Thread: expansion and collisions

i am having a hard time getting my head around this.if the universe is expanding or the space between matter is expanding and objects are moving apart from each other at a growing rate[the further away the faster the speed?],then how can two objects like BH or galaxies collide?i am not disputing the expansion.but how can objects that are moving apart from each other come together ?can someone please explain this or tell me what i am m issing?

I'm new to this forum, and certainly not as well informed as many of the members whose posts I've had the pleasure of reading thus far, so please excuse my layman's attempt at explanation:

I believe part of the actions you're describing may be caused by incredibly vast gravitational fields that act on bodies across vast distances in space, such that items that are all generally moving the same direction over a relatively long period of time (in this case, the entire time continuum from the universe's inception) can still be influenced "in the short term" to move about within a gravitational sphere of influence while still moving away from the center of the universe as a whole. Also, since there have been massive collisions of super-energized bodies occurring throughout the life of the universe, there could in many cases be matter being dispersed at high rates of speed from the epicenter of these collisions outward, running into stuff on their inexorable path outward from the initial collision point.

All of this seems to be easily explained by the same phenomenon of two passengers being on a train travelling sixty miles an hour in one direction, but able to run at each other from either side of a boxcar...both people are travelling in the same direction as a whole, but still seeming to move in opposite directions when viewed on a smaller scale. Hope this makes sense!

I had this exact question when I first heard about Dark Energy, glad to know I wasn't the only one going "wtf?"

Basically the way it works is this: while the Universe is expanding at an accelerating rate with distance and time, the gravitational bond of say, galaxies and clusters of galaxies, can override the expansion at certain levels. The gravitational force between these bodies is enough that even though space is expanding, they remain in their effective positions in their orbits; so even though the Universe itself is "stretching" so to speak, the galaxies and associated clusters and superclusters are bound tightly enough together that they can endure this expansion. Now superclusters that are not that tightly bound WILL move away from each other, but if a system is gravitationally bonded together tight enough, the system itself can survive the expanding Universe (though from what I have read in certain papers, this may change as the acceleration picks up massive amounts of speed during the later epochs of the Universe's history, anyone have info on the topic?)

On that note, this is a question I recently heard answered on another site, I will post the answer here:

"There is enough matter in a galaxy, that locally the expansion of the universe is stopped. You can think of this as the gravity of the galaxy holding it together, but really it's more fundamental than that. The rate of the expansion of the universe depends on the amount of matter (and dark energy) in the universe. If you just consider a tiny fraction of the universe which just includes a galaxy and total the matter in that region, it's more than enough to have already stopped the expansion in that region."

Hope that helps.

ok,now i get it.thank you guys so much.i was actually losing sleep over this[well,having a hard time falling asleep]!

Haha, glad to have helped. It's definitely an interesting topic.

Any other questions on anything like that?

i guess this would be part two of the question.if space is expanding everywhere, in areas with objects with enough mass to produce enough of a gravtational "pull" on each other to overcome this expansion(did i get that right?),wouldn't the space between the objects be trying to expand and therefore become contracted,at least to the space outside of the gravitational field?

Originally Posted by roncj5

i guess this would be part two of the question.if space is expanding everywhere, in areas with objects with enough mass to produce enough of a gravtational "pull" on each other to overcome this expansion(did i get that right?),wouldn't the space between the objects be trying to expand and therefore become contracted,at least to the space outside of the gravitational field?

From my understanding .... space expanding is nothing more than the distance between two points in space becoming greater. The problem is the why and also choosing what metric to use to determine the distances. Is it two objects speeding apart from each other ( Special Relativity or SR ) OR is it because of Gravitational redshifting ( General Relativity or GR ) OR a combination of the two. From my understanding depending on the metric chosen it can be from either SR or GR or a combination. That is why it is taken as its own special case.

Comoving coordinates makes these questions difficult to answer. It is not like space is expanding on a ruler and we can see all of the dynamics going on ... I am not sure this will make it clearer ... but picture yourself shipwrecked in a raft in the middle of the ocean. You look out and see someone else on a raft in the distance. they seem to be moving away from you. Are they paddling away from you OR is the motion of the ocean making you drift apart? Are both of you accelerating in the same direction so that the net distance and velocity between you is growing .... You have no way to tell? Are you at the same point in space when you finished your measurements as you were when you started measuring? What does it even mean to be in the same place without having anything to measure it against.

All we know is that the further something is away from us the more redshifted is the light we receive from them ... is this because it is moving away from us OR is it because back in time there was more gravity because things were closer together? OR both?

Originally Posted by tom

From my understanding .... space expanding is nothing more than the distance between two points in space becoming greater.

ok.so space is not moving objects apart itself,a force is spreading them apart(dark energy,dark matter?).

Space is expanding, yes. As far as we are aware, the Universe itself (and the dimensions associated with it) are expanding outward. So while space between two points is expanding, in gravitationally locked systems the inward pull of gravity effectively cancels out this expansion.

so in these locked systems the outward expansion is balanced or at least severly slowed as to not create a contraction,while the space around these systems continues outward "slipping around"said systems.did i get that right?