Universe Expansion: Does Less Dense Mean Boundaries?

[MagneticMagic]

TL;DR Summary

General question around expansion

If the universe is "expanding", does that mean less dense, or does that mean there are measurable boundaries?

With very far away galaxies that stay "alive" for a "short" time, is most of the very distant observable-from-earth universes now gone?

 

[Ibix]

If the universe is "expanding", does that mean less dense,

Yes.

does that mean there are measurable boundaries?

Not that we're aware of. And any boundaries raise the question of what's outside them, so no boundary (either infinite or finite but closed) is the simpler model.

With very far away galaxies that stay "alive" for a "short" time, is most of the very distant observable-from-earth universes now gone?

I don't know what you're trying to ask here. But it's worth noting that the universe seems to be pretty much the same everywhere, so far away galaxies are expected to be like nearby ones.

 

[MagneticMagic]

To have expansion you need measurable boundaries. I can't find a way around this. If I look at the stretch explanation, like a rubber baloon, a reference point on the surface would see things around it "expanding" away, but this does take away the actual balloon boundary envelope.

If on the other hand I define space-time as being everywhere without a boundary, then all I get is a system that is becoming less dense.

If yet on another hand I look at space-time itself is stretching out without boundaries, things moving in the space-time can appear to be moving faster than light.

And then I wonder about dark energy, massless energy. There's a whole bunch of study around inflation to explain how some galaxies are so far away when there was not enough elapsed time since the "big bang". Why can't massless dark energy travel faster than light? If we follow a BB theory could we not have large sums of dark energy zip out into the abyss of space-time faster than light, magnitudes faster, and then some or that dark energy coalesces into galaxies far far away?

I can't seem to grasp why BB theory wants to wrap in it space-time. Why is the BB simply not an event of energy (mass and massless) in some infinitely large pre-existing space-time?

 

[Ibix]

To have expansion you need measurable boundaries.

Bang a line of stakes in the ground. Come back in a little while and see if the stakes are further apart. If they are then the line is expanding. No need for any boundary.

I can't seem to grasp why BB theory wants to wrap in it space-time. Why is the BB simply not an event of energy (mass and massless) in some infinitely large pre-existing space-time?

Because it's a result of general relativity, which is a theory of the dynamics of spacetime. The big bang cannot be an event in a pre-existing spacetime because if it were it would have a center, and it does not. This is one of several reasons why your speculation about dark energy is baseless.

 

[jim mcnamara]

Thread locked.