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PhinPhan1227
01-27-2009, 12:36 PM
I was looking at some NASA shots of galaxies "colliding". Basically they just passed through each others tracks and gravity did the damage. My quistion is this though...how were they on intersecting tracks in the first place?

If all matter in the universe is moving away from the site of the Big Bang, how are any two galaxies on converging courses? Shouldn't they all be moving out and away from each other? And it's not like this is apparently a vanishingly rare occurance. NASA has several shots of galaxies that are either about to collide, are colliding, or just got done colliding. So what's the deal? If everything is moving outwards on an ever expanding sphere, how do we have colliding galaxies?

eric1589
01-27-2009, 01:13 PM
everything is affected by gravity. just because everything might be expanding from a single point doesnt mean none of that stuff is able to collide. especially when there are LARGE masses out ther with immense gravitational fields.

our own milky way galaxy is on a collision course with the andromeda galaxy. but that isnt supposed to happen for about 4 billion more years. buts its very likely that humans wont even exist any more at that time. so its nothing for us to worry about.

Tetragrammaton
01-27-2009, 01:26 PM
everything is affected by gravity. just because everything might be expanding from a single point doesnt mean none of that stuff is able to collide. especially when there are LARGE masses out ther with immense gravitational fields.

our own milky way galaxy is on a collision course with the andromeda galaxy. but that isnt supposed to happen for about 4 billion more years. buts its very likely that humans wont even exist any more at that time. so its nothing for us to worry about.

This sums it up about perfectly.

I remember hearing about the collision course for the Milky Way and Andromeda and not understanding how it was possible.

Right now the Milky Way and the Large and Small Magellanic Clouds are undergoing galactic cannibalism, where the big one absorbs the small one.

PhinPhan1227
01-27-2009, 01:50 PM
This sums it up about perfectly.

I remember hearing about the collision course for the Milky Way and Andromeda and not understanding how it was possible.

Right now the Milky Way and the Large and Small Magellanic Clouds are undergoing galactic cannibalism, where the big one absorbs the small one.

I could understand to some extent galaxies moving at different speeds and therefore "catching up" to other galaxies(rear ending them so to speak), but again I have to wonder how that is possible. The Big Bang took place in a nano second. So why are some galaxies "behind" other galaxies? If galaxies are being drawn together off their original courses by gravity, shouldn't that same gravity have slowed us down by now, rather than what we are actually experiencing? If "Dark Matter" can cause the galaxies acceleration to continue, why doesn't it also keep galaxies on their original courses?

eric1589
01-27-2009, 03:07 PM
some things will stick to each other and some things will repel each other.
those random instances, mixed with unimaginably large explosions from dying stars can change the way things are laid out in a system. that would change their angle of trajectory based on how much mass is in certain areas.

the short answer is we certainly dont know everything.

as far as we can tell the universe has been around for more then 13 billion years. human societies have only been discovered to be around for last 12 thousand years. and we have only been able to dig so deeply into astronomy for about 150 years.

we have a lot to look back on. and most conclusions drawn about the past are formed by studying the present and finding out how things work now. then we assume a set of rules as to how things are able to work. then we work out possibilities based on those conclusions and try to find the one that makes the most sense.

PhinPhan1227
01-27-2009, 04:14 PM
some things will stick to each other and some things will repel each other.
those random instances, mixed with unimaginably large explosions from dying stars can change the way things are laid out in a system. that would change their angle of trajectory based on how much mass is in certain areas.

the short answer is we certainly dont know everything.

as far as we can tell the universe has been around for more then 13 billion years. human societies have only been discovered to be around for last 12 thousand years. and we have only been able to dig so deeply into astronomy for about 150 years.

we have a lot to look back on. and most conclusions drawn about the past are formed by studying the present and finding out how things work now. then we assume a set of rules as to how things are able to work. then we work out possibilities based on those conclusions and try to find the one that makes the most sense.


Actually, everything except magnets attract each other. It's the universal law of gravity.

eric1589
01-27-2009, 04:36 PM
you misunderstood. maybe i wasnt clear enough.

i meant some things will be repelled by different methods. like the force of an explosion pushing something away. or ateroids impacting each other and pushing each other, and or pieces apart.

PhinPhan1227
01-27-2009, 05:01 PM
you misunderstood. maybe i wasnt clear enough.

i meant some things will be repelled by different methods. like the force of an explosion pushing something away. or ateroids impacting each other and pushing each other, and or pieces apart.


