The 2 galaxies are scheduled to “collide” in about 4 billion years, or about 30% of the time that has elapsed since the Big Bang. How could this be? Aren’t the galaxies in the universe moving away from each other as space expands? Why are these two headed for each other?
I put the word collide in quotes because, from what I understand, none of the stars will literally collide. It’s just that all of the gravitational forces will wreak havoc everywhere as the 2 galaxies coalesce into one. I’ve heard the analogy that if our galaxy were shrunk to the size of the Pacific Ocean, then our sun would be the size of a grain of sand and the closest grain of sand would be a mile away. That’s how much empty space there is out there. Does this mean that density of galaxies in the universe (as measured by the average space between them) is far greater than the density of stars in a galaxy? If that weren’t the case, and that the proportional distances between galaxies in the universe and the stars in a galaxy was the same (or even within an order of magnitude), the probability of 2 galaxies colliding would be as remote as 2 stars with a galaxy merger colliding. Or is it just the immense gravitational masses of the galaxies, with 5 times more dark matter than regular matter, that are causing this collision?
Because they and the other members of our Local Group of galaxies are gravitationally bound together. The expansion of space is not great enough to overcome this gravitational relationship, any more than it will cause the Earth to overcome the influence of the Sun. The expansion of space affects our distance to more distant galaxies that are not close enough for gravity to have much of an effect.
Nitpick: Will the two galaxies coalesce, or will they pass through each other and finish as two separate galaxies with distorted shape? Here’s a photograph of five colliding galaxies. That webpage seems to suggest that the collision might change the galaxies from ‘spiral’ type to ‘elliptical’ type.
4 billion years into the future isn’t much by cosmic standards. Remember, the universe has already existed for about 13.8 billion years. Some estimates say that by about 100 billion years into the future, there will be nothing observable outside the Local Group of galaxies, since everything else in the universe will be so far away from the Earth that it will not be possible to see it. After that, some estimates say that the Local Group will have collapsed into a single galaxy by something like 450 billion years from now. After that, it could be a Heat Death for the universe or a Big Rip, according to various estimates:
I’m fascinated by this picture (click to expand to full size):
It’s what the Andromeda galaxy would look like today compared to the size of the moon, if all the spiral arms were bright enough to see, despite the fact that it’s 2.5 million light-years away!
That sucker is BIG – and getting closer every day! :eek:
The expanding rate is actually very small and is noticeable only over gigantic distances. The expansion is related to distance–at two points 10x further apart, the expansion rate is 10x faster. With two galaxies only a few million light-years apart, the force of gravity causes them to fall towards each other (numbers entirely off the top of my head, not necessarily even on the right orders of magnitude) at 5,000 miles per hour and the space between them is expanding at a rate of .01 mile per hour, for a net speed towards each other of 4,999.99 miles per hour. For two galaxies to be net moving away from each other thanks to the expansion of the universe, they have to be far enough apart that the strength of the gravitational attraction between them is lower than the rate of expansion of the space between them.
It should also be noted that we don’t actually know that we’re going to collide. Right now, we can look at the Andromeda Galaxy and see that it’s getting closer to us: That’s a really easy measurement to make. But then again, when you’re driving down the road, and look ahead at the opposite lane, you also see cars that are getting closer to you. But those other cars almost never collide with yours. To determine whether another car or galaxy or whatever is going to collide with yours, you need to make other measurements, which are much more difficult, and to get enough information out of them you’d need to make those measurements over a span of time that’s at least a significant fraction of the time until closest approach.
Would such a collision result in catastrophic destruction, including the possible collision of another star into ours, and massive acceleration and deceleration forces? Or is there so much empty space and the timeframes are so large we would hardly notice? Not that there would be any humans around to observe it…
We’d notice, in the sense that the night sky would be very pretty. But it probably wouldn’t cause any problems on a Solar System or planetary scale.
And the black holes would eventually merge, which would provide some very interesting scientific data, but again wouldn’t cause any problems unless you were right on top of them.
EDIT: When I say “probably”, there, I don’t mean “eh, it could go either way, but this is the better bet”. I mean “99.if-I-put-in-enough-9s-it’ll-look-like-exaggeration% likely”.
I’m an engineer not an astronomer, but as I understand it, there is some debate over this.
What scientists think will happen is that both black holes will end up in the center of the new combined galaxy, and the two black holes will rotate around each other, getting closer and closer. In order for them to merge, though, they need to lose enough energy that they stop rotating around each other.
This is called the “final parsec problem” (you can google that for details), and theoretically (as I understand it), the loss of energy will take so long that the two black holes won’t be able to merge before the heat death of the universe.
But, just to throw a wrench into things, black hole pairs that are closer than predicted by the final parsec problem have been observed, and merging black holes have also been observed. I believe that they are trying to measure gravity waves from one of these mergers.
So, our knowledge of physics says they won’t merge, but our observations say they could merge.
Stick around for a few billion years and maybe you can answer the question.
In fact, they’re trying to measure gravitational waves from many such mergers, since knowing the rate at which they happen would be very useful for solving that problem, and you can’t get much of a rate from a single event.
And I’d be more inclined to say that our knowledge of physics says that they will merge, but we don’t know exactly how.
Although they also tug on each other gravitationally. This doesn’t matter so much for a single galaxy where everything’s orbiting in a relatively (by now) stable pattern,but you’re talking about mixing in a second set moving in a different set of standard trajectories/orbits. Stars actually colliding is going to be rare, but not as rare as it is in either galaxy alone, and “near-miss” gravitational effects like planetary systems being disrupted/destroyed will be a little less rare.
This same question comes up when people talk about (usually deliberately) colliding things with our sun. It’s actually much harder to do from a body in the solar system (which have existing orbital speeds that have to be overcome) than it is for an extrasolar object coming in. A drunk Andromeda driving by constitutes a significant number of potential extrasolar intruders.
The most likely destructive result would be a burst of star formation from the jostling and merger of any remaining interstellar gas. Shortly (by astronomical standards) followed by the larger short-lived stars going supernova.
Unfortunately, the cars passing each other by on a highway is not a perfect analogy in a significant way. For cars, if they come really close but don’t hit, they just keep going, albeit with a lot of buffeting from the wind of their passage. For compact objects (e.g. stars, planets) in space, the same thing. A close passage (say, during an orbit) will just mean they keep going. As long as they stay out of each other’s Roche limit, they’re good for a very long time.
Galaxies don’t work that way. They aren’t compact objects, but rather very extended objects. So a close passage will mean they raise tides in each other and those tides will steal energy from their orbital motion. That’s why the simulation shows them making a near passage before they finally collide. It doesn’t show the tides on the first passage, but they’ll be there and probably actually be visible, most likely as warping of the outer parts of their spiral arms.
