Expanding Universe-Colliding Galaxies?

[Jimmy9]

Ive heard the space between galaxies grows over time and that the universe is expanding in every direction..ive also seen the ballon explanation were they make dots on a ballon and blow it up to demonstrate the expansion of the universe, if this is true how is it we have colliding galaxies? Something to do with galactic clusters? gravity? Differing velocities? The weakness of dark energy vs gravity at this stage?

[zhermes]

Good question---and you've already given the right answer.

The universe is expanding overall---and thus, on average, galaxies are moving apart. None the less, certain galaxies can be big enough (and close enough) that their gravity will overcome the expansion and pull them closer together.

Its the same idea as what allows galaxies to form in the first place---on small enough scales, gravity is able to overcome the effects of expansion.

Note that this doesn't have anything to do with 'dark energy' which is the cause of accelerated expansion.

[Radrook]

It's mainly the space between the galactic super clusters that is generally expanding and creating vast almost empty regions giving our visible universe a somewhat filamentary bubbled appearance. Galactic clusters such as our own , which belongs to the Virgo Super cluster and includes Andromeda and the Milky Way are gravitationally bound to their super clusters and stay within their boundaries.

Here is a chart of super clusters and the separating voids.
http://en.wikipedia.org/wiki/File:Nearsc.gif

[SHISHKABOB]

but so the long tendrils of galaxies stretching from cluster to cluster, these would be "stretching," correct?

I mean, there are tendrils connecting galaxy clusters, right?

[Radrook]

but so the long tendrils of galaxies stretching from cluster to cluster, these would be "stretching," correct?

I mean, there are tendrils connecting galaxy clusters, right?



They aren't tendriles of galaxies. They are tendriles of superclusters the largest

structures known structues in our universe. They are called supercluster filaments,

complexes, or walls and are considered larger structures than super clusters because

they themselves are composed of many superclusters stretched out in filament fashion.


They appear thinned out because they span greater distances, up to a billion light years.

So it's better to consider superclusters as small nodes along larger supercluster

filaments instead of large nodes to which smaller supercluster filaments are attached.


Being structures indicates that they are gravitationally bound. Which in turn would mean

that the filamentary superclusters would hold their relative positions in relation to each

other while the voids between filaments expand.