanding (stretching) outward, growing less dense as time moves forward toward eventual heat death. Just like everywhere else in the universe is doing. Much the same way that the “extra” 39.6 billion light years of radius of the observable universe is from the expansion of space-time in the observable universe. Otherwise, how could the observable universe have a 93 billion light year diameter if the universe is only 13.8 billion years old. It’s not that new universe is created in the observable universe. It’s just that the space-time itself has expanded (stretched). Incidentally, I realized that in my hypothetical star z scenario, there would be two things in the spacetime beyond star z toward the edge: cosmic microwave background and dark energy. Neither of those strikes me as a particular problem. ---
**Author:** ![EllisDee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ellisdee/32/4531_2.png) [@EllisDee](https://boards.straightdope.com/u/EllisDee)\ **Post date:** [January 3, 2026, 12:52pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/26 "2026-01-03T12:52:07Z")
> [@Half\_Man\_Half\_Wit](#): > > And anything with a boundary of course invites the question: what’s on the other side? My hypothesis is that spacetime expansion hides the edge of the universe in ~~the same way that~~ a similar way to how an event horizon hides the interior of a black hole. I don’t have enough schooling to understand most of the rest of your comment here, but that’s not to say I don’t appreciate it. I will spend some effort on Wikipedia to try and wrap my head around the concepts. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [January 3, 2026, 12:53pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/27 "2026-01-03T12:53:55Z")
> [@EllisDee](#): > > It’s not more universe, it’s the same old universe expanding (stretching) outward, growing less dense as time moves forward toward eventual heat death. Just like everywhere else in the universe is doing. That IS “more universe”. And expanding at FTL speeds again, most likely will involve the creation of huge amounts of energy to go with it. At any rate, such a wildly asymmetrical universe goes completely against modern ideas of how the universe is. ---
**Author:** ![EllisDee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ellisdee/32/4531_2.png) [@EllisDee](https://boards.straightdope.com/u/EllisDee)\ **Post date:** [January 3, 2026, 12:57pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/28 "2026-01-03T12:57:34Z")
> [@Der\_Trihs](#): > > That IS “more universe”. And expanding at FTL speeds again, most likely will involve the creation of huge amounts of energy to go with it. But how is the diameter of the observable universe larger then 13.8 billion light years? It’s because that 13.8 billion light year diameter observable universe has expanded to 93 billion light years, yes? I don’t understand why the edge of the universe can’t behave exactly the same way the observable universe behaves. Are you saying that the observable universe is _not_ expanding at FTL speeds? If so, how is it so large? ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [January 3, 2026, 1:07pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/29 "2026-01-03T13:07:59Z")
> [@EllisDee](#): > > But I don’t recall any of them saying that the big bang was super dense _and infinitely large._ Quite the opposite, actually. Most or all of them said the big bang was super dense and very small. I think the model of the Big Bang as infinitely large and infinitely dense is older than you remember, because you ( **and I** ) have run into far more science popularizers willing to oversimplify the actual theories used by scientists than actual scientists telling us what they have concluded (and also science popularizers who never quite understood what they were popularizing). It is true that everything _we see_ around us was once in a very very small volume, but that volume wasn’t all of the universe, just all of what we can currently see. I suspect that mathematically, it is required to describe the universe as once being infinitely large and infinitely dense to make the theory have all the properties that it is required to have. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [January 3, 2026, 1:08pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/30 "2026-01-03T13:08:10Z")
> [@EllisDee](#): > > I don’t understand why the edge of the universe can’t behave exactly the same way the observable universe behaves. Because that’s not how the observable universe _is_ behaving. The rest of the universe hasn’t vanished at FTL, we can still see other stars. The entire universe is expanding at once, there’s no specific spot you can point to and saying “that spot is expanding faster than light”. ---
