How Can Galaxies Collide if the Universe is Expanding?

[HIGHLYTOXIC]

Researchs show that our universe is expanding very fast. But I have also read that galaxies are racing ahead at great speeds on their way to collission with each other.
If universe is expanding, then how come this is possible?

 

[mathman]

Galaxies which are near each other are attracted by gravitation, overcoming the expansion. Our galaxy and our nearest neighbor (Andromeda glalaxy) are on a collision course.

 

[Phobos]

Good question, HIGHLYTOXIC...and welcome to Physics Forums!

The expansion of space is most significant ("fast", as you said) over VAST distances...like on inter-galactic scales. Smaller than that, like within a galaxy, gravity has the upper hand and keeps things together. So, our galaxy and things within our galaxy, like our solar system, are not expanding with the Universe. Often times, there is still enough gravity around such that galaxies can form clusters. In that case, the expansion of space is more significant in between galaxy clusters (of which there is still LOTS of space to work with). Galaxies within a cluster can move around each other according to their gravity and less influenced by the expansion of space. Our Milky Way galaxy is in a small cluster called the Local Group. The Milky Way has already collided with smaller galaxies in the Local Group. The two biggest kids in our block (Milky Way and Andromeda galaxies) might be on a collision course too...an event which may occur in about 6 billion years.

 

[Nereid]

Superclusters

The Local Group is (loosely?) bound to the Virgo cluster. Generally, clusters can be gravitationally bound into superclusters, which are the largest structures in the universe. Within clusters and superclusters collisions of galaxies are relatively common. In the hearts of clusters, collisions of spiral galaxies produce elliptical galaxies, some of which are extraordinarily massive.

 

[Snipez]

Snipez's friend Chazz writes:

This is more or less on topic but, I have often read that scientists say that there is a missing matter that they can not account for. I believe that is why Dark Matter was theoriozed. We know that at the centre of nearly all galaxies rest a supper massive black hole holding the galaxy together and possibly more black holes in that galaxy since there was porbably "a few" stars that formed black holes. Plus the colisions of galaxies and so on that can account for a lot of black hole in different sizes, mabey even supercluster could colide and form a "cluster black hole" (not sure about this but still) :P anyways if you get where I'm going at there can be a lot of black holes in space. We don't even know how much matter is in these black holes (or do we?).

Wouldn't be safe to assume that the missing matter is inside these black holes or in stars so large that light does not escape them but resists gravity enough to not form a black hole?

 

[selfAdjoint]

The problem is that the astronomers can tell where the dark matter is, by looking at its gravitational effects. One of them is that although the galaxies rotate, they don't fall in on themselves as gravitating masses should. To control this, the dark matter should be in a halo around the galaxy. Black holes inside the galaxy aren't well placed to do the job.

 

[Chazz]

Thanks, this answers my question. I do sometime wonder how much matter are currently in black hole and if one day all matter will be consumed in a black hole but mabey we will see the heatdeath, big crunch (I think that as been disproven by now) or something else that will be a death of our universe.

 

[Nereid]

What is dark energy?

Originally posted by Chazz
Thanks, this answers my question. I do sometime wonder how much matter are currently in black hole and if one day all matter will be consumed in a black hole but mabey we will see the heatdeath, big crunch (I think that as been disproven by now) or something else that will be a death of our universe.

It depends a lot on what 'dark energy' actually is! There was a period recently when cosmologists were starting to get comfortable; they felt they could answer these questions, or could expect to be able to do so before not too long. However, once 'dark energy' was confirmed, all bets were off.

 

[ranyart]

Originally posted by HIGHLYTOXIC
Researchs show that our universe is expanding very fast. But I have also read that galaxies are racing ahead at great speeds on their way to collission with each other.
If universe is expanding, then how come this is possible?

If one looks out into the night sky and, then using data to confirm that Andromeda is 'heading our way', then adds the data confirming our(milky-way) position in relation to the local group, one can state that there is a likleyhood that we will merge with Andromeda, and then at a later time, will be absorbed into the local group.

Now using data that researchers find stating that the Universe Expansion is accelerating, then there is the "Acceleration-Paradox", this may be interesting.

As I have just done some work on a Uni-short-course, one of the questions was about describing a far off QSO (Quasi-stella-object) at a distance of about 10 thousand light years, based on its Redshift signals. Just how much this signal has been disrupted by the SPACE-EXPANSION?..on its long and winding journey to our telescopes?

Lets say we can travel to a Galaxy (B) that is 15 billion light years away (the farthest observation made from the milky-way at present) whilst keeping our observation focused in its direction, we move towards this Galaxy B, which is located in Space that is 15 billion Light years away, the space-expansion between our selves and Galaxy B equliberates as we approach half-way, 7.5 billion Light years, and once we cross this point we will observe the Galaxy to be contracting into a location along with its distant surrounding companion objects, the TIME-LINE of Galaxy B decreases with our approach towards it.

The closer we get to it, the more it contracts , now this is the evolution of every Galaxy in the Universe, Our Milky-Way has gone through the same Expanding evolution process, we make this statement based on our observation from within the Milky Way, so when we arrive at this far away Galaxy and look out into the open Universe we should see a comparable Universe, identical as if we were viewing from the Milky Way? That is the Universe is Expanding (even though as we approach our distant Galaxy B we observe it as contracting to a point of source) the far off Galaxies observed from Galaxy B (including the area of our Milky way-Local Group) will be observed to be an Expanding foreground within a static background (CMB).

Now for the really interesting observations, once we arrive at Galaxy B and get incorporated into its Momentum, we look out and observe the Outside Universe to be Dynamical and in High Expansion mode, but our host Galaxy B appears to be static for those within, we do not get 'giddy' when we look out into the intergalactic arena, we ride along with comfort. We see that far away locations are actually Expanding, so the Andromeda Galaxy does not collide with our Milky way!..this needs to be looked at again, the ANDROMEDA GALAXY does not Collide with the Milky Way!..in fact if we was to turn around when we left our Local Group at the Halfway mid-point of our original journey(Earth to Galaxy B) then we at some point in time would see Andromeda and Milky way Parting company with the Local group(Expansion increases with dstance/Time>>to the farthest points in space-Big-Bang).

But if we now do the same journey from the Milkyway, but this time we keep our observations firmly fixed on to Our Milky way as we leave our Local Group, then we will observe the Milky Way bouncing Away from the Andromeda Galaxy! This seems counter-intuitive to the physics of our current understanding, but a Bounce is as good as a collision! The merging of certain Galaxies in observation terms has to be treated with caution, the location of the observer is of vital importance, and in which direction he/she is looking from.

The location of where one experiences observations can reveal certain paradoxes, the protected Shroud that encompases Galaxies allows one to look out and observe the Universe in its dynamical Motions, but the Physical Laws FOR OBSERVERS, inside looking out, are not the same as the Physical Laws FOR OBSERVERS outside LOOKING IN!

How does the HST observations need to be calibrated?..if we send the HST out of Our SolarSystem and to the edge of our Galaxy, do you think its images would be the same as is currently observed?..lets me say this, How much effect is imposed on to detectors by their local Enviroment, do images get impregnated by enviromental shifts?