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Disclaimer: I’m not a cosmologist or astronomer just a fan.

As as I know, we don’t know. As I understand it we can see ~13.9 billion light years away but not further, and so we don’t know what’s past it.

I’d love if an expert chimed in.

I have recently wondered about this. When we talk about the big bang, the mostly-uniform CMB, accelerated expansion, etc. aren't we always axiomatically presuming the correctness of the cosmological principle (which, in a hand-wavy way says the universe plays the same game everywhere). I would think all we can say confidently is that the principle is working out in our locale.

> analogy that's usually used for this is to think of the 3D universe as the curved 2D surface a balloon

This still requires the balloon to expand into 3D space.

The idea would be by analogy we're expanding in a higher dimensional space.

Imagine a hotel with infinite rooms and there is a guest in each room. A bus arrives carrying infinite guests and you must assign each of these guests to a room. One solution is to ask each guest to move two rooms over and assign the new guests to the odd number rooms.

I am not a physicist, but i do visualize the universe expansion as such an infinity paradox.

But it does not require there to be higher dimensions for the universe to expand into.

It's an analogy that works in our experience that we can visualise.

There is no reason to suppose that the universe requires something "outside" it to expand "into". It could just... get bigger.

> But also in this analogy if the bug travels long enough in what to it seems a straight line, it will wind up back where it started. Does this hold true for the universe as well?

That’s indeed a very important question. The shape of the universe is a subject of study in cosmology.

The property you mention relates to its curvature (hyperbolic, flat or spheric). Experimental data shows that the observable universe is flat which means the universe is either flat or sufficiently big for the observable universe to appear flat as a local area.

> If not, how can the universe have no edge?

What we see of the universe indeed as an edge. Light takes time to travel. It means that the farther you look the earlier you look and we can’t look farther than the beginning of time.

Is it true that if the universe is flat, then either it has to have an edge (meaning a place where you see galaxies in one direction and nothing in the other), or it has to have an infinite number of galaxies?

I’m not entirely sure of that to be honest with you. I seem to remember than counterintuitively you can embed a flat torus in 3D but it goes further than my knowledge.

It means it’s not a “small” hypersphere, that’s for sure.

"Experimental data shows that the observable universe is flat", agreed. However the research put a maximum bound on it's non-flatness. So it's low, but it's impossible to prove it's perfectly flat.

The thing I don't get with that analogy is the ballon IS expanding into something - eg if you put the balloon in a cup, it won't expand (unless you have REALLY strong lungs :-) ).

Also, curious where the extra energy comes from to keep the 'vacuum energy' (zero point energy?) from going down - IIRC, its remained constant even though the universe is expanding

> Also, curious where the extra energy comes from to keep the 'vacuum energy' (zero point energy?) from going down - IIRC, its remained constant even though the universe is expanding

Energy is ill-defined in a global sense: it's the conserved quantity associated with time-translation symmetry, but there's no single choice of "time translation" that can be extended to all of spacetime.

Hmm...This goes beyond what PBS space-time taught me about it :-) My understanding was we have more space (eg space is expanding) but the 'vacuum energy' is staying constant.. If I understand what you're saying, that statement depends on the time frame you measure against ?

edit: s/your/you're/

No, it's just not meaningful. The vacuum energy density is (likely) constant. The total energy of the universe is simply not a thing.

Hmm..ok - thank you. I'm gonna have to work on that more, atm I'm having a hard time comprehending how volume can increase without density decreasing. To google!

Thanks!

Google won't help you here. Popsci stuff is useless at best, and more often actively misleading. You need a GR textbook. An Introduction to General Relativity, Spacetime, and Geometry (https://fma.if.usp.br/~mlima/teaching/PGF5292_2021/Carroll_S...) is relatively accessible.

oh! Thanks!

This article might help: "Energy Is Not Conserved" by Sean Carroll [1]. Here are a couple of paragraphs to give you the gist of it.

> But many people have just this reaction. It’s clear that cosmologists have not done a very good job of spreading the word about something that’s been well-understood since at least the 1920’s: energy is not conserved in general relativity. (With caveats to be explained below.)

> The point is pretty simple: back when you thought energy was conserved, there was a reason why you thought that, namely time-translation invariance. A fancy way of saying “the background on which particles and forces evolve, as well as the dynamical rules governing their motions, are fixed, not changing with time.” But in general relativity that’s simply no longer true. Einstein tells us that space and time are dynamical, and in particular that they can evolve with time. When the space through which particles move is changing, the total energy of those particles is not conserved.

[1] https://www.preposterousuniverse.com/blog/2010/02/22/energy-...

oh! weirder and weirder! Thanks for the link!

edit: that was a good read! Thanks again

I think the balloon analogy is used because people have an easier time imagining that rather than an infinite surface.

That, and stretching a surface intuitively imparts momentum on things on top, whereas it's not the case here.

Doesn't dark energy add momentum?

Why would it?

I thought dark energy was the reason the non-gravitational-bound galaxies are accelerating away from each other? If they are accelerating, that would be a change in velocity, which seems like a change in momentum. ?

The name "acceleration" is just as misleading as "expanding" here, but maybe useful.

Our galaxy is non-gravitationally-bound to a lot of other galaxies, and we "accelerate" away from them. Yet we don't really feel that. We also obviously can't accelerate away from all of them, when they are in random directions. That's because the "acceleration" describes the change of distance, without changing position. Without changing position, there's no change of momentum.

That "acceleration" is a consequence of the expansion/stretching described in the article.

Why is it called dark energy if energy isn't changing?

> But also in this analogy if the bug travels long enough in what to it seems a straight line, it will wind up back where it started. Does this hold true for the universe as well?

It's possible, but the experimental evidence have indicated that it does not - the 2D analogy that more closely matches the measurements is "infinite flat sheet", not "curved surface of a sphere."

Infinite means infinite number of galaxies, right? I watch How the Universe Works religiously. Most of the astrophysicists on that show don't seem to think the universe is infinite.

At this point even if there was an edge you couldn't get to it because points move apart faster than the speed of light and we can't go faster. The points themselves aren't actually moving (rather space is being created in between) so there's no speed limit violation.

Yes but even in these analogy the balloon is still expanding into something. There is space around the balloon and when the balloon expands, there is less of that surrounding space.

does the universe have a finite number of galaxies? We see the 'observable' universe. The universe is even larger than what we can see, but that light can never reach us.