Everything we know about gravity suggests the universe should expand at a decelerating rate, but the opposite is true says Christophe Galfard, Stephen Hawking’s protege.

Christophe Galfard:  So let’s talk for a second about the expansion of the universe. We humans live on Earth. That we all know. And we can look at the stars. There are stars that are close to us and there are stars that are further away belonging to other galaxies. And there are stars that are extremely far away belonging to extremely far away galaxies. Now for about a century we know that we see that far away galaxies which are islands of billions of stars, far away galaxies seem to move away from us. We see that the light that reaches us coming from those galaxies is what the scientists call red shifted meaning when something has been stretched in between those galaxies and us. What has been stretched is the distance that separates us from those galaxies. As time goes by all the distances in space stretch. They expand. That’s what we call the expansion of the universe. It is not actually that those far away stars or galaxies move away from us. It is the distance itself that increases. There is more space and time in between them now than there was before. Just to make that point clear that’s when you are on Earth and look at the stars the expansion of the universe makes all the far away galaxies look as if they were moving away. But if you imagine that you are living somewhere else in another galaxy far away in the universe from there all the stars in galaxies that are far away would again be moving away from you.

To summarize, the expansion of the universe means that distances increase with time. That’s not something that is completely crazy to understand. If you imagine that the universe started with a big bang meaning that everything we can see in outer space right now was condensed in a very much smaller volume a long time ago. Then somehow some energy got released and everything expanded, okay. The universe grew. But if you think about that carefully what that means is that somehow this expansion, this blow up, this big bang thing should slow down with time. When you have an explosion on Earth the objects that are thrown away they slow down because there is friction because there is something happening. Objects don’t keep moving and flying forever. They slow down. The universe it should be the same. In some sense gravity should keep things from moving too far away from one another. If there is enough gravity for instance the expansion should slow down after a while. But there you go. In 1998 astronomers discovered that it is actually the other way around. In space looking at far away galaxies they realized that the expansion, the rate of expansion is accelerating. So there is a force pushing away the far away galaxies faster and faster with time. And that’s something that gravity cannot explain.

Gravity pulls things back towards one another and this accelerated expansion means that there is another force, a force that has been called dark energy. This discovery of dark energy of the accelerated expansion of our universe did change something already in the way we picture the future of our universe. For a long time since we know of the big bang there were basically two possibilities for the future of our universe. The first one is that there is so much matter in there that despite the expansion gravity would work like some kind of rubber band and would bring everything back into what we call a big crunch. Everything will be pulled towards each other at some stage and the expansion would go the other way around and we would have the precise opposite of the big bang which was called the big crunch. The discovery of the accelerated expansion kind of ruled that possibility out in the sense that gravity will never be strong enough to pull things back into a big crunch. We are more into a scenario in which our universe will keep expanding, will keep getting bigger and bigger forever. That is what we can expect as of today for the future of our universe.