Space before spacetime or not?

[bogie]

As I understand it the universe behaves like a plum pudding so that the dough (space) of the pudding expands whilst the plums just move with the dough. So in this scenario the galaxies in the universe move with space rather than through space. If' date=' however, the universe is expanding into already existing space then the galaxies would be moving through space with respect to each other. This relative motion would have the effect of breaking time symmetry so that the time would be desynchronized in different galaxies and it would be meaningless to talk about the age of the universe since no two galaxies would agree from their own perspectives. Also galaxies would increase in speed through space the further they were away from us so that galaxies a certain distance from us would recede at speeds greater than the speed of light which is ruled out by special relativity.

 

R[/quote']I agree with all that. From within the big bang there is no detectable center or edge. Galaxies are moving apart consistently in all directions.

 

The reason it appears that way is due to the curvature of space. Galaxies are moving away from each other in curved paths due to the mass/energy/motion of the rest of the finite universe. Light coming to us from those galaxies is curved as well.

 

Because the universe is expanding, it is increasing in volume, and any volume has dimensions. We just can’t see the surface (edge) or detect a center because of the curvature of spacetime. There is an edge or surface leading the expansion. The curvature of space at the “edge” is so great that light coming from there covers an arc and is red-shifted so it appears further from us than it really is.

 

It wouldn’t matter if we were expanding into existing empty space or if space was being “created” by the expansion. It would look the same from inside.

 

If anybody can refute this with a link from an accredited source or even a brief explanation of why it is not plausible, I would appreciate it.

[bogie]

To relate what Rog replied from another board:

 

Not quite sure what you are getting at' date=' so just to recap: The view of big bang cosmology is that the universe does not have an edge or a center, because if it did it would not be homogeneous and isotropic. The three possible topologies are explained on many web sites – Wikipedia has a good article http://en.wikipedia.org/wiki/Shape_of_the_universe.

 

If the universe has no edge then it doesn’t need to expand into existing space or create new space to expand into but rather its increased volume comes only from the swelling of its internal space as described in my previous post.

 

If the universe did have an edge how would space differ on either side of the edge? What would cause the curvature of space to be extreme close to the edge as you mention - Wouldn’t that imply a much greater mass density near the edge of the universe than in the centre? Also I note that the current best guess from observations is that the universe is flat or very close to flat and so has no/little overall curvature.

 

R[/quote'] Stick it out with me until one of us gets through to the other.

 

Let me rephrase your recap and see if you agree. The view of the big bang cosmology is that the known universe does not appear to have an edge or a center. It appears homogeneous and isotropic because it looks very much the same on a large scale from any vantage point within it. Based on the cosmological principle, also known as the vacuum energy density of the known universe, the shape of the universe is either open like a saddle, flat, or closed like a sphere.

 

Based on General Relativity, mass/energy in motion causes a curvature of space around the mass that is often referred to as gravity. More massive objects curve space to a greater degree.

 

Even though the curvature diminishes, like gravity diminishes, based on the square of the distance, two bodies orbiting each other create a combined curvature. Multiple bodies like a solar system plot out a complex curvature as the orientation of objects in the system move. A galaxy curves space around itself as a result of the complicated pattern of the curvatures of objects in the galaxy.

 

If you think of our known universe as a whole, the curvature of space around it would be great, especially relative to the zero curvature of empty space. The empty space outside the known universe would express no curvature because there is no mass.

 

Now bear with me while I talk about a galaxy near the outer edge of the known universe. Light from such a galaxy would try to head out into empty space, but light bends with the curvature of space. The light that tries to head out to empty space would be curved right back into the known universe. By the time that light is observed from within the known universe it has traveled and returned on a circuitous path. The long curved path of the light as it returns to the observer in the known universe will appear red-shifted and will appear to be coming from a place that doesn’t actually exist, and from a direction far removed from the location of the galaxy where the light originated.

 

So we could be looking out into empty space beyond the known universe and see light coming to us that was so red-shifted that we would think that the objects were billions of light years out there in the direction we were looking. It is an illusion due to the curvature of space and the characteristic of light to bend.

[bogie]

And Rog came back with:

 

I believe this is roughly what happens if you were to shine a light out from inside the event horizon of a black hole (although to be honest I’ve never met anyone who’s done this) i.e. the light is unable to escape and follows the extreme curved space back into the hole. However' date=' black holes are almost unimaginably dense – a teaspoon of material might weigh billions of tons. On the other hand the matter in the universe as a whole is incredibly sparse. If you model the matter in the known universe as a sphere with a density and radius being the current best estimates for the known universe and put it into empty space the gravity (and the associated curvature of space) surrounding the sphere would be tiny. Although the total amount of matter in the sphere is large the average distance of each particle of matter from any point on the surface of the sphere is huge (equivalent to having all the mass concentrated at the center of the sphere) and as you mentioned gravity diminishes with the *square* of distance. Any light beam emitted in an outward direction from near the edge of the sphere would just travel straight outwards into empty space.

 

R[/quote']Your logic is irrefutable.

 

But light emitted from a galaxy at the supposed “edge” of the known universe would be emitted in all directions. Much of the light would be emitted inward, some would be along the edge, and somewhat tangent, and some would emitted only slightly inward or slightly outward. These are the photons that are the best candidates to be curved back at us.

 

There is also a compounding factor when dealing at the edge. The area of the surface of a sphere means that there is an enormous source for the tangential light. Beams emitted at a low angle relative to the surface from anywhere near this vast surface area could be captured and redirected back into the sphere at any other point on the sphere.

 

The extremely low density of the universe is certainly a fact. The extremely weak curvature in the vicinity of the surface is certainly a fact. But if the returned photons emitted by surface or near surface galaxies equals only 54% lets say of the emitted light, there would still be highly red-shifted return light that would appear to be coming from a place that is actually empty space. If we were looking into that empty space we would see highly red-shifted light from galaxies that are actually around the bend.

 

I haven’t thought much about the particle horizon as to how it might affect this scenario. Do you think there could be some effect that the horizon particle environment could play in bending light back toward the known universe?

[J.C.MacSwell]

 

Let me rephrase your recap and see if you agree. The view of the big bang cosmology is that the known universe does not appear to have an edge or a center. It appears homogeneous and isotropic because it looks very much the same on a large scale from any vantage point within it. Based on the cosmological principle' date=' also known as the vacuum energy density of the known universe, the shape of the universe is either open like a saddle, flat, or closed like a sphere.

 

.[/quote']

 

Any vantage point at rest wrt the CBR.

[bogie]

Yes, correct. I see the difference. It brings to mind the peculiar velocity, doesn't it?