universal expansion, how come galaxies collide?

[Nereid]

akirabakabaka asked a very good question (here (http://www.bautforum.com/showpost.php?p=631217&postcount=5)):If the universe is expanding and all galaxies are moving away from each other, why are there so many collisions?As I think it's a question which many folk have, I thought it might be nice to see what answers we get, by starting a thread on just this topic! :)

[phunk]

Since it's space itself expanding, and isn't dependant on mass or anything else, the only factor in how much there will be between two galaxies is the distance between them. When they're relatively close (galactic clusters) gravity is greater than expansion. When they're farther apart, expansion can overpower gravity.

[Nowhere Man]

On a local scale, gravity is strong enough to overcome the expansion of space-time. "Local scale" includes structures up to galaxy supercluster sizes, at least.

Fred

[bigbluestar]

Density I think is the major factor. The period that had the highest amount of collisions would be the really early years after the big bang as abserved with very long exposure images. As the universe got less dense with time, from both canabalism and the expansion of space gravity starts to be the major factor. The mutual attraction of local galactic clusters would cause collision. Just as how we are attracted to the andromeda galaxy.

So for us Gravity will be the cause of the Death of the Milkyway Galaxy. As Andromeda is comming to gobble us up. Lol its like staring down the mouth of lion but really slowly hehe

[George]

Would the following analogy help?

Place two bowling balls opposite, but near, the center of a trampoline; begin adding addtional tension on the material while releasing the balls. What happens? The balls will collide.

Do this thought experiment with two trampolines, the balls will not be able to reach the other's trampoline.

The galaxies on one "trampoline" (super cluster region) will continue to move normally and interact with associated galaxies; but they will not, normally, be able to wander to another "trampoline".

[Blob]

Hum,
i guess that computer simulations of large scale structures have been made with the premise of no expansion (a control) and differing amounts of matter etc in it to mimic what we see.

From the lack of any recent research papers (and the possibility of a noble prize) saying that they have done so, I assume that no one has managed to get it to work.

But a quick google search seems to bring up a host of research that found the sims need dark matter and expansion to diplicate what we see .

[Cougar]

Nowhere Man, despite the hat he wears, is correct. The expansion of space becomes significant only on extremely large scales. The gravitational effect of neighbor galaxies greatly outdoes the expansion effect.

Bigbluestar started out well....Density I think is the major factor. The period that had the highest amount of collisions would be the really early years after the big bang as abserved with very long exposure images. That's right. Like the Hubble UltraDeep Field. (http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/07/)
But then I think Bigblue loses it...
As the universe got less dense with time, from both canabalism and the expansion of space gravity starts to be the major factor.
Maybe this was just a slip. As the universe ages, expansion starts to be the major factor.
The mutual attraction of local galactic clusters would cause collision. Just as how we are attracted to the andromeda galaxy.... So for us Gravity will be the cause of the Death of the Milkyway Galaxy. As Andromeda is comming to gobble us up.
As previously mentioned, the gravitational effect of nearby galaxies can easily overcome the effect of expansion, at least during this Universal Epoch (before the expansion gets accelerating too much), so yep, Andromeda is coming this way. But I wouldn't call the gravitational dance between the Milky Way and Andromeda "death". It's just foreplay before the Great Joining. By the way, during such "collisions", it is exceedingly rare that individual stars actually collide. There is so much space between stars, galaxy collisions are actually collisionless. Pity we won't be around to see what the sky looks like during such times.

[Ken G]

Pity we won't be around to see what the sky looks like during such times.
It would probably be postively cracking with supernovae, for one thing! Galaxy mergers tend to spawn a lot of new, hot stars.

[Dragon Star]

It would probably be positively cracking with supernovae, for one thing! Galaxy mergers tend to spawn a lot of new, hot stars.

I would love to sit in the middle and watch such a thing is super fast forward, how awesome would that be!

[bigbluestar]

Thanks Cougar ;) I was trying to explain why we have less collision with todays galaxy as oppose to earlier galaxies. And that Gravity as oppose to density is the major cause for collisions in todays galaxies. I can see how my crappy grammer would be confusing

And I agree......what a sight it would be say try T - 1000 years Andromeda would be so bright and so luminouse individual clusters would be visable with backyard telescopes it would be awsome.

[RussT]

Bigbluestar;

This is why your first statement was actually correct...see below


Quote 'Chandra Furthers Understanding About Dark Energy
- A mysterious force, which astronomers call "Dark Energy", seems to be speeding up the expansion of the Universe. New observations from the May 18, 2004 Chandra X-Ray Observatory have independently confirmed this expansion by measuring the distances to galaxy clusters. It seems that the expansion of the Universe was slowing down after the Big Bang until 6 billion years ago; at that point the force of this dark energy took over and expansion began to speed up. The big mystery still remains... what is dark energy? ' end quote.

