If the universe is expanding in all directions, why is Andromeda hurtling toward the Milky Way?
MICHELLE THALLER: One of the questions I get asked most commonly as an astronomer is if almost all the galaxies in the universe are flying away from us in space, why is the Andromeda Galaxy getting closer to us? Does that somehow mean that the Big Bang works differently in different parts of the universe? And the answer simply is: no. Space is expanding because of the Big Bang. All of space is expanding in every direction all at once and from our viewpoint that means that it looks like all the other galaxies are moving away from us. But not all the universe around us appears to be expanding. For example, the Earth doesn't seem to be getting any farther away from the Sun. The Sun is not getting any farther away from our galaxy. Thank about smaller scales, like your body; your body (luckily) is not expanding along with the universe. And the reason is that the expansion of the universe is actually a pretty gentle force; you really only notice it out in the middle of nowhere in the vast reaches of space between the galaxies. There's a lot of space out of there, so there is a lot of space to expand and so you really notice this expansion.
But there are things that are stronger than the expansion force. For example, my body is held together by chemical forces and by electrical forces. That is much, much stronger than the tiny little push that space has to expand inside me. I hold together very well. One of the analogies I think about is: You could try to push over the Empire State Building by blowing on it. You are actually exerting a force on the Empire State Building by blowing on it, you can measure that force, but you're not going to blow over the Empire State Building. There are things that are much stronger than this omnipresent but gentle force of the expansion of the universe. The gravity between the Sun and the Earth is stronger than space's pressure to expand over that scale. The force of gravity is stronger than the outward push of the expansion of the universe. That's also true of the galaxy, we are held in orbit around the center of the galaxy. Gravitationally, that's much stronger than any expansion force.
So why is Andromeda different? Andromeda is close enough to our Milky Way Galaxy that the gravity between the two is strong enough for the two to start moving together. Yeah, space is expanding between us and the Andromeda Galaxy, but gravity is accelerating Andromeda toward us faster than that expansion. And, in fact, that means that Andromeda is going to collide with the Milky Way in a couple billion years. And we see this happening all over the universe. There are clusters of galaxies where the galaxies are close enough together that they are merging and colliding. When galaxies are far enough away from each other that the gravitational force is weaker, the acceleration due to gravity is weaker than the outward expansion, then they start moving away. Now one of the intriguing things is that we don't know what the future holds when it comes to the expansion force of the universe. Just recently in the last couple of decades we've measured that the universe is not only expanding but it's actually accelerating. In some ways that force is getting stronger and stronger all the time and we don't know whether that will stop or whether that will stay constant or whether in fact that expansion force will keep getting stronger.
Will there be a day when the expansion force of the universe is strong enough that our galaxy does start to expand and the stars start to move farther and farther away from each other? Will there come a time when the Sun and the Earth are actually pulled apart by the expansion of the universe? And perhaps most intriguingly, will there ever come a time when the expansion of the universe is strong enough to rip apart your atoms, to actually have matter disappear into a little soup of organic particles? We call this idea the Big Rip and it's one of the possible ends of the universe – that the expansion force will eventually get so strong it literally rips everything apart. We don't know whether this will happen yet so we have a lot more investigating to do about this thing called dark energy that may be accelerating the universe and that's one of the best questions we're trying to answer right now.