[Lamprey5 (OP)]

How do we know, when we find galaxies moving away at specific speeds, that it is space that is expanding, and not just that these moving galaxies are just moving within space? It seems that a galaxy that is just moving away would look exactly the same as a galaxy that is staying put within space but space itself is expanding.
Similarly, how do we really know that space itself curves in the presence of mass?

[yor_on]

Define a proposition in where you will see every 'point/galaxy' around you, in a three dimensional space, move away from each other 'point/galaxy' at the exact same 'speed', increasing the same from each individual point of view with distance, without using the mainstream proposition please?

When you've done that one try to explain why light 'diverge' in the presence of mass without losing its 'energy quanta' as is easily proven.

[graham.d]

These are quite difficult questions. If the universe was flat Euclidean space then I expect that it would be the best explanation of the redshift of distant galaxies to assume that the galaxies were all simply moving away from a single point (the big bang). However, General Relativity has been proved to be a reliable theory for correctly predicting other observations and so it is usually accepted that a model of the universe has to be consistent with these ideas. The concept of the expansion of space is really based on how models of the universe would fit with GR which leads to matter (strictly energy and momentum) curving space itself and the idea, really a consequence of Special Relativity, that the universe we see is only explicable in terms of 4 dimensions. Conclusive proof is not available, but it is simply that all the evidence suggests that the universe is a whole lot more complicated than what would be simply deduced from assuming that the world was Newtonian. It may be possible to explain one or two observations without GR (e.g precession of the perihelion of Mercury by making some unlikely assumptions about mass distribution) but the weight definitely favours GR being right. It may well be that it is incomplete, and some recent theories suggest that this may be so, but it is very well accepted that it is a big step beyond the Newtonian view.

[Lamprey5 (OP)]

I see. I've read that the mechanism behind the expansion of the universe (space itself) is a source of negative pressure. This is because negative pressure exerts a repulsive gravitational force, causing objects to spread further apart. However the energy or matter (Dark energy/ Dark matter) has not been observed, only its effects through negative pressure on the expansion of the universe.
But even through this explanation, it sounds as if the expansion of space is indistinguishable from the increasing separation of two objects. Unless of course the negative pressure causes spacetime curvature that increases the actual amount of space that exists.

How would an increase in space appear mathematically? I picture the limits of space being infinite, and when a precise volume k is added to this infinite volume of space, you get ∞ + k, for some constant k, as an answer. Is there some theoretical limit to space that one can understand through mathematics?

[ukmicky]

Im not a science guru but are not objects moving away from each other faster than C and as they are and matter cant travel faster than C it seems logical that the space itself must be expanding

[graham.d]

I see. I've read that the mechanism behind the expansion of the universe (space itself) is a source of negative pressure. This is because negative pressure exerts a repulsive gravitational force, causing objects to spread further apart. However the energy or matter (Dark energy/ Dark matter) has not been observed, only its effects through negative pressure on the expansion of the universe.
But even through this explanation, it sounds as if the expansion of space is indistinguishable from the increasing separation of two objects. Unless of course the negative pressure causes spacetime curvature that increases the actual amount of space that exists.

How would an increase in space appear mathematically? I picture the limits of space being infinite, and when a precise volume k is added to this infinite volume of space, you get ∞ + k, for some constant k, as an answer. Is there some theoretical limit to space that one can understand through mathematics?

It is indeed postulated that space is finite and that the dark energy (not dark matter - that is something entirely unconnected with this) results in expansion of the universe thus creating more space and "revealing" more dark energy. There are competing models for the mechanism though still based on the observed accelerating expansion. Spacetime is not flat, although space is (except in local regions around large masses) to a high approximation.

The maths is hard going and to get a good grasp of it you really need about 3 years studying differential geometry and group theory. All the sensible theories are consistant with General Relativity though. A full explanation may only arise with a proper theory of Quantum Gravity of which the main contenders seem to be "braneworlds" (based on string theory) and "Quantum Loop Gravity". I have been reading much about QLG recently and find it quite compelling.

Im not a science guru but are not objects moving away from each other faster than C and as they are and matter cant travel faster than C it seems logical that the space itself must be expanding

That is true. But it is only a model that suggests that objects are moving away faster than C. We can't see them so it is really a postulate to make the models self consistant.

[Soul Surfer]

This is something that is very difficult to explain in simple language and I am not totally confident that I have got it absolutely right.  If the galaxies at a great distance were actually moving at velocities close to the velocity of light like the fastest lumps of shrapnel flying away from a great explosion they would firstly need lots of extra energy to make them move so fast because of the relativistic increase in mass at high speeds.  Also the doppler shift caused by this high velocity would also be subject to relativistic second order effects.  However if it is space expanding that is stretching out the light the red shift is simple and slightly different.  Remember that the accepted name "the big bang" was a derogatory term that was coined by people opposed to the theory that has stuck in spite of it being a very bad description of what is going on.