Becaue the universe expansion was slowing (maybe even coming to a stop), this would have caused more galaxy interaction in different clusters.

[Cougar]

Becaue the universe expansion was slowing (maybe even coming to a stop), this would have caused more galaxy interaction in different clusters.Well, expansion is expansion, whether it is slow, fast, or accelerating. Things get farther apart, which means they were closer yesterday. Which means, logically speaking, they were really close together 10 billion years ago. As distance is halved, the effect of gravity increases 4 times, so yes, we expect much more interaction between galaxies as we go back in time.

[George]

Becaue the universe expansion was slowing (maybe even coming to a stop), this would have caused more galaxy interaction in different clusters.
This is an interesting point, that I suspect makes sense. However, isn't it really the relative velocities the key here? One could have deceleration immediately after an explosion, but high separation velocites, and still have less interaction, right? [Of course, I am discounting density issues a little to make my point.]

[ngc3314]

This is an interesting point, that I suspect makes sense. However, isn't it really the relative velocities the key here? One could have deceleration immediately after an explosion, but high separation velocites, and still have less interaction, right? [Of course, I am discounting density issues a little to make my point.]

It's a bit counterintuitive, but galaxy interactions are actually more effective (transfer more energy and do more tidal damage) in small groups, and especially bound pairs, than in rich clusters. When the relative velocities are much greater than the internal speeds (as in rich clusters, where typical flyby velocities may be >5 times internal motions) the gravitational effect resembles a single impulse. But when the two are comparable, there are all manner of near-resonance effects giving twin tidal tails and transferring a lot of orbital to internal energy. (Erik Holmberg did analog calculations of this during World War II in a darkened Swedish barn, with specially-made light bulbs! Inverse square acts like inverse square...)

In today's Universe, the majority of tidal encounters happen either in poor groups or in bound pairs; there are not enough galaxies wandering around fast enough for "unrelated" galaxies to make much of a contribution. Furthermore, the pairs we see today are survivors of a once more numerous population, many of which have merged.

In rich clusters, the effects consist of single quick pulses, so you can have modest tidal tails pulled out. However, the difference is that there one has many weak, fast encounters ("galaxy harrassment"), so the spiral disks may be nibbled away over time as these effects conspire with pressure from motion through the surrounding X-ray gas. These two effects happen in the same environments, so it has proven a frustratingly long search to see effects of ram pressure by itself.

[GOURDHEAD]

Are there any decent theories about at what scale the expansion (isotropic stretching of space) ceases to act on the contained mass: neutrino? electron? quark? proton/neutron? multi-neucleonic atoms? molecules? bricks? asteroids? large moons? planets? stellar systems? globular clusters? galaxies? local groups? clusters of galaxies? superclusters of galaxy clusters?

During the inflationary period that existed from BB plus 10^-43 seconds to BB plus 10^-35 seconds when the rate of expansion is said to have exceeded the speed of light, were the various manifestations of mass carried along with the isotropic expansion, did the expansion of space slip pass them, or were additional carriers of mass created as the expansion proceeded? See what a tangled web results when the Shmoo field is dismissed outright.

If the particles did go along for the inflation ride, did the electrons, of much lower mass, in an equipartition of kinetic energy environment, outrun the heavier particles, followed by the mutually repelling protons and then the neutral neutrons, or were they all still in the quark state with speeds commensurate with their respective masses?

At whatever scale the tendency to expand is overcome, is it simply outmuscled by gravity (spacetime warpage) or is it totally absent for some (quantizable) reason?

[Cougar]

During the inflationary period... were they all still in the quark state...I'm not sure, but yeah, I think it was more like a quark gluon plasma at that point.

At whatever scale the tendency to expand is overcome, is it simply outmuscled by gravity (spacetime warpage) or is it totally absent for some (quantizable) reason?My understanding is that it's just outmuscled by gravity. And it is certainly outmuscled by the electromagnetic force between an electron and its nucleus, which has a fixed, quantum value; so atoms are not expanding even a little.

[George]

It's a bit counterintuitive, but galaxy interactions are actually more effective (transfer more energy and do more tidal damage) in small groups, and especially bound pairs, than in rich clusters. When the relative velocities are much greater than the internal speeds (as in rich clusters, where typical flyby velocities may be >5 times internal motions) the gravitational effect resembles a single impulse. But when the two are comparable, there are all manner of near-resonance effects giving twin tidal tails and transferring a lot of orbital to internal energy.
If I undertand, the slower the relative speeds, the better the interaction. This does make sense.

