[thedoc (OP)]

David Spence asked the Naked Scientists:
   My question is quite simple, but maybe difficult to answer?

What exactly is Space?

This, I think, is a Chicken and the Egg scenario. Does Space require Space before it can expand?
What do you think?

[Scott Mayers]

I think of most physical reality as merely "information" manifest in some way, like computer program structures using arrays. Each array (list of numbers) may represent some dimension, that increasingly represents newer dimensions when they become complex.

As to spacial expansion, usually the fourth dimension is treated as 'time'. I prefer to think of space of the first three dimensions limited to a relative 'static' frame, like a single picture. The fourth to me is EXPANSION, which would also be a 'cause' of time and energy at its first phase. But if the information of expansion can't come in as the full spherical idea of a balloon we usually imagine, then it may still be a product of the one of the infinite possible 'linear' directions that add information in lines, since they don't have to act in conflict like 'solids' might. It may be possible too to be planes. But even in that, the information would be contradictory in that it would either have to accelerate information or spread it out thinner as it expands.

[zx16]

Why should we think, that there's a thing called "space" which is "expanding"?

All we observe from Earth, is that galaxies seem to be moving further apart from each other.  Couldn't they do this, by just travelling in opposite directions?

I mean, suppose you look out of your window, and observe two people in the street moving further apart from each other. Do you think: "Ah, the "space" between them must be "expanding".

Or do you think, "They're walking in opposite directions".

[Scott Mayers]

The expansion is to every direction away from you, not simply in a linear bipolar perspective. The question is to where could such space expand into if space is moving away from every other point.

As to the BB interpretation, which treats this space 'finite' at any given moment (a freeze framed perspective), matter stays one 'size' (quantity or mass) and space grows (from nowhere). The same interpretation is equivalent to imagining a fixed space in which space is fixed in time but matter shrinks. This is the problem the OP is questioning. It's not about a simple linear perspective as in the example you give.

[zx16]

Thanks Scott Mayers,  It seems to me that a "simple linear perspective", as you put it, is all we can see from Earth.

We look at the night-sky, and see galaxies apparently moving apart from each other.  Actually, we don't even see that.

What we see is displacements of lines in spectroscopes, the "red-shift", and assume it's caused by the galaxies moving.  Whether that assumption is correct who can say. I wouldn't bank on it.

Couldn't someone just as well argue, that because we see a displacement of the Sun in the sky, from rising in the East to setting in the West, this is caused by the Sun moving round the Earth.

[Scott Mayers]

Thanks Scott Mayers,  It seems to me that a "simple linear perspective", as you put it, is all we can see from Earth.

We look at the night-sky, and see galaxies apparently moving apart from each other.  Actually, we don't even see that.

What we see is displacements of lines in spectroscopes, the "red-shift", and assume it's caused by the galaxies moving.  Whether that assumption is correct who can say. I wouldn't bank on it.

Couldn't someone just as well argue, that because we see a displacement of the Sun in the sky, from rising in the East to setting in the West, this is caused by the Sun moving round the Earth.

"Linear" if we treat EACH direction away from us. I'm confused at your interpretation of the fixed Earth. But  if you are thinking that it 'appears' that we are at the 'center', this is true by the distant stars. But this appearance is equal anywhere in the universe. That is, if you were at any point arbitrarily in the universe, all stars and galaxies at this very moment appear to be accelerating away from us.

[I tried to link you to a demonstration but this site for some reason doesn't allow it and I don't know what is allowed to upload for online images regarding copyrights here.]

[zx16]

As regards the "linear effect", obviously, we can't be sure that it's because our position is at the "centre".

But we do observe things going away from us in all directions, "as if" we were at the centre.  How to we know that our observations aren't true, and we really are at the centre?

You claim that it's just an "appearance", which is equal anywhere in the Universe.  But is that claim based on fact, or just an idea?

[yor_on]

It depends on definitions. 'Space' is in a accelerating expansion according to main stream physics. doesn't matter its possible magnitude for that, how can it 'expand' at all? We define 'space' relative matter, them becoming the 'buoys' that gives us our means of measurements. And as it expands, does it cost 'energy' to do so? or does it go the opposite way? It's one of the weirdest things I know of, presuming 'space' to exist that is. It's about how 'real' the 'geometry' we see is to me. There has to be some way to make sense of it, and I would look at geometries for it.
=

There also is the idea of us 'shrinking'
Yes yes y, No, it has to do with what we observe to be the SpaceTime geometry, as I see it.
although I don't know how.

[zx16]

Perhaps the whole trouble is linguistic, and simply results from our having the word "Space" in our vocabulary.

"Space" is an impressive and nice-looking word, and quite smooth and easy to say.  That encourages us to believe that "Space" is a real, independent, slightly esoteric but physical thing, which can "expand" or "contract".  So we  talk about the "Expansion of Space", without feeling troubled.

However in most cases, can't the word "Space" be replaced by the word "gap".  This is a commonplace word which just means the separation between things. 

I mean, if instead of saying: "There's Space between the galaxies", we said: "There's a gap between the galaxies", would that make us think that there's an actual thing "Gap", a physical thing, an entity, which can expand by itself?

[nilak]

The only way I can see this is that the universe which contains entities and information has a certain size and a shape close to a sphere. Everything within the limits of the Universe moves folowing the internal geomety. Outside of it there is space to expand into. This space has no geometry until it is occupied.

If we imagine another universe near ours, then the space between them could have for example a flat geometry. This scenario is too complex and less likely. Flat geometry, would allow infinite relative speeds between universes. Another possibility is that space between them is the same geometry outside and inside the universe.

[puppypower]

There is than one way to get the same apparent expansion in space-time.

Say we started with mass M confined to a given volume V. If I was to explode that mass so the volume doubles; mass density halves, space-time will expand around the mass due to GR. On the other hand, if we started with M confined to volume V, and expanded space-time, so the apparent volume doubles, it would look like the mass is expanding. Both scenarios look the same, but cause and effect is opposite in each case.

We can expand a mass; bomb,  to make local space-time expand in the lab. But has anyone done the opposite and expand space-time in the lab, first, to show it is possible for this to lead the mass? We do this on paper with dark energy, but dark energy has not been seen in the lab to prove it is real and not just a placeholder.

Say we had a uniform cloud of water vapor, that occupies volume V. Next, we cool the cloud and allow the water vapor in the cloud to condense into millions of little water droplets. This will alter the space-time profile of the cloud.  Space-time will contract, at each water droplet, compared to the original uniform cloud due to increases mass density. While space-time will expand the space, between each droplet compared to the original cloud, due to lowered mass density in the dead space.

[jeffreyH]

Space is another way to say capacity. How much can you fit in. If we inflate a balloon the volume and capacity increase. However, capacity and density complement each other. As mass clustered together after the big bang the gaps in between those clumps enlarged. At this point in the evolution of the universe that is no longer a sufficient description to explain expansion.