**Author:** ![xtenkfarpl](https://avatars.discourse-cdn.com/v4/letter/x/c2a13f/32.png) [@xtenkfarpl](https://boards.straightdope.com/u/xtenkfarpl)\ **Post date:** [January 3, 2026, 1:09pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/31 "2026-01-03T13:09:22Z")
> [@Whack-a-Mole](#): > > There’d be a whole universe identical to ours except one blade of grass was missing from one. (can get more granular than that…infinity is weird and big) I have seen this argument many times, and I think it fails because of Cantor’s hierarchy of infinities. You can postulate a countable infinity (aleph null) of particles, but the number of possible _arrangements_ of those particles increases in a way which in a strict mathematical sense is _larger_ than the number of particles. In mathematical terms, the power set of a set is larger than the set in a definite conceptual way. So a recurrence of a particular object is not inevitable: it is in fact _infinitely_ improbable. ---
**Author:** ![Charlie\_Tan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/charlie_tan/32/12582_2.png) [@Charlie\_Tan](https://boards.straightdope.com/u/Charlie_Tan)\ **Post date:** [January 3, 2026, 1:11pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/32 "2026-01-03T13:11:15Z")
> [@EllisDee](#): > > Google says the observable universe is 93 billion light years across while the universe itself is only 13.8 billion years old > [@EllisDee](#): > > But how is the diameter of the observable universe larger then 13.8 billion light years? My primate brain has been pondering things like this and it makes my primate brain hurt, but… AIUI: If one photon is going that way ← and another in the opposite direction that way -\>, then, despite nothing moving fast than _C_, the distance between them is in fact growing faster than _C_, not violating our understanding of relativity. Then again, a lot of handwaving and comments like “the math says this must be so” and “there was no before, because time didn’t exist,” make my brain hurt. ---
**Author:** ![EllisDee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ellisdee/32/4531_2.png) [@EllisDee](https://boards.straightdope.com/u/EllisDee)\ **Post date:** [January 3, 2026, 1:19pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/33 "2026-01-03T13:19:26Z")
> [@Andy\_L](#): > > I think the model of the Big Bang as infinitely large and infinitely dense is older than you remember, because you ( **and I** ) have run into far more science popularizers willing to oversimplify the actual theories used by scientists than actual scientists telling us what they have concluded I think this is correct. The only popularizer books I read that I know were written by an actual cosmologist are Hawking’s books. (I read both.) When I asked Google AI a few hours ago whether Hawking wrote the universe was infinite in A Brief History of Time, it said yes, he did. It said he didn’t claim it as fact, but he presented it as either the universe is infinite or there are an infinite number of universes. The AI slop summary did not say he wrote that a finite universe is possible. And honestly, that would be the kind of thing that I couldn’t wrap my head around and just ignored. I could have easily read and ignored the same thing in all of those books. ---
**Author:** ![EllisDee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ellisdee/32/4531_2.png) [@EllisDee](https://boards.straightdope.com/u/EllisDee)\ **Post date:** [January 3, 2026, 1:23pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/34 "2026-01-03T13:23:26Z")
> [@Charlie\_Tan](#): > > AIUI: If one photon is going that way ← and another in the opposite direction that way -\>, then, despite nothing moving fast than _C_, the distance between them is in fact growing faster tha But the universe is only 13.8 billion years old, which gives us a maximum diameter of 27.6 billion light years from light going in opposite directions. The observable universe has a diameter of 93 billion light years, and virtually everything appears to be accelerating away from everything else. Therefore, the spacetime in the observable universe is itself expanding. Since we have no idea what could possibly cause that, we have labeled the cause of that “dark energy.” Corrections welcome, and yay, I see Chronos typing. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 3, 2026, 1:31pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/35 "2026-01-03T13:31:27Z")
To the best of our ability to measure, the Universe is perfectly flat. This does not necessarily mean that it actually is, because there are always error bars. But a perfectly-flat Universe fits in most simply with our best models. If the Universe is either flat or negatively curved, and if it is topologically trivial, then it must be infinite. But this is less definitive than it might sound, because we likewise don’t know that it’s topologically trivial. We do know that the observable portion of the Universe is topologically trivial; that is, if the Universe does repeat, it does so on a larger scale than we’re able to observe. But there would be absolutely nothing theoretically wrong or problematic with adding some extra topological structure to the Universe on very large scales, beyond the fact that extra topological structure would be an added complication and hence disfavored by Occam (though, of course, one might also regard an infinite universe as being an unrequired complication). If we do have such a topological complication, then there would still be no edge (a map of the Universe would have an edge, but that’d be an artifact of the map, not of the Universe itself: In the actual Universe, the points we labeled as “edge” on the map would be just like any other points). IIRC, there are about a dozen possible topological arrangements for a flat universe. If, on the other hand, the Universe is positively curved (which, again, can’t be ruled out, because our measurements of the curvature always have error bars), then regardless of its topology, it’s finite (though, again, much larger than the observable Universe). ---
