Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: annie123 on 07/07/2019 19:12:35

Title: How can galaxies collide if they're moving away all the time/
Post by: annie123 on 07/07/2019 19:12:35
I saw a video of simulated collision of Andromeda and our galaxy. how could Andromeda collide with our galaxy if everything is moving away from everything else ? Are some galaxies moving more quickly and so catch up?
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: Halc on 07/07/2019 19:43:13
I saw a video of simulated collision of Andromeda and our galaxy. how could Andromeda collide with our galaxy if everything is moving away from everything else ? Are some galaxies moving more quickly and so catch up?
On a large scale, all galaxies are moving apart in general, but there are always local variations due to gravitational influence of nearby mass.  The nearby great attractor plays a role in keeping the local cluster(s) somewhat together, resulting in commonplace collisions of galaxies near each other.  Andromeda is a large such example, but there are smaller collisions that will happen before we get to Andromeda.
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: evan_au on 08/07/2019 00:23:02
Every galaxy has its own random velocity in some random direction within the local cluster of galaxies.

You need to step a long way into space before the velocity of the general expansion of the universe (as measured by the Hubble constant) exceeds these local, random velocities of individual galaxies.
- If the random velocities of galaxies is larger than the general expansion of the universe, then you can expect to see galaxy collisions
- It is thought that many small galaxies have collided with the Milky Way, as indicated by the presence of globular clusters and stellar streams in and around our galaxy.
See: https://en.wikipedia.org/wiki/Globular_cluster
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: Petrochemicals on 18/07/2019 00:24:02
Interesting point really, it may signify the big bang was not singular within itself,  that further tangent events appened after the big bang.

Of course variances can be generated by gravity ie bigger mass affecting smaller mass, but to have galaxys vectors significantly altered would take alot of mass.  Perhaps the blackhole and where it goes to is capable of pivoting galaxies ?
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: Halc on 18/07/2019 04:10:56
Of course variances can be generated by gravity ie bigger mass affecting smaller mass, but to have galaxys vectors significantly altered would take alot of mass.  Perhaps the blackhole and where it goes to is capable of pivoting galaxies ?
Black holes seem to play no significant role.  The central black hole of our galaxy is only about 3% of the mass of the galaxy.  The (relatively nearby) great attractor is on the order of hundreds of times as massive, plenty to pull us and Andromeda together.  There is another even larger mass (called the Shapley attractor) beyond that.  I think that's it, and beyond that scale, mass distribution becomes more or less homogeneous.
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: Petrochemicals on 19/07/2019 02:25:07
Of course variances can be generated by gravity ie bigger mass affecting smaller mass, but to have galaxys vectors significantly altered would take alot of mass.  Perhaps the blackhole and where it goes to is capable of pivoting galaxies ?
Black holes seem to play no significant role.  The central black hole of our galaxy is only about 3% of the mass of the galaxy.  The (relatively nearby) great attractor is on the order of hundreds of times as massive, plenty to pull us and Andromeda together.  There is another even larger mass (called the Shapley attractor) beyond that.  I think that's it, and beyond that scale, mass distribution becomes more or less homogeneous.
I would think the mass of the black hole would be inconcequential if the black hole is somehow anchored inside its void. That is hypothetical anyway as no one knows about the interior of black holes.

The great attractors would suggest though that the galaxies are not seperate at present, and that the "collision" is predetermined due to a shared vector away from the big bang ?
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: Halc on 19/07/2019 05:14:53
Black holes seem to play no significant role.  The central black hole of our galaxy is only about 3% of the mass of the galaxy.
Boy did I get that figure wrong.  Our central black hole is about 0.003% of the mass of the galaxy, a thousandth of the percentage I quoted above.

While the great attractor without doubt has its share of black holes, I doubt that any one of them accounts for a percent of its mass.

I would think the mass of the black hole would be inconcequential if the black hole is somehow anchored inside its void. That is hypothetical anyway as no one knows about the interior of black holes.
The distribution of mass of a black hole is up to debate, but it doesn't matter.  It has the same mass regardless of how that mass is 'anchored'.

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The great attractors would suggest though that the galaxies are not seperate at present, and that the "collision" is predetermined due to a shared vector away from the big bang ?
You make it sound like the big bang happened at a point in space.  You can point to it, but not in any of those directions.

