How can two galaxies collide in an expanding Universe?

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Offline thedoc

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Rich Baker  asked the Naked Scientists:
My question is about the expanding universe. I have heard that every part of the universe appears to repel away from whatever point you're observing it from. But I have also heard that Andromeda is on a collision course with The Milky Way.

So, how can two galaxies be simultaneously expanding away from each other while also hurling toward each other?

I'm a big fan of your programme. Thank you for a really awesome show to listen to.

Thank you!

What do you think?
« Last Edit: 20/07/2013 23:30:01 by _system »


Offline Pmb

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Re: How can two galaxies collide in an expanding Universe?
« Reply #1 on: 21/07/2013 04:18:29 »
The background radiation defines the frame in which the momentum density of the radiation is zero. That means that the average flow of momentum from this radiation is zero in that frame. The typical galaxy would idealy be at rest in such a frame but the typical galaxy has a small velocity in that frame due to gravitational attraction to neighboring galaxies etc. Nothing's perfect. In the case of Andromeda  the drift velocity is towards our galaxy due to gravitational attraction.


Offline evan_au

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Re: How can two galaxies collide in an expanding Universe?
« Reply #2 on: 21/07/2013 05:48:12 »
Cosmologists model the universe as starting with a hot soup of gas, with the local variations in temperature, pressure & velocity - which you expect to see in any plasma or gas.

In these computational models, this turns into hierarchical gravitational clumpings of superclusters, clusters, galaxies, and stars which all have their own masses, velocities and axis of rotation. It is inevitable that some of them will be on a colliding course, and it is thought that galaxies grow by absorbing smaller galaxies, leaving relics like globular clusters, or galaxies with more than one central black hole.

Overall, space is expanding, but the gravitational attraction between masses means that the galaxies within a supercluster are more likely to remain gravitationally bound, forming large-scale structures similar to soap bubbles, with the matter in thin sheets and filaments surrounding relatively empty voids.


Offline Soul Surfer

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Re: How can two galaxies collide in an expanding Universe?
« Reply #3 on: 29/07/2013 09:05:34 »
Galaxies are quite large compared with their separation, the universe is "stringy" so galaxies tend to be much denser in some places than others and the galaxies they are also attracted to each other by gravity, so collisions are relatively common.  The opposite is true of stars in galaxies which are extremely small compared with their seperation, unless they are formed very close to each other, They hardly ever collide even when galaxies collide.
« Last Edit: 29/07/2013 09:09:03 by Soul Surfer »
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