How is the Universe Expanding?

[Rhondazvous]

I know the galaxies are moving away from each other, but the levels of the universe are somewhat fractal. so are the stars inside galaxies moving away from each other? Are the plants around stars moving away from each other?

2. Since the expansion is a result of the big bang and there were no atoms at that point, at oms would not be a part of the expansion. In fact, the formation of atoms appear to be a reversal of the expansion.

[vorlon13]

static universes (ones that neither expand nor contractor) are not stable, so it's one or the other. At least in regards to having a universe with beings inside to contemplate WTF is going on.

atoms are small and sturdy enough regardless of when they form the expansion will not inflate them (disregarding for the moment some theoretical work that might change that in a distant epoch of this universe)

as a whole, and in isolation, galaxies are stable for VERY long time periods, HOWEVER, as, for instance, our sun orbits our galaxy, it is radiating gravitational energy, and it's orbit is contracting excruciatingly slowly. Wait long enough and if nothing else intervenes, we are going to be absorbed into the central black hole, or something else between here and there.

the long term (until the sun burns out) stability of our solar system is in a slight amount of question. The long term (billions of years) stability of the planet Mercury's orbit is slighty suspect, and it might, if we wait long enough, experience a cranking up of it's orbital eccentricity and collide with Venus, or, mush less likely, earth.

Additionally, IIRC, in some billions of years, it is expected earth might achieve an orbital resonance with Saturn. I don't recall what happens then other than it would be unpleasant.

[Rhondazvous]

static universes (ones that neither expand nor contractor) are not stable, so it's one or the other. At least in regards to having a universe with beings inside to contemplate WTF is going on.

atoms are small and sturdy enough regardless of when they form the expansion will not inflate them (disregarding for the moment some theoretical work that might change that in a distant epoch of this universe)

as a whole, and in isolation, galaxies are stable for VERY long time periods, HOWEVER, as, for instance, our sun orbits our galaxy, it is radiating gravitational energy, and it's orbit is contracting excruciatingly slowly.  Wait long enough and if nothing else intervenes, we are going to be absorbed into the central black hole, or something else between here and there.

the long term (until the sun burns out) stability of our solar system is in a slight amount of question.  The long term (billions of years) stability of the planet Mercury's orbit is slighty suspect, and it might, if we wait long enough, experience a cranking up of it's orbital eccentricity and collide with Venus, or, mush less likely, earth.

Additionally, IIRC, in some billions of years, it is expected earth might achieve an orbital resonance with Saturn.  I don't recall what happens then other than it would be unpleasant.

Doesn't bode well for long term investments. It's enough to get Wall Street scrambling.

[Alex K]

I know the galaxies are moving away from each other, but the levels of the universe are somewhat fractal. so are the stars inside galaxies moving away from each other? Are the plants around stars moving away from each other?

2. Since the expansion is a result of the big bang and there were no atoms at that point, at oms would not be a part of the expansion. In fact, the formation of atoms appear to be a reversal of the expansion.

Distances are stretched through the expansion of space. It's as if space itself is stretched..You can think of it like ants walking on a rubber band that is being stretched.
Two separate ants will move away from each other through the stretching, but the ants themselves keep their length because they are held together by forces which are not changed by the expansion of the rubber beand.
Likewise, in space, all objects which are held together by forces such as gravity or electromagnetism will resist this stretching and keep their sizes because their size is determined by these forces at work in or between them. Two distant objects which are not e.g. bound by gravitation will not have anything compensating the stretching of the space in between and their distance will increase.

[LostLocke]

On that note, something I've always wondered...
Do we know, or have an idea, at what stage or level gravity or other forces can continuously overcome expansion? Or could expansion get to the point where it would tear even objects apart?
Or, is the most direct and short answer, We don't know?

[Alex K]

On that note, something I've always wondered...
Do we know, or have an idea, at what stage or level gravity or other forces can continuously overcome expansion? Or could expansion get to the point where it would tear even objects apart?
Or, is the most direct and short answer, We don't know?

Yes, the measured Hubble parameter tells you how quickly the distance of two points increases, and that can be translated into a small tug. If the simplest concordance cosmology is right the hubble parameter will not ever become so big that everyday objects are ripped apart. But there are the "big rip" models in which H increases dramatically in the future and space expands ever quicker until even the atoms get ripped apart.

[Rhondazvous]

I know the galaxies are moving away from each other, but the levels of the universe are somewhat fractal. so are the stars inside galaxies moving away from each other? Are the plants around stars moving away from each other?

2. Since the expansion is a result of the big bang and there were no atoms at that point, at oms would not be a part of the expansion. In fact, the formation of atoms appear to be a reversal of the expansion.

Distances are stretched through the expansion of space. It's as if space itself is stretched..You can think of it like ants walking on a rubber band that is being stretched.  
Two separate ants will move away from each other through the stretching, but the ants themselves keep their length because they are held together by forces which are not changed by the expansion of the rubber beand.
Likewise, in space, all objects which are held together by forces such as gravity or electromagnetism will resist this stretching and keep their sizes because their size is determined by these forces at work in or between them. Two distant objects which are not e.g. bound by gravitation will not have anything compensating the stretching of the space in between and their distance will increase.

Understood. But apart from gravity and the em force, inertia will still require two objects to maintain their distance unless some outside force causes them to accelerate. Does this suggest a multiverse?

BTW, did you see my last question in the thread about symmetry breaking?

[Alex K]

Distances are stretched through the expansion of space. It's as if space itself is stretched..You can think of it like ants walking on a rubber band that is being stretched.  
Two separate ants will move away from each other through the stretching, but the ants themselves keep their length because they are held together by forces which are not changed by the expansion of the rubber beand.
Likewise, in space, all objects which are held together by forces such as gravity or electromagnetism will resist this stretching and keep their sizes because their size is determined by these forces at work in or between them. Two distant objects which are not e.g. bound by gravitation will not have anything compensating the stretching of the space in between and their distance will increase.

Understood. But apart from gravity and the em force, inertia will still require two objects to maintain their distance unless some outside force causes them to accelerate. Does this suggest a multiverse?

BTW, did you see my last question in the thread about symmetry breaking?

No. There is no inertia to overcome - because universal expansion doesn't actively move objects apart in physical space, it is just space itself becoming more. This is where the rubber band picture fails.

No I haven't...

[Rhondazvous]

No. There is no inertia to overcome - because universal expansion doesn't actively move objects apart in physical space, it is just space itself becoming more. This is where the rubber band picture fails.

Then I must first understand
1. what you mean by space.
2. Tf the objects are not moving, how have scientists come tosee an expasion? I thought the red shift had to do with an object moving away.

[Alex K]

No. There is no inertia to overcome - because universal expansion doesn't actively move objects apart in physical space, it is just space itself becoming more. This is where the rubber band picture fails.

Then I must first understand
1. what you mean by space.
2. Tf the objects are not moving, how have scientists come tosee an expasion? I thought the red shift had to do with an object moving away.

1. Well... the usual meaning i'd say
2. No, the redshift comes from space stretching while the light is travelling, and stretching the travelling light waves with it. The objets aren't moving in space, space is stretching (see my rubber band analogy: two ants can stand still with respect to the rubber band, and yet their distance changes.)