However, RussT's statement got me wondering if the primordial galactic cluster region's velocities, relative to other clusters, was not more important than the expansion rate, when applied to cluster to cluster gravitational entanglements. Even if you allow deceleration in the expansion rate, it would not be as significant if the velocities were high enough, right?

(Erik Holmberg did analog calculations of this during World War II in a darkened Swedish barn, with specially-made light bulbs! Inverse square acts like inverse square...) Sounds look material for your next book. :) Were they tiny bulbs illuminating small, but close, paper?

In today's Universe, the majority of tidal encounters happen either in poor groups or in bound pairs; there are not enough galaxies wandering around fast enough for "unrelated" galaxies to make much of a contribution. Furthermore, the pairs we see today are survivors of a once more numerous population, many of which have merged.
I see, it's not like the dynamic "good old days". ;)

In rich clusters, the effects consist of single quick pulses, so you can have modest tidal tails pulled out. However, the difference is that there one has many weak, fast encounters ("galaxy harrassment"), so the spiral disks may be nibbled away over time as these effects conspire with pressure from motion through the surrounding X-ray gas. These two effects happen in the same environments, so it has proven a frustratingly long search to see effects of ram pressure by itself.Are you saying the pulses are weaker but greater in number, therefore, they can, over time, greatly impact each other. [Sorry, the surrounding x-ray gas ram pressure is new to me.]

[ngc3314]

If I undertand, the slower the relative speeds, the better the interaction. This does make sense.

However, RussT's statement got me wondering if the primordial galactic cluster region's velocities, relative to other clusters, was not more important than the expansion rate, when applied to cluster to cluster gravitational entanglements. Even if you allow deceleration in the expansion rate, it would not be as significant if the velocities were high enough, right?


Early on, the expansion rate was practically the same for all regions, whatever the density, because there hadn't been time for gravity to act much. Over time, one expects galaxies at ever-greater distanecs from the centers of mass concentrations (clusters of galaxies) to reverse their motion away from the center (known technically as turnaround). Within this growing region, the motions are then dominated by internal cluster dynamics rather than the Hubble expansion. The more massive a cluster, the longer it will continue to grow (until the process is shut down in the case of accelerating expansion).


Sounds look material for your next book. :) Were they tiny bulbs illuminating small, but close, paper?


He use small bulbs, each with four photomultipliers around its base to measure the net effect of the other bulbs. By good fortune, the finite size of these photomultipliers gave the equivalent of the softening of the gravitational force law at small distances that numerical techniques often have to use to avoid artificial instabilities from unrealistically close passages for the particles' masses. He used 40-odd test points, concentrating on the velocity behavior (transfer of orbital to internal energy implying decay of the relative orbits during a close passage). This was just barely enough resolution to show the stubs of tidal arms.

I see, it's not like the dynamic "good old days". ;)

Are you saying the pulses are weaker but greater in number, therefore, they can, over time, greatly impact each other. [Sorry, the surrounding x-ray gas ram pressure is new to me.]

Yes. And in addition, material pulled out into a tidal tail or arm is then much easier for a subsequent weak encounter to liberate completely from its parent galaxy. Ben Moore has a simulation of the process available as movie number 5 here (http://star-www.dur.ac.uk/~moore/movies.html).

[George]

Early on, the expansion rate was practically the same for all regions, whatever the density, because there hadn't been time for gravity to act much. Are you saying the expansion rate can be regional due to gravity gradients, or are you speaking of cluster separation rates?

He use small bulbs, each with four photomultipliers around its base to measure the net effect of the other bulbs. By good fortune, the finite size of these photomultipliers gave the equivalent of the softening of the gravitational force law at small distances that numerical techniques often have to use to avoid artificial instabilities from unrealistically close passages for the particles' masses. He used 40-odd test points, concentrating on the velocity behavior (transfer of orbital to internal energy implying decay of the relative orbits during a close passage). This was just barely enough resolution to show the stubs of tidal arms. Amazing and ingenious.

Yes. And in addition, material pulled out into a tidal tail or arm is then much easier for a subsequent weak encounter to liberate completely from its parent galaxy. Ben Moore has a simulation of the process available as movie number 5 here (http://star-www.dur.ac.uk/~moore/movies.html). Cool site. It appears the smaller, and more lethargic, ring-shapped structure actually captured parental material.