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Tank_AT OP t1_jc6xv1a wrote

Reply to comment by Aseyhe in Does space expansion occur uniformly in all directions and dimensions? by Tank_AT

That sounds plausible. So to further and solidify my understanding, space itself is not expanding, just that physical entities (e.g. galaxies and therelike) are moving apart carried by their initial momentum.
But what I don't quite understand is the notion that this expansion (or motion) seems to accelarate relatively to each other, to a point where distant objects become unreachable for us even with we could travel at the speed of light. Or am I mixing things up here?

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Aseyhe t1_jc7jfq5 wrote

> That sounds plausible. So to further and solidify my understanding, space itself is not expanding, just that physical entities (e.g. galaxies and therelike) are moving apart carried by their initial momentum.

Exactly.

> But what I don't quite understand is the notion that this expansion (or motion) seems to accelerate relatively to each other, to a point where distant objects become unreachable for us even with we could travel at the speed of light. Or am I mixing things up here?

Dark energy induces gravitational repulsion, which accelerates the expansion of the universe and also creates a cosmological event horizon. Indeed the mathematics are very similar to that of a black hole, but inverted, so that whereas things too close to a black hole cannot escape, with dark energy, things that are too distant (with respect to any observer) cannot return. (When thinking about unreachable distant objects, it is we who have become too distant from the object.)

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_mizzar t1_jc9l0si wrote

Is there any functional difference between the misconception you speak of in your initial post and the reality you describe above in terms of the physical outcome?

For example, if “space was expanding” except for things that are close enough overpower it with gravity, how would that result be different from dark energy creating a repulsive force except for things that are close enough to overpower it with gravity?

Also, how can galaxies be “carried by their initial momentum” as you confirmed above AND all be moving away from one another at the same time. Maybe I’m misunderstanding something but that doesn’t really make sense. The only way all galaxies that are not locally bound to one another could be moving away from one another is if that space between them is somehow expanding, similar to the “dots on the surface of a balloon” metaphor that is often used. If we instead imagine an “initial momentum” scenario, it suggests a single point of origin in space, similar to an explosion. But in the case of an explosion, there is a center source, which the universe doesn’t appear to have. Also, the red-shifting caused by an explosion would not be so uniform, but have many items moving away at much faster speeds than others relative to the observer.

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Aseyhe t1_jc9qu7c wrote

> For example, if “space was expanding” except for things that are close enough overpower it with gravity, how would that result be different from dark energy creating a repulsive force except for things that are close enough to overpower it with gravity?

The main problem is that dark energy's repulsion has no intrinsic link to the global expansion of the universe. For example, as I noted in another comment, in Einstein's static universe (in which matter's attraction balances dark energy's repulsion), there is a repulsive force even though the universe is not expanding.

More generally, the repulsion from dark energy has no link to the cosmic expansion rate at a given instant. It does connect to the rate of change of the cosmic expansion rate (i.e. the acceleration), but this link is only partial, since dark energy is only one of the factors controlling the rate of cosmic acceleration (the other main one being matter, which decelerates expansion).

> Also, how can galaxies be “carried by their initial momentum” as you confirmed above AND all be moving away from one another at the same time. Maybe I’m misunderstanding something but that doesn’t really make sense. The only way all galaxies that are not locally bound to one another could be moving away from one another is if that space between them is somehow expanding, similar to the “dots on the surface of a balloon” metaphor that is often used. If we instead imagine an “initial momentum” scenario, it suggests a single point of origin in space, similar to an explosion. But in the case of an explosion, there is a center source, which the universe doesn’t appear to have.

The universe can originate from a single point in spacetime while still having no center in space. Due to the principle of relativity, any observer departing from that point has an equally good reference frame, so there is no way to decide that one observer is better than another.

(You could still suggest defining the center in terms of "distance from the edge". There doesn't have to be an edge, though.)

> Also, the red-shifting caused by an explosion would not be so uniform, but have many items moving away at much faster speeds than others relative to the observer.

That's not a fundamental limitation, it's just intuition from the explosions you know. The initial "explosion" doesn't have to be messy (for example if it arose from inflation). Also note that in an expanding system, velocities self-sort because if an object is moving rapidly with respect to the material near it, it's not going to stay near that material. Instead it will gradually end up near material moving at the same speed. (This effect is essentially the cosmological redshift applied to massive particles.)

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nowayjose081 t1_jc8qcfn wrote

I might be wrong but you can think of it like this: Between us and andromeda there is 'x' amount of dark energy repulsion. Between andromeda and the next galaxy beyond that there is also 'x' amount of dark energy repulsion. And so on. So between us and some galaxy thats 100 times farther away than andromeda is, there will be 100x amounts of dark energy repulsion.

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