MICHELLE THALLER: One of the questions I most often ask as an astronomer is if almost all the galaxies in the universe fly from space to us, why is the Galaxy Andromeda approaching us? Does that mean that the Big Bang works differently in different parts of the universe? And the answer is simple: no. Space is expanding due to the Big Bang. The whole space expands in all directions at once and from our point of view it means that all other galaxies are moving away from us. But not the entire universe around us seems to expand. For example, the Earth does not seem to be moving away from the sun. The sun does not move away from our galaxy. Thanks for the smaller rocks like your body; your body (fortunately) does not expand with the universe. And the reason is that the expansion of the universe is actually a gentle force; you really notice it only in the middle of nowhere in the vast spaces between the galaxies. There's plenty of space there, so there's plenty of room to expand and that's why you really notice this extension.
But there are things that are stronger than the force of expansion. For example, my body is held together by chemical forces and by electrical forces. This is much, much stronger than the little push that space has to expand inside of me. I'm very good together. One of the analogies I think is: You can try to defeat the Empire State Building by blowing it. You are actually exerting power over the Empire State Building, blowing on it, you can measure that power, but you will not blow the Empire State Building. There are things that are much stronger than this ubiquitous but gentle force of the expansion of the universe. The gravity between the Sun and the Earth is stronger than the space pressure to extend beyond this rock. The force of gravity is stronger than the external impetus of expanding the universe. This is true of the galaxy, we are in orbit around the center of the galaxy. Gravitationally, it is much stronger than any power of expansion.
Why then is Andromeda different? Andromeda is close enough to our Milky Way galaxy, that the gravity between the two is strong enough for them to start moving together. Yes, the space expands between us and the Andromeda galaxy, but gravity accelerates Andromeda to us faster than this expansion. And in fact, it means Andromeda will face the Milky Way for several billions of years. And we see this happening throughout the universe. There are groups of galaxies where galaxies are close enough to merge and collide. When galaxies are far enough apart that the gravitational force is weaker, the acceleration due to gravity is weaker than the external expansion, then they begin to move away. One of the intriguing things is that we do not know what the future is when it comes to the power of expanding the universe. More recently, over the last few decades, we have measured that the universe is not only expanding but actually accelerating. In a sense, power is getting stronger and stronger, and we do not know whether it will stop or whether it will remain constant or, in fact, that the force of expansion will continue to grow stronger.
Will there be a day when the power of the expansion of the universe is strong enough that our galaxy begins to expand and the stars begin to move farther and farther apart? Will there come a moment when the Sun and Earth will actually be torn apart by the expansion of the universe? And perhaps most intriguing, is there ever going to be a moment when the expansion of the universe is strong enough to tear your atoms, in fact, the matter disappears into a small soup of organic particles? We call this idea the Big Rip and this is one of the possible ends of the universe – that the power of expansion will eventually become so strong that it literally tears everything. We do not know whether this will happen yet, so we have a lot more research on this thing called dark energy that can speed the universe and this is one of the best questions we are trying to answer right now.