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hagendaz
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If the universe is expanding per the inflating balloon example, how come the planets within our own (and every other) solar system aren't spreading further appart.
hagendaz said:If the universe is expanding per the inflating balloon example, how come the planets within our own (and every other) solar system aren't spreading further appart.
By "pulled apart" do you mean that there are 'groupings' of individual galaxies that because of the proximity of the individual galaxies they would be expected to behave as a bound system, but are observed to be moving away from each other? If so, is this behavior primarily due to the velocities of the galaxies involved -- or is there more to it?marcus said:I understand there are borderline cases where what looked like a wannabe cluster got pulled apart, it wasn't really gravitationally bound to begin with.
Not if they were parallel to each other.Assuming gravitational interaction ignored, they will fall apart at the standard expansion rate of 1/140 % per million light yrs.
Ich said:Not if they were parallel to each other.
Space expansion per se (the first derivative of the scale factor [tex]\dot a[/tex]) does not affect anything at all. It is merely a set of initial conditions, saying that at a certain starting time nearby objects have a (measurable) velocity v=H*d, if "d" is distance.The space expansion does not matter it is parallel or not, but it affects everything evenly.
Yes, and what I'm saying is that it will not increase.What I am saying is the distance between bullets will increase due to space expansion.
Space expansion applies exactly to those objects that are subject to a certain set of initial conditions.Basically the space expansion applies quite fairly to all objects, unless there are disturbing forces like gravitation or electric or other forces.
"expansion" usually means that objects are moving away from each other. They could do so wihout a cosmological constant or gravity.I thought that the cosmological constant is the same as spacetime expansion
Whatever "parallel" and "when they are fired" means at these distances. Better stay close, so that the bullets can be described in a common nearly flat frame.Dusty_Matter said:Okay, let's say that the bullets to be fired are not a million light years apart, but are now 13 billion light years apart, but they are still parallel when they are fired onto their paths.
I'm saying that neither wil happen. They will eventually spread abart, but more slowly than nearby comoving objects.Dusty_Matter said:Are you saying that they still will not spread apart along with the expansion of the universe, but will stay parallel with each other?
Expansion is unable to alter the distance of any two objects, unless these objects are initially prepared with some relative velocity. (where "expansion" has the meaning I use in this thread.)hagendaz said:So does space expansion only act as a "tailwind", but is unable to expand the distance between two objects lateral to their path of motion?
Yes, cosmological distance between observers at rest wrt each other would initially increase under some circumstances. But "cosmological distance" is a coordinate construct, not what one would call a "distance" in any realistic operational way.Would the expansion of space not increase the distance they would have to travel to hit each other?
Yes, that's an usual picture. I dont'tlike it at all. The picture of space dragging things along with it, or the notion of motion through space as opposed to (and physically different from) motion of space, are awfully inadequate and misleading.I believe that when they talk about the universe expanding, a lot times they use the example of raisin bread rising in the oven to show how galaxies are spreading apart. They are in motion with space/time which is expanding.
Yes, and every observer will think that the explosion happened exactly at his position. It's just a matter of coordinate choice:The galaxies are not flying apart due to any velocity on their part, but due to the expansion of the universe. If it was due to an intial explosion of some sort, then there would be a central location from which they are leaving.
Not quite, if you count gravity as a force, it will change the rate of expansion.All the continued expansion is a result of the original acceleration. There is not a continued force acting on objects.
There is a continued force, but its effect is minuscule, and the bullets will stay at essentially the same distance for a long time, when comoving particles will long have increased their distance measurably. Comoving particles are those which had the proper relative velocity from the start.Like your bullets. if you fire them parrallel they will remain parallel. The only reason objects get further apart over time is they were on a different path originally. There is no continued force... ?
No, object the size of galaxy clusters and below are gravitationally bound, they do not expand.The universe is expanding, then how about the size of individual galaxies, say, our Milkyway is increasing in size?
Ich said:No, object the size of galaxy clusters and below are gravitationally bound, they do not expand.
pixel01 said:You mean galaxies in the universe do not influenced or being influenced gravitationally?
pixel01 said:You mean galaxies in the universe do not influenced or being influenced gravitationally?
v2kkim said:My understanding is that a galaxy does not expand, even possibly contract due to gravitation.
However the space expansion is quite universal, so applys to each space in a galaxy. But in a galaxy after each tiny expansion between objects, they adjust their motion due to gravity, and we know in this case gravity influence is much much greater than space expansion in a galaxy distance. Therefore we can ignore space expansion within a galaxy.
Wallace said:... continuing to move apart following the initial kick given by inflation. ...
marcus said:I see, you picture it as "continuing to move".
In which direction did this initial kick point?
Wallace said:Marcus, you surprise me. I thought you were beyond this!
* Note in fact that the expansion has slowed of course due to gravity, and is now once again accelerating due to dark energy, which like the inflaton has negative pressure. The point is that forces operate on the second derivate of motion, not the first.
Wallace said:... continuing to move apart following the initial kick given by inflation. ...
marcus said:I see, you picture it as "continuing to move".
In which direction did this initial kick point?
Wallace said:...
Imagine a very large (possibly infinite) blob of dust of uniform density. Let it start completely from rest. What will the blob do? Because of gravity, all bits of the dust will be attracted to every other bit and the blob will start to contract, in the sense that everything gets closer to everything else (thus even if it is infinite we can still say that is contracts). Now, if you do the sums you will see that the material in the blob will be following Hubble law contraction (same as Hubble flow, but contracting). We don't need any funky relativity here, this thought experiment is entirely Newtonian. If you want, you could describe this situation as 'contracting space' as long as you knew that you were just inventing a metaphor, not describing physics. You could derive co-moving co-ordinates and all the rest.
Now, imagine that we turn gravity OFF after a small amount of time. What happens? Does everything stop moving? No, of course not, the dust blob continues to follow a Hubble law contraction and space continues to contract in the invented metaphor. The second derivative of the contraction rate would go to zero, but the contraction would continue.
So, in this situation, in which direction was the kick that was applied before we turned gravity off? Easy, every particle can say they every other particle received a kick towards it. All kicks are inwards to every point. This is an inevitable consequence of Gauss's law and the homogeneity of the material. It is Aristotelean to suggest that motion requires something to perpetuate it, so let's get with the Renaissance and realize that mass has momentum and we are done.
Now, how do we go from this thought experiment to our Universe? Easy. Inflation was (we think) driven by some inflaton field that had negative pressure. This means that it acts as a kind of 'anti-gravity'. For some reason the inflaton field turned off after a small period of time, and ever since* the expansion has been simply due to momentum originally given by that kick. You just need to go back to the original though experiment and apply anti-gravity in the beginning. You still get the same answer, all 'kicks' can from each point be viewed as going outwards in all directions.
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