[Bill S (OP)]

    Certainly, a common image is that of an expanding sphere, like an inflating balloon.  This, of course, is not to be confused with the “balloon analogy” in which the Universe is represented by the two-dimensional surface, rather than by the whole balloon.

    Let’s think briefly about this visualisation.  Without in any way suggesting that the Universe actually has a physical boundary, the mental image will almost certainly have one.  You, of course, will be at the centre of this sphere, and will see the rest of the Universe moving away from you.  The most up to date figure for the expansion rate of the Universe is “67.3 kilometres per second per megaparsec”.  (A megaparsec is defined as a distance equal to 3.26 million light years.) 

    Just to make the arithmetic easy, let’s say our imaginary sphere has a radius of one thousand megaparsecs.  Obviously, this is much too small, but it is just a simple thought experiment. This means that the imaginary boundary will be separating from you at 67,300 kilometres per second.  The galaxies are being carried along with the expanding space, so any celestial body that is within the last 67,300 km. on this side of the boundary now, will be outside the present boundary position in one second’s time.  It will be in newly created space; space that was not there a second ago; at least, it will be if the expansion of the Universe really does involve the constant creation of new space.
    Think a little more about this, though.  An observer, say, on a planet that has, from your perspective, just passed into the newly created space, will perceive himself as being at the centre of a spherical universe, and will think that you have just passed into newly created space.

    This tells us a couple of things:
 
(1) If the Universe is homogeneous, and in order to do any meaningful cosmology we must assume it is, we cannot define a physical boundary. 

(2) We cannot define any part of the Universe; rather than any other part; as being the newly created part.  This must mean that every part of the Universe is expanding: new space is coming into existence everywhere in the Universe, all the time.  No part is newer than any other part.  Newness is relative! 

   I would welcome comments on these random thoughts.

[yor_on]

1 makes sense to me although you might be able to argue that just as with a balloon there always will be a boundary on which we are 'stuck', or 'inside'.

2 too.

It's a matter of what perspective you use, with the 'eyes of a God', presuming 'it' to be able to perceive everything instantly, everything must happen at a 'same time'. No 'patches of space' can be newer, due to distance etc.

[chiralSPO]

One can imagine an infinite 3D array of points in a crystalline lattice. No boundaries, but you can only see so far from any particular point. This lattice can expand or contract in many ways (depending on the symmetry of the lattice, there could be unique axes along which it could dilate independently (some would not be orthogonal).

As far as I know, the cosmos is not crystalline, in terms of where the matter is. But because it is "uniform" on a large enough scale, we can consider that to define the "unit cell" of our mental crystal.

cubic lattice: http://gph.is/1e5hi7F

[jeffreyH]

Consider a lattice where the cubes are replaced by spheres that all expand at the same rate. They will appear to expand at different rates the further they are away from the observer. Just think about that for a moment. The distances between spheres will appear smaller the further away they are. Since they will have expanded less in the past. Isn't this the opposite of what we see with the current state of the universe?

[chiralSPO]

Consider a lattice where the cubes are replaced by spheres that all expand at the same rate. They will appear to expand at different rates the further they are away from the observer. Just think about that for a moment. The distances between spheres will appear smaller the further away they are. Since they will have expanded less in the past. Isn't this the opposite of what we see with the current state of the universe?

I'm not sure what you mean by "they will have expanded less in the past" ... In both the real world and in the model more distant observations would necessarily be of older events (speed of light and all).

This lattice model accounts for the increasing red-shift of increasingly distant objects, if that's what you mean (if all cubes or spheres are expanding at the same rate, longer distances will grow faster than shorter ones.)

[jeffreyH]

Ok so say the profile circumference of each sphere is detectable. Then the redshift tells us at what distance each successive sphere was when the light left it. We can then determine that spheres were smaller in the past. All well and good so far. However this would not reflect what Hubble found. Am I missing something?

[jeffreyH]

Just to stir the pot a little. As the mass density required to create a black hole falls with increasing mass then the whole universe could be within its own event horizon. Yet all the mass doesn't form a singularity. It is actually expanding.

[yor_on]

I'll stir the pot too :)

Do you mean expanding 'inside' or 'outside'?
Who's the 'observer'?

[jeffreyH]

I'll stir the pot too

Do you mean expanding 'inside' or 'outside'?
Who's the 'observer'?

That is a very good question. I am currently in a cinema coffee shop so I will make a better reply later.

[Bill S (OP)]

Thanks for the comments, folks. There’s quite a bit to think about there.

A related thought towards which I was drifting was this:

If the surface of last scattering is the furthest back we can “look” in terms of the age of the Universe; then whenever we look towards it, we are looking towards the Big Bang.

 Every direction in which we look is towards the surface of last scattering, and therefore towards the BB.  This will apply to observations made from any, and every, point in the Universe.
 
Is this a reasonable response to the question as to where the BB happened?
 
It seems to establish conclusively that it must have happened everywhere. 

