Well, most galaxies are moving apart, but most of them are doing it as clusters that are bound together by gravity. The Big Bang may have started everything out in a straight line (though I’m not even sure if we know that for a fact), but gravity wants to pull things together. Within the clusters of galaxies, they could be traveling in just about any direction at this point in time, even directions that could cause a collision.
One way to think about it is like throwing a baseball up. It’s moving away from the Earth when it starts, but gravity turns the path in a parabola that causes it to collide with the Earth again.
And simulations of galaxy collisions do show large numbers of stars flung off into deep space. If you do some Google searches, you should be able to find animations of it.
This site has some fantastic supercomputer simulations and modeling of galactic collisions. If you have the time and bandwidth, download the full resolution videos (rather than the YouTube versions) and be prepared to be mesmerized.
Escape velocity from the Milky way is on the order of 550 km/sec
The Hubble constant for expansion is about 70.1 ± 1.3 (km/s)/Mpc.
Thus Milky way sized galaxies would have to be separated by about 7.8 megaparsecs (25.6 million light years) apart in order to for the expansion of space to free them from each others gravity.
I wish people would quit quoting this as established fact. All we know for certain is that we’re currently approaching each other. But this does not imply that we’re going to collide: It’s rather more likely that we’ll just peacefully pass by each other. In fact, the two galaxies have probably passed close by each other this way several times since they were both formed.
To clarify, we know that Andromeda is moving towards the Milky Way radially through fairly straightforward observations of redshifting (err blueshifting), but “side-to-side” tangential velocity is harder to pin down.
My understanding is that given the amount of radial velocity, for the two galaxies to pass without interacting, they would have to be unbound gavitationally. There’s probably enough tangential velocity that the two will make a small loop before they collide. But they’ll be close enough that there will be tidal interactions which will steal orbital energy and cause the collision.
It’s probably true that the two galaxies have looped around each other two or three times since they formed, but that’s not enough for the orbits to have stablized.
The above is based on a fairly recent article in Sky&Telescope which was based on computer simulations. But since then it’s been found that the Milky Way is more massive that was previously thought. However, the additional mass most likely means the collision will occur sooner than previously expected.
Remember the example of the universe being like a balloon with the galaxies represented by magic marker dots, and how as it is blown up, the dots move away from each other?
Does anyone want to explain how these ‘dots’ get to cross each others paths?
The Big Bang occurs and all matter is sent blasting from a specific point, in every direction. At some point, the mysterious property gravity, begins to draw some of the matter together, which developed into blobs we call galaxies. Such was the velocity these particles were travelling from their initial point, that these galaxies continued travelling “outwards”, and even seem to be speeding up.
So, if you rewound all these galaxies to their starting point, when would their paths have criss-crossed, and why are they likely to start doing it in the future if they haven’t already?
The Big Bang was not a firecracker explosion that sent matter away on an empty space. There was no space there to begin with. Have you seen those “art” pieces where they squirt paint on a canvas and then they make it spin and the paint spreads over the blank parts of the canvas? The Big Bang was not like that. The canvas itself is growing.
That is how the balloon example works. It is not that the matter is the balloon and it is expanding propelled by an explosion filling the empty room. The balloon is the universe expanding. Gravity won’t make the balloon collapse again. Gravity moves the dots around the balloon surface.
Am I making any sense or just further confusing this?
This appears to be the case, but it’s a consequence of the particular conditions of the Universe, not the way the physics operates. A decade ago, it was not known whether gravity would cause the balloon to collapse eventually or not.