Do Black Holes Equal Universes?

I think it does. In the analogy of the cannonball, the cannon would be at the centre of the universe. If there was such a centre, where all matter exploded outwards from, then there is pre-existing space in order for the matter to move into. Not only is there a centre, there would be a leading edge or boundary of matter, beyond which is empty space. The most powerful telescopes do not see empty space at the limit of observation, they observe the cosmic microwave background radiation. There is no empty space waiting for matter to whiz through.

While it is true that you can think of each point in the universe in a relativistic sense as being the centre of the universe, I do not think the analogy of all matter (cannonball) exploded out from a source (cannon) is compatible with this. If the Milky Way can be considered the centre of the universe, then where is the cannon that the Milky Way came from? If it is equally valid that Andromeda is the centre of the universe, where is its cannon etc.

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It is the distance between sufficiently distant galaxies that is expanding. In the analogy of the cannonball, there is no explosion in the cannon that propels the cannonball. The distance between the cannon and the cannonball grows over time. There is no energy imparted from the cannon to the cannonball. The cannonball becomes more distant from the cannon due to an unseen force.

It is becuase that it is correct to think there is no true centre, that removes the idea that there is an outward explosion of matter in pre-existing space.

This is why I was unsure if KE and PE could be used.

Yes, I admit I struggled with that problem as well. You effectively need to translate a 3D analogy into a 3-plus-whatever D reality. And it is not certain what the effect would be. That's the hurting head part.

Still, questions remain:
Would our universe in the absence of any mass at all revert to a 3D universe?
Is the universe infinite in an absolute sense, or does it only appear so to observers because it is curved and we simply can't see past 14 billion light years?
Or is it strictly finite and will it only become infinite after an equally infinite amount of time has passed?

An infinite number of Planck sized cannon perhaps?

Am I correct in assuming that the concept of seeing the expansion of the universe as a geometrical effect rather than as a force is comparable to what Einstein did to Newtons concept of gravity? To mere mortals falling out of tall buildings gravity still feels like a force(or rather the shedding of accumulated KE upon landing does)

I still need to think about this. If we launch a rocket we can surely use KE and PE, no question. If we consider the orbits of planet around stars and even of stars in galaxies we can still use them. The concepts are even used in the virial theorem. Why would they not work on the macroscopic level?

I still need to think about this. If we launch a rocket we can surely use KE and PE, no question. If we consider the orbits of planet around stars and even of stars in galaxies we can still use them. The concepts are even used in the virial theorem. Why would they not work on the macroscopic level?

I am not saying that they would not work, I am questioning the use of the term KE for two sufficiently distant galaxies that the region of space between them is expanding, as the galaxies have not had energy given to them by an explosion that propelled them outwards from each other, as in the analogy of a cannon and cannonball. Energy is being supplied to the space between these two galaxies and not the galaxies themselves.

Depends on what you call short...

Let me see if I get this right.
Assume two galaxies a long way apart.

1. Because of gravity they will accelerate towards each other.
2. Because of the expansion of space they will be pushed apart.

Both effects would occur simultaneously. The actual redshift measured would be the result of 1+2.

Do you mean by 1+2 occuring simultaneously, that the two galaxies appear stationary to one and another?