True enough. But you have to ask how they came into contact in the first place. If all matter is flying outwards from that center point, how did anything come into contact in the first place? We have already seen that gravity hasn't overcome the initial force of the Big Bang, so if everything is traveling outwards and upwards, how did anything hit anything else?

eric1589
01-27-2009, 11:28 PM
becasue everything is affected by the gravity of other things.
an asteroid orbiting our sun is binded to sun by gravity and put into orbit.
other things like planets also orbit the sun, but they are not all on the same path.

many things will orbit at different distances and speeds. eventually some things are bound to collide.

it is thought that having a large planet like jupiter in our solar system has saved planet earth from many asteroid impacts because of how jupiters gravity can affect the path an asteroid is traveling.

eric1589
01-27-2009, 11:33 PM
also, its not as if planets and stars and galaxies just instantaneously spat out from a single point in their current form.

i think hydrogen was the only thing at the begining. maybe there were some other elements that were also thought to be there, but i cant recall.

hydrogen would slowly bond with other hydrogen and form gaseous clouds and thier increased gravity would attract even more hydrogen untill eventually things got hot enough, through friction, for fusion to start. then hydrogen was turned into helium and other things happened such as super nova explosions that could have turned helium into other elements and so on.

Dolphan7
01-28-2009, 02:31 AM
To me....if the big bang started this outward push, and all objects moving away from the center or source of that push went "spiraling" out into the universe, that spiral or spinning would I think alter the course of lets say a galaxy. So the trajectory of a spinning galaxy could at some point intersect the trajectory of another spinning galaxy, even spinning back toward the center of the explosion would be possible.

Does that make sense?

PhinPhan1227
01-28-2009, 08:42 AM
also, its not as if planets and stars and galaxies just instantaneously spat out from a single point in their current form.

i think hydrogen was the only thing at the begining. maybe there were some other elements that were also thought to be there, but i cant recall.

hydrogen would slowly bond with other hydrogen and form gaseous clouds and thier increased gravity would attract even more hydrogen untill eventually things got hot enough, through friction, for fusion to start. then hydrogen was turned into helium and other things happened such as super nova explosions that could have turned helium into other elements and so on.


Sure, all solid matter coalesced from the gases ejected during the Big Bang. But again, all that matter was traveling outwards at the same rate. When it "clumped", it should have stayed on the same basic path. I'm sure there was some shifting, it just seems strange to have so many colliding galaxies.

eric1589
01-28-2009, 03:55 PM
one outward projecting mass would have a gravitational affect on other things also projecting outwards. so everything out there potentially had affects on any number of other things that could have changed their trajectory.

and i mentioned explosions durring the formation and destruction of stars as well. those things would push a lot of stuff in any direction that was opposite of the blast point. regardless what direction it would have been moving previously.

1 dol fan
02-15-2009, 03:00 PM
Maybe they just circumvented the universe... Maybe the universe isn't endless or forever expanding but instead round and able to be circumvented.

Everything going out from the big bang would then act like continental drift on our own planet.

Maybe there are ways to disprove this but i is simply something I have thought about a bit for the past hour or so.

1 dol fan
02-15-2009, 03:04 PM
also, its not as if planets and stars and galaxies just instantaneously spat out from a single point in their current form.

i think hydrogen was the only thing at the begining. maybe there were some other elements that were also thought to be there, but i cant recall.

hydrogen would slowly bond with other hydrogen and form gaseous clouds and thier increased gravity would attract even more hydrogen untill eventually things got hot enough, through friction, for fusion to start. then hydrogen was turned into helium and other things happened such as super nova explosions that could have turned helium into other elements and so on.
I believe that some stars actually get so big and eventually use iron as for fusion and that makes the star heavy and the super nova that takes place would account for metals of that sort.

Eshlemon
02-26-2009, 05:36 PM
http://curious.astro.cornell.edu/question.php?number=354



My question is how do galaxies collide? If things started in the Big Bang - moving away from every thing else - what would cause a mass the size of a galaxy to change direction and move into another galaxy?

It's interesting that you should ask a question about that picture. It was taken with the Wide-field Infra Red Camera (WIRC) at the Palomar Observatory. That camera was built by astronomers here at Cornell and a good friend of mine was involved in the observations in which that picture was taken.

Anyway it's quite natural for galaxies to collide even though the universe is expanding - although I could see why you might get confused about it. What happens is that there is a battle between the forces of gravity between the two galaxies (which is trying to pull them together) and the expansion of the universe (which is trying to pull them apart). With galaxies that start out quite close together, it is almost always gravity that wins, so in the end the galaxies will collide. This will most likely happen to the Milky Way and the Andromeda galaxy (our nearest large neighbour) in a few billion years.

Good question...apparantley gravity wins most battles over expansion force.

http://www.finheaven.com/images/imported/2009/02/070807galaxycollision_big-1.jpg

While looking for an answer came across, the biggest bang up with 4 galaxies crashing into each other...expected to result in one super galaxy 10 times the size of the Milky Way.


Galaxies clash in four-way merger

http://news.bbc.co.uk/2/hi/science/nature/6933566.stm

Wonder if gravity beats expansion most of the time...as we get more 'super galaxies' and their gravity sucking each each other together as this would establish the 'horizon' of expansion. As eventually the great galaxies reach a balance of pulling on each other, while may not necessarily bringing them together crashing together anymore, keeps them going out any further.

