[johnnujler]

Forgive my ignorance here. What do we mean when we say the universe is expanding, expanding into what. My untrained brain is trying to look at it like the inflation of a balloon that takes up more space when it is inflated, but if the universe is all the space there is, what would it even mean for it to expand. Is there space outside our universe that is like unfilled or something?

[vecter]

Your balloon analogy is almost correct but it's missing one important piece. In that analogy, the surface of the balloon itself is the entire universe. It is a two-dimensional universe. Imagine that there is no other space except for that surface of the balloon. No emptiness inside and no space outside that it's "expanding into".

The entire space is the infinitesimally thin surface which is the balloon surface, yet it is still expanding. Dots on that surface that used to be X distance apart are now > X distance apart in the future. That is all you would know and be able to measure.

[johnnujler]

Ah! That makes sense. I realise that talking about complex concepts in terms of analogy can be misleading, but is it like the fabric is elastic and long enough and is only getting stretched? Is there a way to reconcile this with the (3 dimensional) inside and outside aspect of our experiences.

[brodie]

I think it’s more like the balloon analogy or baking bread analogy is required because there’s no good way to explain what we observe without either analogies or math or unintuitive descriptions.

That doesn’t mean there can’t be a higher spatial dimension that our observable universe is part of; we just haven’t observed it or can’t observe it.

I guess we’re sort of like ants crawling on the surface of an opaque, seemingly-indestructible balloon. We’re pretty clever ants, but we probably aren’t gonna peer inside the balloon anytime soon.

[dotancohen]

  > That doesn’t mean there can’t be a higher spatial dimension
  > that our observable universe is part of; we just haven’t
  > observed it or can’t observe it.

Maybe we can not observe higher spatial dimensions, but we can observe a force that seems to propagate in higher spatial dimensions, thus there seems to be evidence of those dimensions.

Light intensity, gravity, and electromagnetism decay at 1/r^2, which is the same rate at which the area of the surface of an expanding 3D bubble grows. So each unit of light / gravity / electromagnetism could be seen as taking up a specific "patch" of expanding area.

However, the strong nuclear force decays much faster than 1/r^2. In fact, at some distance it goes negative and then decays back to zero. This could be (wild speculation) a force that propagates in many more dimensions. And it's not the only force that does this, the weak nuclear force also decays much more rapidly than 1/r^2 with distance.

I'm not a physicist and I would love nothing more than to hear what holes could be punched into this hypothesis.

[alok-g]

>> I would love nothing more than to hear what holes could be punched into this hypothesis.

I'm not a physicist either but believe that this is a well-known idea.

[brodie]

You may not be a physicist but you’d make a convincing string theorist!

I say that partly pejoratively, with a Sabine Hossenfelder sort of jocular but judgmental tone.

I’m personally excited to see how astrophysicists will take dark matter out of its darkness and cast it into intuitive lights, like some have with the rising dough analogy for cosmic expansion.

[Razengan]

But what's filling the space between the dots?

Is it new space?

If I have a long ass string, is the string gradually getting longer, or will it break apart?

[vecter]

More space. It's pretty nuts. Imagine you were stretching a rubber band. What's in-between two points on that rubber that was stretched? Just rubber, albeit thinner and less dense.

In our universe, however, the space that's created is the same as the space that was just there. It'd be like if the rubber band stretched, but didn't lose any density while it did so.

Insane, I know.

[Razengan]

So, is it like, the number of pixels on the screen stay the same, but it takes more “ticks” for a variable representing coordinates to go from 1 to 2: x += 0.09 instead of x += 0.1

[bencollier49]

So as the universe grows, am I growing as well?

How much is the space inside me growing on a daily basis?

[bbbobbb]

No. The expansion is only apparent on intergalactic scale. On human scale, and even much larger scales, it does not overcome the fundamental forces. You're not expanding.

[dotancohen]

So, it was the donuts?

[insulanus]

The string won't break apart, because the forces that are holding it together are so much stronger than the "force" that is pulling our universe apart.

Unfortunately, it doesn't answer your more fundamental question of "if I make 2 marks in space, and then those expand apart, what is in the new space, and how do I know they really are farther apart?"

I have never heard a good description of this on human scales, only at stellar and galactic scales.

[bmcn2020]

i guess my question would be about the analogy

in these analogies, the balloon surface is made of rubber, or the stretchy thing is made of whatever stretchy things are made of

is space something tangible? what is it made of? what is its physical property? how can it stretch?

[cygx]

That's where the analogy breaks down. If you take Einstein relativity at face value, there's no priviledged spatial slicing (contrasting with, say, Lorentz ether theory), and the balloon is not really a thing.

The most obvious interpretation of general relativity is in terms of B-theory of time: Spacetime is some 'pre-existing', 'eternal' thing over mich matter is distributed. This also fixes the geometry (things like lengths and angles) via Einstein's equations, which more or less state that energy-momentum ∝ Ricci curvature.

In our universe, that distribution comes in layers, ie there's a spatial slicing where the matter distribution appears homogeneous. In that sense, there is a priviledged slicing, which has the unfortunate side effect of making people forget the lessons of special relativity.

Now, the average density of matter changes from layer to layer, and, if our universe were described by the 'closed' Friedmann model, so would the (finite!) volume of the slice. That change is not arbitrary: The layers can be labelled by cosmological time, and with its increase, the average proper distance between galaxies increases as well. That's called the metric expansion of space, because in our idealized model, the metric (a thing defining distances and angles) within a given slice is just a scaled version of the one in a different slice.

[nathanyukai]

Nobody knows. Dark matter and Dark energy probably, but again no one knows what they are either :)

[aliceryhl]

No, space is not something tangible. It is not really made of anything.

[cstejerean]

You can look at it the other way. Ignore for a moment the surface of the balloon and imagine the insides. As the balloon expands everything inside gets farther from everything else. That’s how I understand the universe expanding.

[ellis-bell]

think more like you're on the surface of the balloon as it's getting inflated. the "fabric" or elastic material on which you're standing is the thing that's expanding while the balloon inflates. you're still constrained to the surface of the balloon, however.