What Is Our Universe Expanding Into?
nautil.us
nautil.us
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?
Let's say the universe stopped expanding tomorrow, and didn't shrink. If someone got in a spaceship and traveled in (what seems to them) a straight line for long enough, would they wind up back where they started?
If not, how can the universe have no edge? How can a you always see another galaxy straight ahead, in a universe with a finite number of galaxies? The universe doesn't just create new galaxies. You have to start recycling galaxies at some point.
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."
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.
That, and stretching a surface intuitively imparts momentum on things on top, whereas it's not the case here.
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.
This still requires the balloon to expand into 3D space.
I am not a physicist, but i do visualize the universe expansion as such an infinity paradox.
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.
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.
It means it’s not a “small” hypersphere, that’s for sure.
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.
edit: s/your/you're/
Thanks!
> 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-...
edit: that was a good read! Thanks again
My problem with an infinite isomorphic universe is that it implies infinite mass. I don’t know enough about cosmology or other physics to understand all the implications, but to me it seems like it would make models of the early universe janky - ie why would it not just immediately collapse in on itself?
As I understand it, the standard cosmology includes inflation, which stretched out the "full" universe to much larger than the observable universe. This also made the observable universe very, very close to flat, which makes it almost impossible to observe the difference between "flat" and "not flat, but very close to flat in the observable part of the universe".
So if you think there's evidence that strongly indicates "truly flat", I'd like to hear what it is. Ditto for evidence that strongly indicates "infinite" as opposed to "finite, but considerably larger than the observable portion".
[Edit: If you want to say that inflation is a just-so story, I'm not sure I will disagree. And if you say that inflation looks like it was specifically designed to be untestable, I could probably agree with that, too. But those are different from saying that you have sufficient evidence to rule out inflation.]
> We now know (as of 2013) that the universe is flat with only a 0.4% margin of error. This suggests that the Universe is infinite in extent; however, since the Universe has a finite age, we can only observe a finite volume of the Universe. All we can truly conclude is that the Universe is much larger than the volume we can directly observe.
Sure, why not?
> I thought the Big Bang implied a finite mass.
What makes you say that? How so?
It is conceivable that our 'universe' is just one regional phenomena within a much larger universe. Multiple such 'universes' in the same physical space could seem to be isolated until they expand to the point of overlap and begin to collide.
This image from Max Tegmark illustrates this idea well: https://space.mit.edu/home/tegmark/multiverse.jpg
As shown in that figure, a level 1 multiverse is simply whatever other areas exist beyond our universes light cone. There could very well be 'stuff' out there that we are expanding into!
That "boundary" is not a spacial boundary, but simply the farthest distance we can "see".
The universe is expanding fast enough that any given space-time point can only "see" a finite region of the greater universe.
But it is all connected. We can see objects far away, whose citizens (if they exist) can see beyond our visible boundary, and so on.
And our area is not expanding "into" other areas. Think of the expansion as equally inflating every area, including our own. Pushing distant areas to be more distant. All in a smooth way across the region of the universe we can see and beyond.
Maybe the universes that are expanding inside a larger universe would not overlap but repell when they encounter each other, i.e. push each other out of the way instead changing the shape of a universe.
The shrinking people don't perceive they are shrinking, or that anything else is shrinking. But the distances between things not held together by significant forces (like chemical or gravitational bonds) appear to be growing and growing.
And space isn't expanding >into< anything. It is just relative changes between bond forces and space, creating the experience of more space. There is no impact of this expansion "outside" the universe (that we know of, of course).
This seems to come from a Rust programmer.
Every C programmer knows that the universe expands into void and the size of void is machine and inplementation dependent.
There is no "into" without the concept of "space".
The universe is space (and time).
Thus it's not expanding into anything as there is no space outside of the universe.
QED.
Every second we have another second* of time in most of the universe.
* May be less time near large amounts of mass.
How could this be due to the expansion of space, which should expand our galaxy, and my foot-long ruler, to the same extent that it expands the empty space between our galaxy and the next?
If you decide that what used to be 1cm is now 2cm, everyone will understand that everything is now twice as far. Now, imagine that instead of suddenly going from 1cm to 2cm, you do it gradually during say, 1 minute. Well, congratulations, that’s expansion right there.
I never understood the balloon analogy. It’s just a bad one.
And there's a good metaphor: The surface of Earth has no center. Therefore we can imagine that space similarly has no center if we imagine it having 4 dimensions.
But what proof is there that universe is actually 4-dimensional? Or > 3 dimensional in general?
Describing the universe's shape is similarly done from within, so we can't see the outside and don't need any "outside" to see that the Universe is expanding. It's a description of how our 3d world is behaving, seen from within, and not relying on any "outside".
The word "expand" is not describing the exact same physical phenomenon as what we are used to seeing, because when we expand something, we typically cause it from the outside. That's why it can't be satisfying: it's a similar but different thing.
Note the title of the article "What is our Universe Expanding Into? Is it expanding into the 4th spatial dimension?
(I believe this is a different thing to the various higher dimensional cosmological theories that exist - branes and the like - but such things are beyond me.)
If you aim at a direction in the sky and teleport past the last object or energy in that direction, you yourself becoming an edge of the universe, where will you go if you expand in the direction of the expansion of the object behind you? My answer: into nothing, your very progression has updated the boundary of reality. If you were a dot in a two dimensional window,your arrival at pixel 0x0 and attempt to move past it simply updates where 0x0 is relative to all other points.
As to teleporting to the edge, there is as much an edge to the Universe as to the Earth, so the answer is: you can't teleport there.
Note the Big Bang is a point in time where all paths started, as stated in the article. It’s a mathematical singularity. It’s not an explosion as you think of it. It was a sudden expansion of all points in space creating vastly more space between the points. But since space has no boundary it didn’t get bigger, and it wasn’t smaller.
This makes zero sense.
Because the universe follows general relativity, space itself behaves like a fluid. When the universe "expands", that's just space itself growing bigger over time.