What Is the Universe Expanding Into?

[Mark M]

to imagine an entity as vast as the present observable universe to have emanated from a
single point (a.k.a. big bang theory) before which nothing existed;not even empty space
since space is something is a practical and scientific impossibility.if nothing existed before
the big bang what did the exploding matter expand into?--------!

This is totally incorrect. The big bang model doesn't hold that anything 'exploded', just that the early universe was very hot, and very dense. Nor does is attempt to assert the size of the universe, which as phinds said, may be finite or infinite.

See here:

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

 

[audioloop]

So then is it like the universe is infinite and everything inside it, is just moving further apart from each other, that is, if the universe is really infinite.

read this thread

https://www.physicsforums.com/showthread.php?t=632836

 

[Brainiac2]

I didn't say its expanding into anything other than itself, its falling outward.

Its dimensionless, it has no definite size to begin with and therefore its perfectly happy to expand and not get any bigger. I think looking at the properties of an infinite size number and realising you can't make it bigger by adding to it is a good analogy for expanding space. It gets rid of the idea that its expanding into something else.

No matter how much it expands its still the same size.

That's mind boggling!

 

[marcus]

Mark had an interesting point here that I want to bring up again. Most people participating here do I think realize that in standard cosmology the expansion of distances between stationary observers does NOT imply that there is any "surrounding space" that space is "expanding into".
Mark summarized that early on, but also made the additional point that this has nothing to do with the (unnecessary) assumption that space is infinite volume. It is a common misconception that the boundarylessness of space comes from its being infinite. The math model used in cosmo can easily be finite volume and still not be embedded in some surrounding space. I'll quote Mark's summary as a reminder and then a subsequent post regarding this.

... I want to very strongly emphasize a few points.

1. The universe has no 'edge'. This is because we model the universe as homogeneous and isotropic. That is, there are no special places. Also, the universe at large is described by general relativity, which describes spacetime as a smooth manifold, which excludes the possibility of an edge.

So, one possibility is that the universe is infinite, it just extends forever. However, it may also be finite. If this is the case, then it's either simply or non-simply connected. That means it would be like the three-dimensional analogue of the surface of the Earth - it's definitely finite, but it has no edge. So, in a finite universe, going far enough will bring you back to your original position.

So, you can see from that why your question is flawed - there is no such thing as 'outside' of the universe because it has no edge.

2. The universe has no center, and the big bang was *not* an explosion. This will make more sense in a little bit, but it's key to remember that the universe has no center - it expands from every point in intergalactic space. The big bang was the point in time when the entire universe was filled with a hot, dense, plasma and radiation. As space underwent expansion, the radiation redshifted and cooled, and the plasma cooled into hydrogen gas. This then accumulated into clumps, which became galaxies. The space in between these galaxies continues to expand.

So, what does it mean that the universe is expanding? It means that the distance between bound objects (i.e. galaxies) is increasing. The galaxies themselves aren't moving (well, they have a very movement that has nothing to do with expansion, but this is besides the point), they just appear to be because the distance between them is getting larger. So, you can see that 'expanding' in this context is a lot different than how you think of it - the distance in between all galaxies getting larger.

...
I highly recommend reading this article, it should clear up a few misconceptions:

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

Its to do with the properties of infinity.

Infinity +100 = infinity

You can't make it any bigger by adding to it, its already infinite.

So if the universe is infinite in size and its expanding its still the same size (infinite), basically its just expanding into itself.

No, this isn't right. Even if the universe is finite, it isn't expanding into anything. See my above post.

If I understand what he's saying here, Mark is making an important point. It is still an open question whether U is spatially finite or spatially infinite. We don't have to assume infinite in order to avoid the idea of a U with boundary that is "expanding into" someplace that is not the U.
Boundary and "space to expand into" are not logically necessary to what cosmologists mean by expansion.

So if anyone finds this puzzling, here is a good opportunity to get a more accurate idea of what they mean by Hubble law---the pattern of expansion of internal distances--and the related idea that geometry (experienced from within) can be dynamic and not a fixed rigid framework in which things happen. Geometry interacts with the rest of nature.

 

[Lino]

... Just the fact that there is now more distance in between all bound objects.

Mark, I thought that it would be more distance between "unbound" objects. In what sense are the objects bound?

Regards,

Noel.

 

[marcus]

Mark, I thought that it would be more distance between "unbound" objects. In what sense are the objects bound?

Regards,

Noel.

Heh, I think I see what you are driving at, Lino---but what I see is more a verbal ambiguity than a disagreement. A distance between bound objects (in Mark's words) could mean the distance between two objects, each of which is bound, but the two objects are not bound gravitationally to each other.

So the distance could be the distance between two rocks, or two galaxies, or two gravitationally bound clusters of galaxies. Each of them separately is a "bound object" because held together by its own internal forces.

But it's assumed (as I read Mark's post) that the two bound objects do not form a larger bound system. So it is an "unbound" pair of bound objects.

Ordinary language is frustrating, so many chances for ambiguity.

 

[skydivephil]

I thought the best way to define the expansion of the unvierse is the distance between galaxies is growing. It doesn't mean the unvierse is getting bigger (it might have been infinite at the start of the expansion). Also it doesn't mean the distance between objects smaller than galaxies is expanding as these are gravitationally bound. However I have just realized a difficulty here that I am not sure how to square. The propblem is if we define the expansion this way, what do we mean by the expansion before galaxies formed? Woudln't that imply the distance between unbound obejcts is growing and hence invalidate the earlier description or deny that the unvierse was expanding before galaxies formed. Both seem wrong to me although obviously the latter seems a little more absurd. not sure how to make my deifnition correct though.

 

[Lino]

Thanks Marcus (& Mark). Much appreciated.

Regards,

Noel.