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NyyDave
2008-Oct-17, 05:16 AM
I came across this question posted on Frasier's profile and I wanted to get it out into the forum.


If the universe exploded from one single point, and everything travels away from that point how do galaxies collide? Do they move at different speeds, or change direction?

Thank you for your time, Michael (Liverpool, England)

I understand how small objects get thrown off and change course, does the same apply on a larger scale for entire galaxies? And since everything is supposed to be moving further out does that mean eventually collisions will not occur?

Thanks, Dave.

Neverfly
2008-Oct-17, 05:21 AM
Collisions between galaxies are not common, but they do happen.
For one thing, remember that the Big Bang is not appropriately named.
It wasn't a Gigantic Space Explosion- It was a sudden expansion.

But Large galaxies also have Large Gravity which can attract two together and result in a "Collision."
There is so much space between the objects in a galaxy that even collision is a bad description. They tend to flow through eachother and effect eachother gravitationally rather than slam into one another.

The expansion is observable between galaxies/ Galactic Clusters. Within though- it's effect is overwhelmed by gravity.

matt.o
2008-Oct-17, 08:23 AM
Collisions are common - that's how galaxies grow! Major collisions between roughly equal mass galaxies are rarer, however it is thought that this is how ellipticals are formed.

Also, the stellar component of galaxies remain collisionless during a merger, however, the gaseous component does not (assuming the galaxies have gas left). During a gas rich (commonly called a 'wet' merger) there is much interaction with the gas causing violent starbursts and driving gas towards the centre of the galaxies, forming bulges and feeding black holes.

Neverfly
2008-Oct-17, 08:25 AM
Maybe we need to define the word "common" :p

matt.o
2008-Oct-17, 08:30 AM
Well, more commonly mergers are distinguished between 'major' and 'minor' where major mergers are (roughly) those occurring between galaxies with a 1:3 mass ratio, and minor where one of the galaxies is less than one third the mass of the primary one.

matt.o
2008-Oct-17, 08:49 AM
If the universe exploded from one single point, and everything travels away from that point how do galaxies collide? Do they move at different speeds, or change direction?

Thank you for your time, Michael (Liverpool, England)


I understand how small objects get thrown off and change course, does the same apply on a larger scale for entire galaxies? And since everything is supposed to be moving further out does that mean eventually collisions will not occur?

Thanks, Dave.

Okay, firstly the "explosion from a single point" is not a correct interpretation/representation of the standard theory.

Second, the expansion occurs on large (i.e. >100Mpc) scales. This is where the isotropy and homogeneity come into play allowing the GR field equations to be solved and to predict the expansion. On smaller scales, gravity is the dominant force, thus galaxies are free to merge. This occurred more in the past since the universe was denser.

As to your last point - whether collisions will occur in the future really depends on the nature of the dark energy, and whether it is truly constant or changes with time.

Eroica
2008-Oct-19, 01:13 PM
And since everything is supposed to be moving further out does that mean eventually collisions will not occur?
You have to make a distinction between those systems which are now gravitationally bound and those which are not gravitationally bound.

In clusters of galaxies, the galaxies are gravitationally bound. So the space between member galaxies is not expanding with the general expansion of the Universe - gravity is counteracting the Hubble expansion. The peculiar motions of the galaxies through space dominates over their relative motions due to the expansion of space. Here collisions and mergers can still happen. But over time clusters gradually become gravitationally relaxed or virialized (ie the galaxies settle into stable orbits about a common centre of mass), and as a result collisions become less probable.

In larger structures such as superclusters, which are not gravitationally bound, the Hubble expansion dominates over peculiar motions, and galactic interactions are becoming less and less probable as time passes.

Veeger
2008-Oct-19, 01:24 PM
Here is a previous discussion on BAUT forums they may be interesting: http://www.bautforum.com/questions-answers/73011-how-do-galaxies-move.html

First, in my opinion, you must clear your head of the idea, that the universe exploded at the Big Bang. Matter did not explode outward into space as there was no space. The BB was an expansion of space-time, not an explosion of matter.

Galaxies move by gravitation and inertial forces and will occasionly pass through one another. Since galaxies are mostly made of space, actual star collisions are probably rare.
:)

mugaliens
2008-Oct-19, 05:08 PM
The initial expansion was almost perfect. Perfect, but not quite perfect. The result of that imperfection, spread out over the last umpteen billion years, are the stars, galaxies, and systems we see today.