Galaxies collide for the same reason planets do (like our little encounter with Theia).  Their paths cross mostly due to the pull of a local central mass (one of the attractors) that prevent the objects from distancing themselves from each other.
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: Petrochemicals on 19/07/2019 18:58:24


While the great attractor without doubt has its share of black holes, I doubt that any one of them accounts for a percent of its mass.
and what is the great attractor ?
The distribution of mass of a black hole is up to debate, but it doesn't matter.  It has the same mass regardless of how that mass is 'anchored'.

if the black hole is anchored somehow to subspace ? The mass would be incalculable.
You make it sound like the big bang happened at a point in space.  You can point to it, but not in any of those directions.

Galaxies collide for the same reason planets do (like our little encounter with Theia).  Their paths cross mostly due to the pull of a local central mass (one of the attractors) that prevent the objects from distancing themselves from each other.
You can point to it ? Are parts of space expanding at different rates to others ? That would be the only way concievable that could lead to collisions.

Planets orbit stars, stars galactic centres, if galaxies orbit attractors they are not falling away from each other vis-à-vis the OP's point. Thus the larger affects the smaller, or vectors at conflict interact,  this would lead them to be in freefall collision unlike objects on vectors that are distancing without exterior influence.
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: Halc on 19/07/2019 22:25:20
and what is the great attractor ?
It's the center of mass of the Laniakea Supercluster, of which we are a part.

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if the black hole is anchored somehow to subspace ? The mass would be incalculable.
No idea what you mean by this.  Mass is quite calculable due to speed that things orbit it.  For this reason, it is considerably more difficult to determine the mass of any body that doesn't have anything orbiting it.

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You can point to it ?
The big bang?  Sure.  That's easy.  Crack your knuckle.  That's one event. Go to dinner.  Now crack a different knuckle to define event B.  Draw a line in spacetime from B through A and on.  That line points to the big bang.  Easy peasy.

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Are parts of space expanding at different rates to others ?
Yes.  The space far from any matter expands more than does space with nearby mass.
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That would be the only way concievable that could lead to collisions.
Peculiar velocity explains it.  Galaxies would never collide if they didn't have any peculiar velocity.  Non-uniform mass distribution (like the great attractor) is what accounts for anything (anything big at least) having peculiar velocity.

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Planets orbit stars, stars galactic centres, if galaxies orbit attractors they are not falling away from each other vis-à-vis the OP's point. Thus the larger affects the smaller, or vectors at conflict interact,  this would lead them to be in freefall collision unlike objects on vectors that are distancing without exterior influence.
All true, but there are exterior influences.  We've got Andromeda close by and nothing on the other side to balance it.  So we accelerate in that direction.  In less time than it took life to evolve into humans, we will hit it.
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: abrooks051 on 20/07/2019 01:23:12
About us and Andromeda, I had the same thought. I appreciate the comments above and for this reason when "scientists" and TV scientists talk about everything flying away from everything else in the Universe, they should clarify that general statement to prevent confusion.
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: Bill S on 20/07/2019 02:04:24
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....when "scientists" and TV scientists talk about everything flying away from everything else in the Universe, they should clarify that general statement to prevent confusion.

Absolutely! but we all succumb to the tendency to "abbreviate".  If we all talked about galaxy groups moving apart, then the effects of peculiar velocity within the groups would be subject to less misunderstanding.
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: yor_on on 30/07/2019 17:31:20
Yep, soon we're going to meet our neighbors, just some 150 million light years away :)
Heh, they probably have better coffee too.
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: Halc on 30/07/2019 17:40:48
Yep, soon we're going to meet our neighbors, just some 150 million light years away :)
Andromeda is about 60 times closer than that.
Title: Re: How can galaxies collide if they're moving away all the time/
Post by: Bill S on 31/07/2019 15:51:32
https://www.nature.com/articles/d41586-019-02198-z?WT.ec_id=NATURE- 

This doesn’t address the OP question, but may be of related interest.
Title: How can galaxies collide if theyre moving away all the time/
Post by: AlekceuhoX on 06/08/2019 15:43:54
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