Expanding universe versus galactic collision

[buzzinfinity]

Hello everybody,

I have been watching the history channel's show "The Universe" . In one of the seasons "Most Dangerous places on the universe" they say our galaxy is moving towards our neighboring galaxy Andromeda and they will collide after some billion years. But in another season "Dark matter and Dark energy" they say that the universe is expanding due to the presence of some dark energy and the galaxies are continuing to move apart. I find both these information conflicting each other. How can galaxies collide, if they are moving apart?. Any ideas ?

[ajb]

The expansion of the universe is a large scale global phenomena as where the colliding of galaxies is a small scale local phenomena.

 

It is a question of scales. When one talks of galaxies receding from us one should have in mind clusters of galaxies (or even clusters of clusters) each with their own internal dynamics. The local gravitational field can overcome the expansion of the universe. Thus there is no conflict of ideas here.

 

Andromeda is in our local cluster. The galaxies in this cluster are all gravitationally bound. In the case of Andromeda it is indeed moving towards us and not away. It is blue shifted and not red shifted, in technical language. Again, no conflict of ideas here, just differing scales.

[Sisyphus]

Picture thousands of rubber balls bouncing around a room. Now steadily increase the size of the room. On average, the balls are spreading out, getting farther apart. But any two could still be getting closer, and there would still be collisions between them.

[Royston]

Picture thousands of rubber balls bouncing around a room. Now steadily increase the size of the room. On average, the balls are spreading out, getting farther apart. But any two could still be getting closer, and there would still be collisions between them.

 

I think that's rather misleading, because it implies collisions based on likelihood rather than distance, and forces et.c

 

buzzinfinity, I'm sure you know the electrostatic force is stronger than gravity. So, when you rub a balloon against your head and stick it to the ceiling of a room, you can see that the electrostatic force overcomes the gravitational force of the Earth.

 

However, if you repeated the same experiment, and held the ballon a metre away from the ceiling, it would fall to Earth, because it's too great a distance from the ceiling, i.e gravity then governs the direction of the balloon.

 

Now just apply that to two galaxies, if they're close enough, they're gravitational forces bring them together, too far apart, and dark energy overcomes the gravitational force (as it's too weak at large distances) and they are then subject to the expansion of space between them instead.

[Sisyphus]

I think that's rather misleading, because it implies collisions based on likelihood rather than distance, and forces et.c

 

True. But the OP's difficulty seemed to be in imagining how individual bodies can collide when the group as a whole is becoming more rarified, and it seemed like a simple visualization to address it. And, to be fair, while the motions of galaxies are driven by deterministic forces, the "initial conditions" are random for all intents and purposes, no? Like a room full of rubber balls on both counts! And, for a big enough room, it would indeed be the case that you could approximately measure a given ball's distance by it's velocity, even as it would locally be too randomized.

[buzzinfinity]

Thanks all.

Each of the posts explained the matter in a slightly different perspective and it helped me to get the general idea that an event as enormous as this would have so many factors to consider with all the randomness and fuzziness of the universe.

[J.C.MacSwell]

Fro Woody Allen: (Annie Hall)

 

Mother(to doctor): He's been depressed. All of a sudden, he can't do anything.

Doc: Why are you depressed, Alvy?

Mother: Tell doctor Flicker. It's something he read.

Doc: Something you read, heah?

Alvy: The universe is expanding.

Doc: The universe is expanding?

Alvy: Well, the universe is everything, and if it's expanding, someday it will break apart and that would be the end of everything!

Mother(shouting): What is that your business? (to doctor) He stopped doing his homework.

Alvy: What's the point?

Mother: What has the universe got to do with it? You're here in Brooklyn! Brooklyn is not expanding!

[Royston]

And, to be fair, while the motions of galaxies are driven by deterministic forces, the "initial conditions" are random for all intents and purposes, no?

 

That's true, I wasn't really picking holes in the analogy itself, more, it could have been misconstrued by somebody new to the subject. Still, that was a nice afterthought / elaboration, on your part.

[PhysicsGroupie]

I've never fully understood where the scale of the expanding universe is limited on the lower end. That is to say, what is the distance (scale) boundary of expansion? For example, why isn't the matter contained inside of a galaxy expanding? Further, why isn't the matter inside our solar system expanding? Further, why isn't matter on the tip of my finger expanding? If everything were expanding relative to everything else, we wouldn't notice any difference. But we do since the expansion is only prevalent in the large scale structure of the universe, but why?

[insane_alien]

space is expanding everywhere. its just that local forces are able to overcome the pseudo force generated by the expansion.

 

in your fingernail, the electromagnetic bonds between molecules hold you together.

 

in the solar system, galaxy and galactic clusters, it is gravity.

 

there is no hard and fast boundary, just a gradual change of dominant forces.

[Airbrush]

Interesting how often this question is asked. Even in this discussion, after it is explained, the same question is asked again.

 

Here is my question. Do superclusters (which are clusters of clusters) expand at all? Or is all expansion BETWEEN superclusters?

[Royston]

Here is my question. Do superclusters (which are clusters of clusters) expand at all?

 

Well, how do you think super clusters were formed, that will give you a hint.

 

Or is all expansion BETWEEN superclusters?

 

AFAIK, it's quite hard to determine whether some clusters are truly isolated from superclusters, i.e expansion isn't necessarily between all (10 million or so) superclusters. I'm pretty certain there are a number of isolated examples. Maybe someone knows better.

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i.e expansion isn't necessarily between all (10 million or so) superclusters. I'm pretty certain there are a number of isolated examples. Maybe someone knows better.

 

Oops, should read 'all expansion isn't necessarily between superclusters (AFAIK).'

[Airbrush]

Well, how do you think super clusters were formed, that will give you a hint.

 

.....Oops, should read 'all expansion isn't necessarily between superclusters (AFAIK).'

 

My bet is that superclusters are gravitationally bound and ALL expansion is BETWEEN superclusters.

 

The reason so many question expansion by saying "Hey look at Andromeda, it is not moving away from us!" is because not enough emphasis is placed on large-scale structure of the universe.

[Royston]

My bet is that superclusters are gravitationally bound and ALL expansion is BETWEEN superclusters.

 

I'd personally be careful with 'gravitantionally bound', think of galaxies undergoing a dynamic slingshot type of effect. It depends on the velocity of the galaxies, and their trajectory.

 

The reason so many question expansion by saying "Hey look at Andromeda, it is not moving away from us!" is because not enough emphasis is placed on large-scale structure of the universe.

 

True, but Andromeda is a simple (observable) case, now try applying the same principle, to less obvious cases.