How is the Universe Expanding?

[Rhondazvous]

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.)

Gulp. And you shall know the truth, and the truth shall make you go "YIKES." If the expansion is not subject to inertia, then it's also not subject to gravity, so there can be no end to the expansion—no eventual collapse. back to big bang size. Unless the ants discoonect from the rubber band and their gravity makes them snap back.

[bennyboy]

HOWEVER, as, for instance, our sun orbits our galaxy, it is radiating gravitational energy

. . . errmmm. . . is that a thing? It sounds to me like something that isn't.

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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.

Alex, if space is expanding, wouldn't the rate of separation of objects be a function of their original distance from each other?  I mean, if I put two dots 1cm apart on a rubber sheet and stretch it to 2x its original size, I'd expect them to be now 2cm apart.  But if I put them on opposite edges of a 1m sheet, I'd expect them now to be 2m apart-- a 100x difference in the apparent rate of separation even thought the original rubber "Universe" is expanding in exactly the same way both times.  But in the 1cm apart dots, it seems it wouldn't matter WHERE on the sheet they were-- near the middle or right up near the edge-- the apparent change in position relative to each other would be identical, no?

In other words, from one of those dots, I don't think I could necessarily figure out where the center of the rubber sheet was based on the changing relative position of any of the other dots.  So. . . how do they arrive at the Big Bang singulariy, rather than on a position that the Universe just expands forever and ever?

[Anomalocaris]

The way I visualize it is if you start with two point particles that are some distance apart but at rest with each other, then somehow cancel out all the effects of all the fundamental forces upon the each of them, including any forces acting between them, you would still find these two particles would not stay at rest with respect to each other, but would somehow gradually accelerate away from each other.

What is more, you would find this rate of acceleration is completely independent of any attributes of these particles, but increases in direct proportional to the distance between these two particles.

So it is like negative gravity is acting on these particles, except it is not proportional to mass, nor does it decrease with inverse square law,  Instead it increases in proportion to the amount of space between any pairs of particles.  So it is not localized, and permeates all space.  Hence it is as if space is stretching.

Now put fundamental forces back in.  If two particles are sufficiently close to each other such that their gravitational acceleration with respect to each other surpasses the acceleration away from each other caused by expansion of space, then they will not draw apart.   So the ants, the earth, the solar system, the Milky Way, or perhaps even the local cluster of Milky Way, andromeda and M33 are probably safe from being ripped apart by expansion of space.  But as distance increases force of gravity decrease while acceleration by expansion of space increases, there could be defined a radius from any concentration of mass and gravity such that the aggregate gravity of all mass within a spherical region of that radius about this mass concentration would produce an inward acceleration exactly equal to the outward acceleration of expansion of space at the surface of this sphere.  Anything outside of the sphere that started at rest with respect to The sphere will inexorably be torn away faster and faster from everything inside that sphere by expansion of space, whereas everything inside that sphere that started at rest with respect to the sphere will be drawn towards the center of the sphere.

[vorlon13]

HOWEVER, as, for instance, our sun orbits our galaxy, it is radiating gravitational energy

. . . errmmm. . . is that a thing?  It sounds to me like something that isn't.  

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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.

Alex, if space is expanding, wouldn't the rate of separation of objects be a function of their original distance from each other?  I mean, if I put two dots 1cm apart on a rubber sheet and stretch it to 2x its original size, I'd expect them to be now 2cm apart.  But if I put them on opposite edges of a 1m sheet, I'd expect them now to be 2m apart-- a 100x difference in the apparent rate of separation even thought the original rubber "Universe" is expanding in exactly the same way both times.  But in the 1cm apart dots, it seems it wouldn't matter WHERE on the sheet they were-- near the middle or right up near the edge-- the apparent change in position relative to each other would be identical, no?

In other words, from one of those dots, I don't think I could necessarily figure out where the center of the rubber sheet was based on the changing relative position of any of the other dots.  So. . . how do they arrive at the Big Bang singulariy, rather than on a position that the Universe just expands forever and ever?

Once the gravitational wave detectors detected gravitational waves, the whole gravitational wave thing took on quite a bit of credence.

As for how long it will take the sun to spiral down into the core of the galaxy?  Be thinking a very long time.  I suspect your growing finger nails would get there first.  (sorry, I'm not going to do the math)

[Ben Davis]

Everything that Alex said but he didn't touch on some of the implications. The expansive force is increasing space at a rate equivalent to a speed greater than the speed of light. That means we're living in a curious time when we're lucky enough to still be able to detect other galaxies. There will come a time when conglomerations of matter will be so far apart, an observer in one would see no others. Cosmology would be all but impossible. This will happen long before the heat death of the universe, too.

In related matters, the latest experiments to discover the Inflaton, the elusive particle predicted to be responsible for universal inflation, have failed so utterly as to rubbish all predictions. The Inflaton, if it exists at all, is nothing like it needs to be to make our current models work. That means our current models reliant on its existence are probably utterly wrong.

[BrianSoddingBoru4]

Picture the Universe as a giant rubber sheet. This will in no way help you understand expansion, but it's good to know that if the Cosmos has a 'little accident', the underpinning Foundational Reality won't get all soggy.

Boru

[Iroscato]

*Seizes Alex by the collar, boggle-eyed and fearful*

So where's the extra space coming from Alex?! WHERE'S THE SPACE COMING FROM?!?

[Anomalocaris]

Everything that Alex said but he didn't touch on some of the implications. The expansive force is increasing space at a rate equivalent to a speed greater than the speed of light. That means we're living in a curious time when we're lucky enough to still be able to detect other galaxies. There will come a time when conglomerations of matter will be so far apart, an observer in one would see no others. Cosmology would be all but impossible. This will happen long before the heat death of the universe, too.

If I am not mistaken, what you describe is not an implication of merely space expanding.  It is an implication of the suggestion that the rate of expansion is increasing without bound. We actually have no theoretical basis for that extrapolation, because we have no theory for the mechanism responsible for the acceleration.

[Alex K]

. . . errmmm. . . is that a thing?  It sounds to me like something that isn't.  

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Alex, if space is expanding, wouldn't the rate of separation of objects be a function of their original distance from each other?  I mean, if I put two dots 1cm apart on a rubber sheet and stretch it to 2x its original size, I'd expect them to be now 2cm apart.  But if I put them on opposite edges of a 1m sheet, I'd expect them now to be 2m apart-- a 100x difference in the apparent rate of separation even thought the original rubber "Universe" is expanding in exactly the same way both times.  But in the 1cm apart dots, it seems it wouldn't matter WHERE on the sheet they were-- near the middle or right up near the edge-- the apparent change in position relative to each other would be identical, no?

In other words, from one of those dots, I don't think I could necessarily figure out where the center of the rubber sheet was based on the changing relative position of any of the other dots.  So. . . how do they arrive at the Big Bang singulariy, rather than on a position that the Universe just expands forever and ever?

Once the gravitational wave detectors detected gravitational waves, the whole gravitational wave thing took on quite a bit of credence.

As for how long it will take the sun to spiral down into the core of the galaxy?  Be thinking a very long time.  I suspect your growing finger nails would get there first.  (sorry, I'm not going to do the math)

Come on, third derivative of the quadrupole moment, don't be a wuss and grab your pencil

[Alex K]

*Seizes Alex by the collar, boggle-eyed and fearful*

So where's the extra space coming from Alex?! WHERE'S THE SPACE COMING FROM?!?

Duude chill out! You can simply picture everything in fixed space shrinking if that helps.