**Author:** ![EllisDee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ellisdee/32/4531_2.png) [@EllisDee](https://boards.straightdope.com/u/EllisDee)\ **Post date:** [January 3, 2026, 1:39pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/36 "2026-01-03T13:39:01Z")
> [@Chronos](#): > > If the Universe is either flat or negatively curved, and if it is topologically trivial, then it must be infinite. This is the part I don’t understand. Unfortunately, I get the sense from this… > [@Half\_Man\_Half\_Wit](#): > > Well, I’m not sure it’s accurate to say that it _must_ be infinite. However, according to our best current theories, universes with this property _are_ infinite—unless you do something like introducing a boundary ‘by hand’ (more specifically, I’d guess there are no solutions to the Einstein equations that are flat, topologically trivial, geodesically complete, and have finite spatial volume, since all solutions obeying the first three should be isometric in their spatial part to R3, thus infinite; …that understanding it may require higher math than I am able to comprehend. (eg: What is R3? That’s rhetorical.) I do have a straightforward question, though: Is the observable universe expanding from an unknown force that we have labeled dark energy? If so, follow-up question: Is there a correlation showing that the further away something is, the faster it is accelerating away from us? ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [January 3, 2026, 1:53pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/37 "2026-01-03T13:53:15Z")
> [@EllisDee](#): > > …that understanding it may require higher math than I am able to comprehend. (eg: What is R3? That’s rhetorical.) Sorry, ‘R3’ is just ordinary Euclidean three-dimensional space extending infinitely in every direction. R is the real number line, R2 is the plane obtained from putting one real number line at a right angle to the first, and R3 is doing that again. And again, I suppose one could add a boundary ‘by hand’, but that would just introduce something arbitrary, somewhat like saying there is a highest number, _k_—why that number, why not 2_k_, etc.? It just leaves you with some inexplicable brute fact, which physicists generally don’t like. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [January 3, 2026, 1:54pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/38 "2026-01-03T13:54:34Z")
I’m not sure how that works. Is there an “Explain it Like I am Five” for Cantor’s hierarchy? (I am genuinely curious, also genuinely not sure what he is on about.) As I mentioned above with the Poincare Recurrence Theorem if you have a finite number of particles (as we see in the _observable universe_) then there has to be a finite number of ways to arrange those particles. It is an almighty big number that may feel infinite but it is still finite. If you have infinite time to keep re-arranging particles then you will get repeats of any given arrangement. You simply have to. E.G. How many ways can you arrange these numbers? 1, 2, 3 Six ways. On seven you get a repeat. I suppose you could say one arrangement will never repeat but that gets unlikely fast. ---
**Author:** ![xtenkfarpl](https://avatars.discourse-cdn.com/v4/letter/x/c2a13f/32.png) [@xtenkfarpl](https://boards.straightdope.com/u/xtenkfarpl)\ **Post date:** [January 3, 2026, 2:01pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/39 "2026-01-03T14:01:42Z")
> [@Whack-a-Mole](#): > > I’m not sure how that works. Is there an “Explain it Like I am Five” for Cantor’s hierarchy? (I am genuinely curious, also genuinely not sure what he is on about.) The key take-home is that in a mathematical sense, some infinities are strictly larger than others. Wikipedia on Cantor is reasonably helpful, though it may require a certain amount of mathematical background to be fully comprehensible. Try giving that a read, and if you still have questions come back to us and we’ll pick it up again. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 3, 2026, 2:14pm UTC](https://boards.straightdope.com/t/infinite-universe/1026295/40 "2026-01-03T14:14:14Z")
> [@EllisDee](#): > > I do have a straightforward question, though: Is the observable universe expanding from an unknown force that we have labeled dark energy? If so, follow-up question: Is there a correlation showing that the further away something is, the faster it is accelerating away from us? Yes to both. > [@xtenkfarpl](#): > > The key take-home is that in a mathematical sense, some infinities are strictly larger than others. That’s not relevant for repetition in an infinite space, though, because any given arrangement of matter is finite. [Previous page](https://boards.straightdope.com/t/infinite-universe/1026295.md?page=1) [Next page](https://boards.straightdope.com/t/infinite-universe/1026295.md?page=3)