[CPT ArkAngel]

This idea of the possibiity of space expansion came with the Big Bang Theory which says that everything, including space, time and energy emerged from a single point in space.

If there was no space, it has to expand from that point. But if there was already a universe before the Bigbang and our Universe is just a small part of a bigger universe, the space expansion becomes irrelevant. If space expands, should  matter not expand with it? No one can explain this...

[CliffordK]

Isn't the idea of Space expanding based on the theory of red-shift that is used to determine the distance an object is in space.

I.E.  The more red-shift, the further away a celestial object is believed to be.  But, it obviously confounds velocity and distance which would only be true if everything had a central point of origin, and everything moved more or less in the same direction like spokes in a wheel...  (is Earth the central hub?)

But, then for a spokes in a wheel concept, a "blue-shifted" object couldn't exist....  so Andromeda can't exist.

Are there alternatives to explain the apparent red-shift, distance relation?  For example a stretch or tension on space which might cause a decrease in frequency as light travels through distance?

One other thing to keep in mind.
What we perceive as the edge of the visible universe is light that is (presumably) 13 billion years old, or less than a billion years from the hypothesized beginning of the universe.

There is no way to know what is happening today.

In fact, as I mentioned in a different chain.

If the universe has expanded from a point source to at least 26 billion lightyears in diameter in a time period of less than 1 billion years, then we must conclude that there is either a flaw in the theory or calculations, or that the expansion of the universe was decelerating prior to the emission of the light from the most distant stars and galaxies.

And, if the expansion of the universe was RAPIDLY decelerating during the first billion years, then it would be hard to conclude what is happening on the outer fringe of the universe today.

Could Andromeda and other blue-shifted celestial objects be a sign of a collapsing universe?  I'd rather conclude more complex movements than spokes on a wheel.

[Soul Surfer]

Clifford:    Galaxies have their own independent relative velocities inside clusters on this scale they are of the order of one thousand kilometres per second  (a lot less than light at 300,000).  This is caused by their gravitational attraction and residual angular momentum.    Our galaxy and the Andromeda galaxy are the nearest big galaxies and are very likely to collide eventually because of their gravitational attraction for each other.

The Hubble constant is about 70 km/sec per  megaparsec The Andromeda galaxy is about 0.5 megaparsecs away so the expansion effect is about 35km/s only a few percent of the independent velocities of galaxies.   Any effects associated with the expansion of the universe are much smaller than the local velocities it is only when you get well beyond the local cluster of galaxies and the local supercluster of galaxies in Virgo that the expansion of the universe becomes detectable over the velocities of the galaxies themselves.

[graham.d]

If space expands, should  matter not expand with it? No one can explain this...

I'm not sure this necessarily follows from all the theories. Quantum Loop Theory suggests that space is expanding by adding quanta of spacetime and thereby extending the mesh of quantum foam. It is not as though this foam, and the matter embedded in it, is stretching in any way.

[imatfaal]

If space expands, should  matter not expand with it? No one can explain this...

As an alternative to Graham's answer.  The expansion of space through the an action of a cosmological constant would provide for huge changes over huge distances - but the distances/forces involved over molecular/atomic/nuclear scales are completely insignificant compared to eletrostatic/strong etc.  A metre rule stays the same - but the 10^20 metres it was used to measure a few million years ago has changed substantially. 

[Bill S]

If space expands, should  matter not expand with it? No one can explain this...

Mark McCutcheon devotes a whole book to trying to explain the expansion of matter, but I have to say I was not overly impressed.

[yor_on]

Yes, it's explained. It's gravity. The expansion is constricted through the stronger gravity inside the galaxies. It's not that the expansion isn't working there, but gravity negates it.
==

Maybe one could look at it this way. If space had a density like air we would hear a swooshing sound as it 'space' got 'pushed' away by the forces keeping matter in check Not a very good example, was it.

Or you could look at it as a question about what distance is? Space contain it, but space is also 'nothing at all' at our macroscopic plane. So assume that the amount of space in front of my nose 'expands'. Space having no resistance and no 'friction', how would I ever notice it doing so? And if gravity is stronger than this resistance, which it definitely have to be, then that should be enough to keep the marbles, ah, heavenly objects at place, shouldn't it?

But as Space also have this weird 'distance-thingie' that we all expect, cherish and love, except when in a glass of beer, it will still need to add 'distances' between the galaxies. and now I've explained what I still don't know if I understand It's truly a weird universe.