I need clarification of expansion

[greenestar]

Simply , I would like to understand why if when we look through the most powerful telescopes everything seems to be getting further from each other, but on the other hand from what I understand it's a fairly regular occurance that galaxies collide & merge. I do not understand the contradiction. For example if our universe is 13.7 billions years old and during this time everything expanded away from each other how can galaxies be merging?

[Peter B]

Simply , I would like to understand why if when we look through the most powerful telescopes everything seems to be getting further from each other, but on the other hand from what I understand it's a fairly regular occurance that galaxies collide & merge. I do not understand the contradiction. For example if our universe is 13.7 billions years old and during this time everything expanded away from each other how can galaxies be merging?

G'day Greenestar

Good question. Galaxies collide and merge because they're gravitationally attracted to each other. At the scale these events happen (tens of millions of light years, give or take an order of magnitude), gravity is a stronger force than the expansion of the universe. But beyond that, gravity is too weak to overcome the expansion. As a result, things which are (relatively) near each other gather together, but become separated from other clumps of "stuff".

In the distant future, distant galaxies will be receding from us so fast, and have receded so far, that they'll be invisible; and all the galaxies in our part of the universe will have coalesced into one giant galaxy. People living then will therefore almost certainly assume the galaxy is the only thing in the universe, and will have no way of knowing about the other giant galaxies beyond their visibility.

[Hornblower]

Simply , I would like to understand why if when we look through the most powerful telescopes everything seems to be getting further from each other, but on the other hand from what I understand it's a fairly regular occurance that galaxies collide & merge. I do not understand the contradiction. For example if our universe is 13.7 billions years old and during this time everything expanded away from each other how can galaxies be merging?
Correction: Most things over cosmic distances are getting farther apart, not absolutely everything. The distribution of matter is lumpy, and in regions where the overall density is relatively high, the gravitation is strong enough to overpower the large-scale tendency toward expansion. Thus we have gravitationally bound clusters of galaxies, but if the clusters are far enough apart they will separate while the galaxies within them remain bound within their respective clusters.

[gzhpcu]

Yes, similarly to planetary systems, many galaxies form "groups" of galaxies, and then "supergroups", bound by gravitational attraction.

Our galaxy is part of what we call the "local group":

The Local Group is the group of galaxies that includes our galaxy, the Milky Way. The group comprises over 35 galaxies, with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy

http://en.wikipedia.org/wiki/Local_Group

[greenestar]

Correction: Most things over cosmic distances are getting farther apart, not absolutely everything. The distribution of matter is lumpy, and in regions where the overall density is relatively high, the gravitation is strong enough to overpower the large-scale tendency toward expansion. Thus we have gravitationally bound clusters of galaxies, but if the clusters are far enough apart they will separate while the galaxies within them remain bound within their respective clusters.

okay, well if this is the case, then 13 billion years all matter would have been much closer together hence being much more dense. So if the density was so high it would have stopped the universe from expanding in the first place. How can you explain this?

[Peter B]

okay, well if this is the case, then 13 billion years all matter would have been much closer together hence being much more dense. So if the density was so high it would have stopped the universe from expanding in the first place. How can you explain this?

It would only have happened if the gravity was stronger than the force driving the expansion. You can't just assume that gravity would have been the stronger force. However, I'll leave it to the boffins to explain it better.

[Murphy]

In the distant future, distant galaxies will be receding from us so fast, and have receded so far, that they'll be invisible; and all the galaxies in our part of the universe will have coalesced into one giant galaxy. People living then will therefore almost certainly assume the galaxy is the only thing in the universe, and will have no way of knowing about the other giant galaxies beyond their visibility.

Is that true? That all the Galaxies in our group will one day merge into one Super-Galaxy? Cool, I never heard that before.

Any idea on when this is likely to happen (i.e. in a few Billion years or in a few Trillion?)

[Grey]

Is that true? That all the Galaxies in our group will one day merge into one Super-Galaxy? Cool, I never heard that before.

Any idea on when this is likely to happen (i.e. in a few Billion years or in a few Trillion?)Probably about 3 billion years for the Milky Way and Andromeda, which are the two big ones. Here (http://www.galaxydynamics.org/tflops.html) are some simulations of what it might look like.

[Grey]

okay, well if this is the case, then 13 billion years all matter would have been much closer together hence being much more dense. So if the density was so high it would have stopped the universe from expanding in the first place. How can you explain this?As Peter B says, that depends on the detail of the rate of expansion, the strength of gravity, and so forth. One might say that gravity on Earth is pulling down, so there should be no way to escape. But even though anything you throw upward would be constantly slowing down from Earth's gravitational influence, launch it fast enough and it will never fall back.

Our best theory of gravity, general relativity, can be used to model that expansion from a high density, hot universe into today's lower density, cooler universe. We can compare the predictions for what the universe should look like today based on that model with what we actually see, and when we do that, the model agrees very well with the observations.

[alainprice]

Here's an analogy.

Take a bucket of marbles and have the bottom trip open on a timer. The timer is the big bang. When the bottom falls out, the marbles all hit the floor at about the same time and start to spread out. The spreading is expansion. Now watch it frame by frame. You will catch some marbles moving towards the center of the floor after a hit(retrograde to the expansion). You'll also catch a few colliding.

Overall, galaxies are receding. Individually, they are free to do what they want.

[Murphy]

Grey:
Probably about 3 billion years for the Milky Way and Andromeda, which are the two big ones. Here are some simulations of what it might look like.

Very nice images, but I'm aware of the Andromeda-Milky Way collision, what I was asking about was whether all the Galaxies in our local group (or cluster, or whatever it's called) would one day merge, and if there was a timeframe on that.

[speedfreek]

Very nice images, but I'm aware of the Andromeda-Milky Way collision, what I was asking about was whether all the Galaxies in our local group (or cluster, or whatever it's called) would one day merge, and if there was a timeframe on that.

Around 100 billion years, according to the Scientific American article - "The End of Cosmology? (http://www.sciam.com/article.cfm?id=the-end-of-cosmology)"

The article is by the authors of the recent paper:

The Return of a Static Universe and the End of Cosmology (http://arxiv.org/abs/0704.0221)

[WayneFrancis]

okay, well if this is the case, then 13 billion years all matter would have been much closer together hence being much more dense. So if the density was so high it would have stopped the universe from expanding in the first place. How can you explain this?

Well there is the Nobel Prize winning answer. If you can tell us what the initial cause of the BB was and what is driving the apparent increasing expansion of the Universe then you will mark yourself into history as one of the greatest scientists of all time.

I can't explain what causes gravity either but I can see its effects. I can see the effect of the universe expanding and can say this effect must have either been strong enough to over come gravity in the early universe or, for some even stranger reason, kicked in on a large but static universe that just happens to appear that it was only created about 14 billion years ago.

[Murphy]

Around 100 billion years, according to the Scientific American article - "The End of Cosmology? (http://www.sciam.com/article.cfm?id=the-end-of-cosmology)"

Ah, cool thanks for the info, that's quite interesting. I should really get round to reading a general Cosmology book that details all these kinds of things, if I can find the time.

[zhamid]

[QUOTE]I can see the effect of the universe expanding and can say this effect must have either been strong enough to over come gravity in the early universe...QUOTE]

Well, bigbang would have created an expanding universe, but gravity would have eventually slowed it down and caused it to collapse. With time though, however, objects got far enough just on the initial inertia that the "dark energy" could start accelerating them.