Expansion of the Universe

On a scale above millions of light years, everything is retreating from everything else.--Percy

There's no clear line of demarcation because with increasing distance the effects of gravity diminish gradually while those of expansion increase gradually. The larger the scale the more easily expansion will win out over gravity.I was just responding to your statement that, "Objects farther away move faster away from us then closer objects, but not from each other." All observers in the universe would see precisely the same thing that we see from our vantage point here on Earth, that the further away an object is the faster it is retreating. This is because all points in space are retreating from all other points in space, but what's contained in that space, such as light and matter, will have their own motions and will attract each other gravitationally.--Percy

Along a single line of sight this would be true, trivially true in fact. It wouldn't be possible for it to be any other way.--Percy

Hi NoNukes,I realize now that there was no way to tell what I was responding to. In the paragraph I quoted there was the OP excerpt that Slevesque quoted, and then there was his interpretation of it. I should have just quoted the OP excerpt instead of the entire paragraph, because that's what I was responding to. I didn't address Slevesque's comment because it seemed like a post hoc rationalization of what he wrote earlier, and he shifted the context to observed recession speed, which has a relativistic component, as Cavediver noted. Of course, this must also sound like a post hoc rationalization. But let me repeat what you said, because I think it's the important point about the expansion of space. It seemed like it was getting muddled earlier in the thread, and that was my original reason for posting. Changing your wording only slightly: --Percy

Hi Slevesque,I think NoNukes and I agree with Cavediver that as observed from Earth, two distant objects at radial distances x₁ < x₂ will have a smaller measured radial Δv than two closer objects of the same radial separation.But that's just because of relativistic effects from our observer position. Ignoring local motion, two objects at separation x from each other will be retreating from one another at the exact same velocity no matter where they are in the universe.--Percy

Hi Slevesque,Here's the original statement/response in your Message 3: You said the "from each other" part was incorrect. I can see now that you meant, "They *are* moving away from each other, but not faster than they're retreating from us." It seemed at the time that you were saying, "They're not moving away from each other." That's why I responded that on large enough scales everything is moving away from everything else.And of course if you ignore local motion, everything is moving away from everything else regardless of distance.One reason you might think you see a contradiction is that you were thinking of objects along a single line of sight, in other words, that are lined up radially when looking outward from the Earth, but you never said that. Two distant objects, one further away than the other, could easily be retreating from each other faster than they're retreating from us if they were in opposite directions when looking outward from the Earth. What you were saying is only true if the distant objects are lined up in the same direction.Regarding the NonUkes "It isn't true at all" comment in his Message 13 reply to me, my reply looks wrong because it has to be interpreted in the context of what you said earlier in your Message 3 about the "from each other" part being incorrect. Cavediver's Message 14 is also correct where he notes that the actual observed separation velocities will be slower for the far pair than the near pair because of relativistic effects. Once relativistic effects are taken into account then this anomaly goes away. When quoting you can use the form [qs=NoNukes], and then you won't have to type things like "Nonukes replied".--Percy

Space is expanding everywhere throughout the universe, but the Milky Way and Andromeda galaxies are close enough together that their gravitational attraction is more than sufficient to overcome any retreat due to the expansion of space.There's the old example of two ants on a rubber band. If you stretch the rubber band the two ants get further apart. But if you first tie the two ants together with a thread (analogous to gravity) then their separation distance will not increase as the rubber band is stretched.Objects sufficiently far apart have so little net gravitational attraction (this would be the attraction resulting from adding up all the gravitational pulls from all other objects in the universe, which for distant objects will tend to average out and be a net zero) that the expansion of space is sufficient to carry them further apart.--Percy

I am not myself aware of any mechanism that would produce so large a local velocity.--Percy

The big bang is not responsible for the expansion of the universe. There's something more fundamental going on, something that played a significant role in both the big bang and the accelerating expansion of the universe. I suppose that initially possibilities would be explored within existing theory, such as a serendipitous sequence of this galaxy being whipped around other galaxies in a manner similar to the way we accelerate spacecraft by whipping them around planets. If none of those pan out then and it turned out to be inexplicable within any current paradigm of cosmology then we'd have to seek modifications to current theory, or even replacement. It would be an exciting time within science.--Percy