Mile High Fin
03-19-2009, 04:47 PM
Also, keep in mind that the matter of the universe isn't "flying" away from the big bang in the traditional sense of a regular explosion. It's space itself that's expanding. The matter is more-or-less going along for the ride.

The best analogy I've heard is placing dots 1 inch apart on a balloon that's not inflated. Then, when you blow up the balloon, all the dots move "away" from each other and are then 2 inches apart. The balloon's surface represents "space" itself (not outer space, just all "space", like the distance between your face and the monitor screen). If you are on one of those dots (galaxies) everywhere you look, others are moving away (overall).

However, if 2 dots started out very close together (or had some other interaction long ago), then the gravity could pull them together. Also, even if we can see 1,000 galaxies colliding in various areas, that's 0.000000000000000001% of the galaxies in existance, so it IS INDEED a very rare occurance. Most galaxies (almost all) are far enough apart that they do NOT collide.

That's the best I can do for you.
Suffice to say, the most brilliant minds in science are not shocked by this apparent contradiction, so I think there's no mystery.

Tetragrammaton
03-22-2009, 10:35 PM
The best analogy I've heard is placing dots 1 inch apart on a balloon that's not inflated. Then, when you blow up the balloon, all the dots move "away" from each other and are then 2 inches apart. The balloon's surface represents "space" itself (not outer space, just all "space", like the distance between your face and the monitor screen). If you are on one of those dots (galaxies) everywhere you look, others are moving away (overall).

However, if 2 dots started out very close together (or had some other interaction long ago), then the gravity could pull them together. Also, even if we can see 1,000 galaxies colliding in various areas, that's 0.000000000000000001% of the galaxies in existance, so it IS INDEED a very rare occurance. Most galaxies (almost all) are far enough apart that they do NOT collide.

That is a good explanation, but images are always cool to have.

http://en.wikipedia.org/wiki/File:Raisinbread.gif

PhinPhan1227
03-23-2009, 11:39 PM
Also, keep in mind that the matter of the universe isn't "flying" away from the big bang in the traditional sense of a regular explosion. It's space itself that's expanding. The matter is more-or-less going along for the ride.

The best analogy I've heard is placing dots 1 inch apart on a balloon that's not inflated. Then, when you blow up the balloon, all the dots move "away" from each other and are then 2 inches apart. The balloon's surface represents "space" itself (not outer space, just all "space", like the distance between your face and the monitor screen). If you are on one of those dots (galaxies) everywhere you look, others are moving away (overall).

However, if 2 dots started out very close together (or had some other interaction long ago), then the gravity could pull them together. Also, even if we can see 1,000 galaxies colliding in various areas, that's 0.000000000000000001% of the galaxies in existance, so it IS INDEED a very rare occurance. Most galaxies (almost all) are far enough apart that they do NOT collide.

That's the best I can do for you.
Suffice to say, the most brilliant minds in science are not shocked by this apparent contradiction, so I think there's no mystery.

Obviously observation trumps theory, and since we can see galaxies collide, we know it happens. But your balloon model presents the same dilema. The dots on an expanding balloon will move away from each other as the balloon fills. Of course gravity will have an effect, but if the forces at work(dark matter maybe) are keeping the universe accellerating in it's expansion, I find it strange that in some areas those forces wouldn't be enough to keep some galaxies from falling through each other.

Obviously the effect takes place. I just wonder at our grasp of the dynamics at work.

Mile High Fin
03-27-2009, 02:41 AM
Obviously observation trumps theory, and since we can see galaxies collide, we know it happens. But your balloon model presents the same dilema. The dots on an expanding balloon will move away from each other as the balloon fills. Of course gravity will have an effect, but if the forces at work(dark matter maybe) are keeping the universe accellerating in it's expansion, I find it strange that in some areas those forces wouldn't be enough to keep some galaxies from falling through each other.

Obviously the effect takes place. I just wonder at our grasp of the dynamics at work.

I think you've just answered your own question.
Like I said, it is a rare occurance. Once in awhile (like 0.000000001%), gravity will win out if the galaxies are sufficiently close together.
99.99999999% of the time the galaxies DON'T collide -- due to expansion of universe.
I think we (scientists/physicists) understand this issue much more than you think.
I understand the apparent contradiction, and I like that you are versed enough to even realize it and ponder it (95% of population wouldn't even be able to ponder such apparent contradictions). But I don't think this is a big mystery to the professionals.

Also, FYI - the acceleration of the universe is NOT caused by dark matter (in fact, dark matter is detected from its GRAVITATIONAL pull on holding galaxies together), it's caused by dark energy. That's the mysterious repulsive force at work (sorta like Einstein's cosmological constant).