[Bill S (OP)]

presuming 'it' to be able to perceive everything instantly, everything must happen at a 'same time'.

If everything happens simultaneously, time is not involved; everything simply “is”.  Surely. This would describe infinity/eternity. (?)

[sunshaker]

I'll stir the pot too

Do you mean expanding 'inside' or 'outside'?
Who's the 'observer'?

We are the observers, we stand between above & below
I see this universe, as a quark within a proton, (proton = multiverse), meaning we are walking talking, multi-versal entities.

[jeffreyH]

I'll stir the pot too

Do you mean expanding 'inside' or 'outside'?
Who's the 'observer'?

I am going to revisit my book on galaxy formation. There are some views of the early universe that are surprising.
https://www.space.com/866-surprising-view-early-universe.html

[Bill S (OP)]

Just to make the arithmetic easy, let’s say our imaginary sphere has a radius of one thousand megaparsecs.  Obviously, this is much too small, but it is just a simple thought experiment. This means that the imaginary boundary will be separating from you at 67,300 kilometres per second.  The galaxies are being carried along with the expanding space, so any celestial body that is within the last 67,300 km. on this side of the boundary now, will be outside the present boundary position in one second’s time.  It will be in newly created space; space that was not there a second ago; at least, it will be if the expansion of the Universe really does involve the constant creation of new space.

I suppose the last sentence would be correct if, and only if, new space were being created only at what we might consider, from our individual perspective, to be a boundary.  On reflection, such seems unlikely, because each, and every, observer sees a boundary in a different place.

However, this means that if, potentially, there is an observer at every point in the Universe, there must be a perceived boundary at every point, also.  Therefore, if new space is being created at "the boundary", it is being created everywhere in the Universe, so no part can be defined as newer than any other.

[jeffreyH]

We can equate new space with new vacuum. Since the vacuum can be considered to be a condensate then this is fine. Since adding or removing from a condensate doesn't change the condensate. It just gets bigger.

[Bill S (OP)]

OK; but the question still remains: does new vacuum appear everywhere; i.e. uniformly across the Universe?

To this is now added: What happens to the vacuum energy when new vacuum is created?

1. Is preexisting vacuum energy diluted?
2. Does the new vacuum come with its own vacuum energy?

[jeffreyH]

OK; but the question still remains: does new vacuum appear everywhere; i.e. uniformly across the Universe?

To this is now added: What happens to the vacuum energy when new vacuum is created?

1. Is preexisting vacuum energy diluted?
2. Does the new vacuum come with its own vacuum energy?

The vacuum shouldn't change. That is the nature of a condensate. What exactly is the sum of all the vacuum energy? That is a good question to research.

[Bill S (OP)]

The vacuum shouldn't change. That is the nature of a condensate.

If new space is created which has lower vacuum energy than pre-existing space, would that not change “the vacuum”?

What exactly is the sum of all the vacuum energy? That is a good question to research.

Run away!

[jeffreyH]

You would still have the same amount of vacuum energy. That is why people think they can get free energy from it. They can't.

[puppypower]

One problem that I see with this visualization and others similar ones for the expansion of the universe, is connected to time dilation experiments. For example, with a moving clock, there will be a permanent change in time that we can measure. However, when you investigate the clock after the experiment the clock does not show a permanent change in size or distance.  The size change was more of a reversible mirage.

In the twin paradox, one twin will have age slower. However, no where is it said the slower aging twin will also remain shorter or thinner due to the motion. The distance contraction is inferred by the light we see. However, it does not persist when we examine the matter. Only time change is permanent.

What this tells me is that time is the dynamic variable and distance is a passive variable. We measure the universe based on its energy/light. We cannot directly measure the matter, but have to infer its properties from energy. With time the dynamic and permanent variable, it makes more sense that the frequency of the energy will show a permanent change. The passive wavelength is carried along, since the product of wavelength and frequency has to remain constant; speed of light. With matter, like two twins, distance and time do not have to multiple to a constant so only time will change, while distance does nothing permanent, when we examine both after the experiment is over. We are confusing a dynamic time shift with motion induced Doppler shift, due to using second hand light data to define the state of the matter, Has any direct matter experiment even been done that shows a permanent change in distance or size? We can take two boxes which are exactly 1 meter cubes. 

The bias of tradition calls the relationship between time and space, space-time, placing distance before time. But this is not what we see when you run experiments with matter.  Time should have top billing. It should be called time-space to set the mind properly in terms of what is the permanent change in the material systems, which give off the energy, by which we infer.

In time-space there are four dimensions of time. There is 0-D, 1-D, 2-D and 3-D time. 0-D time is a point in time such as taken with a photo snap shot. 1-D time is a time line such as connected to velocity; d/t and special relativity. 2-D time is connected to acceleration and therefore all the forces; d/t/t. While 3-D is an acceleration of an acceleration; d/t/t/t. This is connected to the apparent accelerated expansion of the universe in spite of the acceleration due to gravity. What we appear to see is connected to 3-D time leading energy via wavelength.