Ever expanding universe

ziggy

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Son Goku

Nothing.

General Relativity describes the Universe with regards to its own intrinsic geometric properties.
So the expansion doesn't make reference to anything outside the universe.

Think of the Universe as the Earth.

Each Longtitude up from the south pole to the North pole is the Universe at a given time.
The South Pole is the Big Bang for Example and the North Pole is the Big Crunch.



Each Circle is the Universe at a different time.

The Universe at a given time (Each circle) gets bigger away from the South Pole.
To us this looks like Inflation because we crawl up each bit of longtitude and see the circle getting bigger.
In reality the Universe was always been this shape and is static.



The Circle seen here would be the Universe at its maximum size, half way between the Big Crunch and Big Bang.

As you can see, whether there is space outside the Earth or not doesn't matter when discussing this.
A similar case applies to the Universe.

(I know there doesn't necessarily have to be a Big Crunch, but this is just to make the Example simpler.)

ziggy

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causal

ziggy67 wrote:

If the universe is endless and a vaccuum then what is it expanding into?

It's not a vacuum - we're all in it

causal

ziggy67 wrote:

If the universe is endless and a vaccuum then what is it expanding into?

Isn't it a contradiction, paradox at best, to consider the universe is both endless AND expanding.

causal

Son Goku wrote:

General Relativity describes the Universe with regards to its own intrinsic geometric properties.
So the expansion doesn't make reference to anything outside the universe.

Then it would appear that General Relativity doesn't make reference to the question asked.

As you can see, whether there is space outside the Earth or not doesn't matter when discussing this.
A similar case applies to the Universe.

But it does matter when someone asks a specific question about the 'space outside'.

causal

Son Goku

causal wrote:

Then it would appear that General Relativity doesn't make reference to the question asked.

causal

No, thats the answer.

Since the expansion doesn't discuss any outside space in which we are embedded, whether there is an outside space or not doesn't matter.

In other words, it's not the sort of regular expansion into something else.

As I pointed out above it has "always", in a timeless sense, been this shape. Expansion is an artifact of us seeing spacetime, one 3-space slice at a time.

Professor_Fink

causal wrote:

Isn't it a contradiction, paradox at best, to consider the universe is both endless AND expanding.

Expansion means that the distance between points in the universe is increasing, not that the universe has a measureable radius that is changing.

Son Goku's explanation is pretty good though.

To accurately describe what is happening you need to use quite a lot of differential geometry, so it can often be hard to explain these things to someone who may not have a background in the area.

Son Goku

Exactly, the simplest way to describe what is truly happening is to use many Schwarzschild zones fitted together to make a closed Universe.

Using this you could see what expansion truly means, but it is very difficult to describe, if you aren't well versed in vanilla General Relativity.

As Professor_Fink said.

causal

*returns from hibernation*

Can I ask what ye mean when ye say "Universe"?


causal

Son Goku

The Space-Time manifold we inhabit, be it multiply connected or not.

causal

OK, because what I find really interesting is 'space', as opposed to the Universe as you mean it; or the General Relativity model defines it, whichever.

By 'space' I mean a 3D volume:
- space doesn't need matter
- space doesn't need energy
- space doesn't need time
- space isn't created (there's nothing to create)
- space has no boundary
- space exists (we're here)

What's a mind bender is:
- how can space exist? (why is there space at all)
- how could space not exist? (even if there was nothing - that is space)

Any thoughts? Please feel free to comment.

causal

Son Goku

causal wrote:

- space doesn't need matter

I wouldn't say that with confidence.

causal wrote:

- space doesn't need energy

Well space does posses energy, whether it needs it is debatable.
(Over the appropriateness of the word need)

causal wrote:

- space doesn't need time

It most certainly does, they're the same thing practically.

causal wrote:

- space isn't created (there's nothing to create)

Space isn't a nothingness, it is a deformable entity, that has definite existence and could very well have been(in a atemporal sense) created.

causal wrote:

- space has no boundary

Einstein's Boundary condition, it could very well have a Boundary.

causal wrote:

- how can space exist? (why is there space at all)
- how could space not exist? (even if there was nothing - that is space)

A Nothing (Nullness) is definitely not the same thing as space.
What exists outside the universe is a nothing, the universe however is made of space, which is not a nothing.

causal

Son Goku wrote:

A Nothing (Nullness) is definitely not the same thing as space.
What exists outside the universe is a nothing, the universe however is made of space, which is not a nothing.

OK. Maybe I should say 'a space' in a mathematical sense meaning a 3D volume; rather than 'space' in the physical sense.

So 'a space' may contain space, or it may contain nothing (nullness), or it may contain matter, or energy etc.

Rehashing my last post using 'a space':
By 'a space' I mean a 3D volume:
- 'a space' doesn't need matter
- 'a space' doesn't need energy
- 'a space' doesn't need time
- 'a space' isn't created (there's nothing to create)
- 'a space' has no boundary
- 'a space' exists (we're here)

What's a mind bender is:
- how can 'a space' exist? (why is there 'a space' at all)
- how could 'a space' not exist? (even if there was nothing - that is 'a space')

I'm not acting the eejit here, I think this is as fundamental as natural philosophy gets.

causal

Son Goku

Ah, cool.
Sorry for misunderstanding you there.

You mean flat Euclidean 3-D space as an mathematical abstraction.

I've read that at the moment the the idea of an infinite geometry is something which is troubling matheticians.
Both infinite in extent and in detail.

Apparently there are attempts to show a space which limits itself naturally because it is based on Prime numbers.

Don't know much more about self-limiting geometry than that, I'll found out and come back when I do.

causal

In fairness the misunderstanding was down to my not using correct terminology, my physics is quite rusty
If you do come across more info that'd be cool

causal