Is the universe really expanding?

[Garnier Fructis]

I also have a question. If we do exist within an expanding universe, how would the universe not be expanding outwards from a center?

I don't have enough intuition to answer the OP's question, but I can answer this one. I'll answer from a different perspective.

First, consider an explosion. At any point in time after the explosion, the shrapnel is flying outwards from the center of the explosion. If you pick a piece of shrapnel and follow it, you'll see that pieces next to it are not really moving away from it very fast, whereas pieces on the other side of the center are moving away from it very quickly.

Now let's go back to space. If the universe was expanding outwards from a center, then you would be able to observe differences in the large scale motion of galaxy clusters depending on which direction you look. But when you look out, you find that everything is moving away from you in the same manner on large scales.

That seems to imply that your galaxy is the center of the universe. But every galaxy sees the same thing, and so every galaxy would conclude that they are the center of the universe. So there are 'centers' all over the place, which isn't really compatible with the idea of a center.

[Dhrizzle]

I also have a question. If we do exist within an expanding universe, how would the universe not be expanding outwards from a center?

It is, but just like with a balloon the centre it expands from is not on the "surface", and from the perspective of an observer on the "surface" it seems as if the universe is expanding away from them in every direction, giving the illusion they are at the centre of the universe.

Remember that the balloon analogy only works if you remember the 2 dimensional surface curving through a 3rd really represents a 3D space curving through a 4th.

[Yvaelle]

Legend, regarding your first post - we assume that the Conservation of Mass-Energy occurs for closed systems. What we aren't sure about, and actually are leaning away from - is that the Universe is the entirety of the system. Instead, if a multiverse is possible - then it may also be possible that some energy is able to transition between multiverses. This would mean the universe is an open system, and only the collected multiverses - the Omniverse - is truly closed (or is it?).

We joke about the infinite regress, the "Turtles all the way down" lady - which sounded silly at the time, but more and more seems to be true: both microscopically and macroscopically.

Returning from the philosophic for a moment, since we can't characterize 95% of the energy in our universe (dark energy) and 80% of the mass (dark matter) - and we're increasingly sure our universe is one amongst many - which may be overlapping or interconnected - theoretical physics isn't ready to assume our universe is closed - and observational physics can only tell us that things appear to be moving away from us, and then explain why that might be the case.

If you were to paint grid lines onto the expanding universe, the number of grid squares would not increase - expansion would occur equally everywhere as you say - but the length of the grid lines themselves would noticeably increase. This is because when we say the expansion of the universe is 'equal everywhere' we don't quite mean it the way you may think - at the largest scales - between galaxies and superclusters and stars - expansion rates appears equal.

Yet, all the expansion is actually occurring in the Void - Earth isn't expanding, we are not getting taller or heavier as the universe expands - what we mean is only that the infinite black is growing. It could have been pouring into our universe through a reverse black hole - which would cause unequal expansion - and yet that is not the case - all of the Void is expanding at an equal rate - but the observable matter within the universe is not expanding.

[Garnier Fructis]

It does as far as i know. Only, there is no way for us to find out where it is because of the universe being so large and its expansion so fast that its expanding faster then the speed of light. And regardless of where in the universe you are everything will flow away from you. Similar to a grid on a ballon but applied to a 3d model (immagine that ballon grid going inwards aswell to the center. all those dots in 3 dimensions whould be flowing appart from eachother. and if ur anywhere inside the ballon all you whould se are the closest dots that arent yet far enough away to have gone beyond your maxviewdistance)

All observations indicate that this isn't the case, but what if it was true?

A center would imply the existence of a preferred frame of reference, and I believe this violates the general principle of relativity.

[Dendrek]

It does as far as i know.

Your explanation above is reasonably good, and also proves this statement to be false.

If you take an infinite number of blocks, all the same size, all evenly spaced across an infinitely sized table, and then spread every one of those blocks out one meter from its neighbor, then from the perspective of any block in that set, every other block will be moving away from it. From its vantage point, it is at the center of the expanding universe. And yet, from the perspective of any other block, that other block would think it's the center. If the universe is infinite, there can be no center of the universe.

As far as the balloon analogy is concerned: that analogy only applies to the surface of the balloon. We pretend that the surface of the balloon represents space, and as long as we do, the analogy is pretty consistent. If we start also talking about the inside or the volume or the center of the balloon, the analogy breaks down and becomes meaningless. There is no "center" on the surface of a balloon.

[Garnier Fructis]

It is, but just like with a balloon the centre it expands from is not on the "surface", and from the perspective of an observer on the "surface" it seems as if the universe is expanding away from them in every direction, giving the illusion they are at the centre of the universe.

Remember that the balloon analogy only works if you remember the 2 dimensional surface curving through a 3rd really represents a 3D space curving through a 4th.

No, you're attributing more to the balloon analogy than you should. The balloon analogy does not imply the existence of a center. Moreover, in the bolded part of your post you highlighted the glaring issue with trying to attribute a center to the universe in this fashion: it exists in a higher dimension.

Yes, spacetime is 4 dimensional. But there are only 3 space dimensions and 1 time dimension. The center would have to be in a higher spatial dimension, of which there are none to be found in any established science.

As I see it, there is no scientific or observational motivation to declare the existence of a center. Especially when doing so requires one to speculate and invent things that can't currently be tested.

[dak1]

So if we hypothetically assume that our 3-dimensional space is fully divided into cubic volume elements and demand that no mass ever leaves or enters this lattice system (i.e. mass should overall be conserved), then it is obvious that the density can not decrease in all volume elements simultaneously.

Space itself is expanding.

Energy and momentum evolve in a precisely specified way in response to the behavior of spacetime around them. If that spacetime is standing completely still, the total energy is constant; if it’s evolving, the energy changes in a completely unambiguous way.