Expanding Universe?

[Robb Phoenix]

If the Universe is expanding then an observer at one point in the universe would see all the points moving away from him ( like the dots on an inflating baloon) , then why do Galxies cross in the path of each other and merge????????

[mibn2cweus]

The acceleration towards each other due to mutual gravitational attraction between them exceeds the local rate in the expansion of the universe.

[jake-the-peg]

In other words although the general trend is seen as expansion, you get individual motions that cal be in any direction.

If you imagine an expanding gas, all the molecules are not heading out in a straight bee line but bouncing about with a general outward trend.

In our own local group of galaxies some show a blue shift in that they are actually coming towards us.

[Robb Phoenix]

Does this mean that the analogy that the Universe is expanding like the seperating dots on an inflating baloom is wrong?.
I see such an analogy given in almost every book that tries to explain the big bang (like Stephen Hawkings, 'A Brief History Of Time'). It is also given in many textbooks that:
"Over a large scale an observer at one point in the UNiverse would see other points moving away from him":

[kwicky]

If the universe is 13 billion years old and expanding like a balloon you have to ask the question 'when will the balloon burst' and plunge us all into chaos?

[mibn2cweus]

Jake, I thought you nailed it with the expanding gas analogy. Great example . . . oh well

Robb, There's a reason that sentence begins with,
"Over a large scale . . . "

The fact that the universe overall is expanding, does not rule out two nearby objects from moving towards one another.

When you inflate a balloon, the speed at which two points move away from each other is in proportion to the distance between them. Two adjacent points will separate at half the speed of two points that are twice as far from each other.

The 'attraction' between objects due to gravity is inversely proportional to the square of the distance. This means that two identical objects placed twice as far from each other would only 'feel' one/fourth the acceleration of gravity.

Two galaxies can be close enough to each other that the rate of expansion of the universe is insufficient to overtake the attraction of gravity. It is only "On a large scale . . . " that it becomes readily apparent that the separation between more distant galaxies is increasing as the rate of expansion becomes more evident and the motion of galaxies through space becomes comparatively less significant.

I suppose that until you have done it, the motion of galaxies within the expansion of the universe (or of particles moving relative to each other within an expanding cloud of gas) can be difficult to visualise. Can anyone else think of something more available to common experience to relate this to???