[acecharly (OP)]

If we took a 1m cube of space would it be bigger the following day as the universe expands?

If this is right does this mean that all the galaxies that they say are moving away from us are not moving away but the space between us and them is getting larger, such as the dots on a balloon would move apart as it were blown up?

I heard the dots on a balloon example somewhere else and thought id try this as i had 5 minutes to spare. Something else i noticed was the dots got larger as the balloon expanded so are all the bodies in the universe also getting larger in the same way as those enlarging dots?

[imatfaal]

If we took a 1m cube of space would it be bigger the following day as the universe expands?

By very little - but you could think of it like that, yes.  However, how do you measure space?  how would you know it had changed? But yes empty space is expanding.

If this is right does this mean that all the galaxies that they say are moving away from us are not moving away but the space between us and them is getting larger, such as the dots on a balloon would move apart as it were blown up?

Both!  They are moving away from us AND the space is expanding.

I heard the dots on a balloon example somewhere else and thought id try this as i had 5 minutes to spare. Something else i noticed was the dots got larger as the balloon expanded so are all the bodies in the universe also getting larger in the same way as those enlarging dots?

  You gotta love experimenters - great idea to actually do it.  It shows a flaw in the balloon as a model.  The dots represent galaxies/clusters/superclusters etc that are gravitationally bound to themselves - but not to their neighbour.  What this means is the the gravity within the cluster is enough to stop it expanding - but not enough to stop it getting further from its neighbour.  If the model was perfect the dots would stay the same size (or actually get smaller!) as the spaces between all the dots expanded.

[acecharly (OP)]

Thanks for a great answer, I should have put a little more thought in to gravity there  ......

Any ideas how this might work in a black hole?

1. Does space contract inside a blackhole?

2. Does matter inside contract in volume relative to the space contracting or just the space?

[imatfaal]

Blackholes are just difficult - we have almost no model of what happens to space (or almost anything else) once you get past the event horizon.  Our best models come out with dangerous infinities and singularities which people tend to think must be wrong.

Whether space contracts at a blackhole - I see no reason that it should, but it is way over my paygrade.  If we look at a blackhole as the end of a spectrum of matter getting denser and denser - with gas clouds at the other end - then matter within the blackhole will be massively compressed.  The density of whatever is behind the event horizon is enormous - that is, if it is matter of some description.

[greeniemax]

Space does expand like distance between two galaxies is increasing at accelerated speed, but the thing is gravity has opposite effect on this expansion, thus galaxies themselves aren't expanding at same speed even if they are expanding at all.

The balloon experiment doesn't give a right picture because are we not on outer edge of universe but somewhere inside, thus it doesn't give right idea.

Black holes are like this more matter you put into it, smaller they get because of increased gravitational pull, this also implies that size of the galaxy might be getting smaller.

[acecharly (OP)]

I have wondered before if the universe is flat and that matter is what gives us the 3D perspective we get. I was thinking about the gravity well that the sun makes in spacetime meaning we orbit and dont fly off. We cant see this warp in space but its there. If space was not flat surely ther would be an upside down well that would cancel out the one we orbit through? or am i missing something?

[MikeS]

Black holes are like this more matter you put into it, smaller they get because of increased gravitational pull, this also implies that size of the galaxy might be getting smaller.

Is this correct?

[JP]

Black holes are like this more matter you put into it, smaller they get because of increased gravitational pull, this also implies that size of the galaxy might be getting smaller.

Is this correct?

No.  The more mass you put in, the bigger a black hole gets (as defined by its event horizon).

[imatfaal]

Space does expand like distance between two galaxies is increasing at accelerated speed, but the thing is gravity has opposite effect on this expansion, thus galaxies themselves aren't expanding at same speed even if they are expanding at all.

  pretty much by definition, if it is a galaxy then it is gravitationally bound and it is not expanding.

The balloon experiment doesn't give a right picture because are we not on outer edge of universe but somewhere inside, thus it doesn't give right idea.

  Much as I will slag off the "dots on a balloon" model - it is not that bad.  Implicit in the model is that it is a 2d representation of a 3d reality.  the first line of any intro to the model should be along the lines of "imagine the entire universe can be pictured as the surface of a balloon"  - you can see that is changing 3 dimensions of volume space into 2 dimension of a surface.  All the dots on the balloon move away from each other - that is the important message, and it works ok.

Black holes are like this more matter you put into it, smaller they get because of increased gravitational pull, this also implies that size of the galaxy might be getting smaller.

mike and jp have already fielded this.

[greeniemax]

Hi JP,

You see black hole is a problem we don't know if should we use QM on it or GR because its too small for GR and because its not a very small atom its not easy to work with QM on it.

You must have heard world "singularity" it means one single entity and possibly its a single atom, we haven't see a black hole directly to know what it really does we have some idea that it might be there and certain features of it.

Its small because its very massive and is crushed under its own gravitational pull, event horizon is where light actually stops moving, it could be really large or it could be really small depending on the massiveness of black hole itself. Thus more matter you put in a black hole its event horizon would increase in size but actual black hole would get smaller.

[JP]

Actually, greeniemax, a singularity doesn't mean "one single entity and possibly its single atom," it means a point where equations of GR yield infinities no matter your reference frame.  In the case of a non-rotating black hole, this singularity is a point and has no size, and no matter how much mass you have, it is still a point of zero size.  In reality, it probably isn't zero size, but we have no idea what size it is or what properties it has, since we don't have a theory covering it and we don't have any experimental observations of it.  We can't say if it get bigger or smaller as we add mass since we don't know anything about what it actually looks like.

On the other hand, we can see the event horizon of a black hole, and that's what defines the "point of no return" when you approach it, and it's the place where Hawking radiation is emitted.  I think most physicists would agree that the best description of black hole size is the event horizon.  We do know that it gets bigger as you add mass.

[yor_on]

Mass is a weird property. Invariant mass is what we can touch, and see but when it breaks down and start to compress it passes from state to state, with different pressure forces as electron degeneracy pressure acting outwards as 'brakes' for the compression. If they will pass what's called the 'neutron degeneracy pressure' which is the last 'brake', creating 'neutron stars', they are well on their way to becoming a black hole.

If you think of a what a sun does, creating all those elements building up matter and us, it does it due to gravity. Gravity acts as a inwards pressure on any object of matter, as a planet. Newton defined it as all planets gravity could be seen as being concentrated to each planets center in his 'spheres', relativity defines it slightly different but on the whole its a valid picture. So our sun is very much bigger than earth, and gravity's 'pressure' is strong making it create a lot of light elements. The bigger the sun the heavier the elements it can produce, a human is made out of a combination of light and heavy elements, so our own sun wouldn't be able to produce us as I understands it. For a human you need suns about  three times, or more, our own sun mass to create those rare heavy elements building us. And a final explosion 'compressing' even more.

So in this case we're in fact discussing 'pressure' as well as gravity but as soon as the black hole is created, its event horizon apparent to an observer, we're only talking gravity again, at least as I understands it. And with the neutron degeneracy pressure passed there is no definition, that I know of, describing what type of 'matter' there should be at that center, if there is any. Think of how those stars create us, and then ask yourself what they used.

Pressure and Energy.
=

That doesn't answer the question of how something as weird as a star can ever exist, of course. But there is a theory called the Big Bang, in where you will find an enormous lot of 'pressure' at that first instant, creating 'clouds' of different elements in space, that after gravitating towards each other, built up mass to then act as 'pressure cookers' aka suns, creating more elements. And so kick start life..