So what is this other dimension? Or is that the thing we don’t know?

The real head mash is when you realise that beyond the visible horizon of space, the speed of the universe’s expansion exceeds the speed of light (as it did in the period of cosmic inflation). There’s a ‘bubble’ of the current visible universe; behind the bubble’s edge (or horizon) everything is moving away at less than the speed of light and so light, whilst taking its sorry ass time, does reach us. But anything beyond that bubble’s horizon is receding at faster than the speed of light and so is eternally lost to us. Of course, with the rate of expansion speeding up (something we’ve only recently discovered) that horizon is actually shrinking. Ultiamtely, every part of the universe will be invisible to every other part of the universe and there will be nothing but an eternal long dark night of the soul.

eternal long dark night of the soul

Just how eternal? Roger Penrose’s speculations suppose that big bangs could cycle, triggered once the universe becomes an energy only entity after all the black holes have evaporated.

So what is this other dimension? Or is that the thing we don’t know?

You need to read Flatland, or better still a book about that book called Flatterland by Ian Stewart which is where I read about it.

You might as well ask a cartoon character what depth is. There could be another dimension of space just like the three we know, but we just can’t see it. It isn’t anything in particular, it’s just like length and width.

If we measure in cm:

The area of a rectangle is width X length in cm2

The volume of a rectangular prism is width X length X height in cm3

The hypervolume of a rectangular hyperprism is width X length X height X (dimension 4) and is measured in cm4

And so on. The maths works, just extrapolate. You don’t have to visualise it. This is why maths becomes so important in physics, there is no such thing as ‘weird’ in maths.

We see homogeneity in the observable universe. No direction shows any difference in content. The CMB is as ancient in all directions.

If there is a central origin in 3d space (4d spacetime) it isn’t within our observable universe.

Ahhhhhhhh we are talking at cross purposes. I wasn’t talking about a “central origin”. I was talking about a centre (as in “does the universe have a centre”, see above). As far as I can see, the pudding is proof by counter-example that the statement “something which expands homogeniously (like we observe in the universe)* cannot have a centre” is not true. Because puddings do, and do.

*At a local enough level, ie assuming you can’t see all the way to the edge. If there is an edge, which appears to be the case for all known puddings, but not universes.

The maths works, just extrapolate. You don’t have to visualise it. This is why maths becomes so important in physics, there is no such thing as ‘weird’ in maths.

This is why I got frustrated and gave up studying physics. I’m lazy at maths but could deal with things I can visualize (so the maths maps onto something I can relate to). When there’s just maths but nothing I can visualize, I struggle.

As far as I can see, the pudding is proof by counter-example that the statement “something which expands homogeniously (like we observe in the universe)* cannot have a centre” is not true.

I think you’re making the same mistake. As I understand it, depending on the density, the universe is warped in a dimension that our 3D experience can’t describe. It can be described mathematically though. The universe doesn’t have an edge, it’s finite but unbounded because it curves back on itself in a higher dimension.

@thols 2 I agree, I am talkng about the nature of expanding puddings, not of the universe.
In particular I am responsing to @seosamh77:

No, because measurements tell us that every point is expanding the same way. If there was a centre it would be easy to at least locate the direction of it, but in that sense the Universe is uniform, from what ever point you measure it.

and

But if you could see the currants and the seeds, and they were still in their expanding state, you could calculate where the original dough ball started.

I do not think this is the case. The fact that everything is expanding evenly does not prove there is no centre*, because you can’t identify a centre if you are somewhere in an expanding “pudding” (assuming you cannot see as far as the edge). Same goes for the football/balloon, the surface doesn’t have a centre but the fact that all the surface is expanding evenly is not what proves that. The material could be a flat sheet being being stretched out evenly from the edges by a cunning frame arrangement, if you can’t see that far you can’t tell.

*just centre, not “centre of origin” whatever that means.

The balloon analogy or the cake analogy aren’t actually analogies, they are just representations to try and get a specific point across. As analogies, they don’t work at all, cause something completely different is happening.

You need to stop thinking of the cake as a 1:1 representation of the universe.

When people thought the earth was flat (2D), they all thought they were at the centre. After it was understood to be 3D, the notion of a centre on the surface became meaningless. So it is with the 3D understanding of the universe.

I think pretty much every definition of “analogy” you might find includes something to the effect “this is not a 1:1 representation”. The balloon and the cake are analogies, loose representations which have some similar features (in the present discussion, the relevant ones being that they contain stuff and are capable of expanding).

But OK, lets ignore the cake and talk about the universe. I am saying that the fact that “every point is expanding in the same way” and also that “the Universe is uniform, from what ever point you measure it” (and I do not disagree with either) do not demonstrate (to me) that the universe has no centre (or edge). Other evidence may do so, but not, on its own, the even nature of the expansion.

Or to put it another way, the issue is not what it is (as to which I do not need persuading), but why we think it is like that.

the issue is not what it is (as to which I do not need persuading), but why we think it is like that.

Because the mathematical models that best agree with what we observe point to the universe being finite, but unbounded, as I understand it. It has no center and no edge because it curves back on itself in a dimension that we can’t perceive directly. We can only model it mathematically.

On the centre:

does this work – where is the centre of the surface of a globe?  It doesn’t have one

Av it…

Because the mathematical models that best agree with what we observe point to the universe being finite, but unbounded, as I understand it. It has no center and no edge because it curves back on itself in a dimension that we can’t perceive directly. We can only model it mathematically.

Thanks @thols2. I also appreciate everyone trying to explain “not having a centre”, though I already understood that bit. I suppose the next step would be to find out what the observations you refer to are, but I don’t think this thread would be the place for that.

You can read Wikipedia articles about it but you won’t actually understand why physicists came to those conclusions if you can’t follow the mathematical reasoning. I did well at physics at high-school, did ok at first-year physics at university, but it was obvious I would have to spend two years doing some serious maths study if I wanted to keep taking physics. I was too lazy and was more interested in philosophy of science so I just did a philosophy degree because it was much easier. When you get to things like string theory, you aren’t going to understand it if you aren’t fluent in the mathematical language.

Too right about the maths. One of my lockdown hobbies was to do try to understand a couple of those subjects that I dropped during undergrad final year back in ’92. Starting with Quantum Mechanics. Done a few of those MIT youtube lecture series on all kinds of useful topics, but the quantum one seems to be an entire semester of lectures manipulating the Schroedinger equation. Crazy how the Math(s) courses seem like somewhere between A-level and undergrad physics/engineering, but the quantum physics one feels like math(s) overload and brain explosion.
Some of the Feynmann stuff was pretty good though. Good to see how the understanding and analogies evolved over the years as science progressed rather than all in with current maths.
Still remember my first A-level physics class (although I think it’s absolutely true for Maths too, given that Physics is just Applied Maths). “Physics is a load of b******s! It’s just a load of rules that seem to work”.

I agree (which was why I only wanted to know what the observations were that lead to the conclusion – I know I wouldn’t be able to understand the logic). I studied quantum chemistry at university 40+ years ago. I am pretty sure I remember doing a thing called “solving the eigen functions of the shift operator” (or something like that). Now, not only can I not do it, I don’t know what an eigen function (?eigenfunction) is and, as for the shift operator, not a clue.