# What happens as the universe expands?

28 November 2010

## Question

The universe is expanding at an ever-increasing rate. a) How fast is that now? b) Presumably the limit will be the speed of light - what will happen then? Will the rate of acceleration slow down as we approach the speed of light or just suddenly stop? Will the universe begin to contract after that or just keep on expanding?

We put this to Carolyn Crawford, from the Institute of Astronomy at Cambridge University... Carolyn - The most straightforward part of that question is, how fast the universe is expanding now because this is something we actually measure. We call the current rate of expansion something called the Hubble constant, and that is really just a measure of how fast bits of the universe are moving away from us as a function of their distance. As you go further out away from the Earth, bits of the universe are travelling ever faster. For example if you go to 30 million light years away, maybe you've got a bit of universe receding away from us about 700 kilometres per second, go 10 times further to 300 million light years away, it's receding at 7,000 kilometres per second, and if you carried on going further and further away, yes, you do get a point where you're far enough away and that bit of the universe is moving away from us at speeds greater than the speed of light. So it is not a fundamental speed limit.

The key thing here to realise is that the expansion of the universe is caused by stretching of space and that can be at any speed it likes, much greater than the light speed. The thing that is constrained by the light speed is the rate that things can move through space. So the expansion of the universe is the stretching of space, and it's carrying the galaxies along for the ride, but any light, any signal or information from those galaxies can't travel faster than the speed of light.

In fact, the expansion of space is not constrained by how fast it's moving. It's constrained by how much stuff is in it, and that's going to determine whether the universe will contract or carry on expanding forever.

Diana - If space objects are moving away from us at a speed faster than light then the lights from them cannot travel fast enough to reach us.

Carolyn - All the evidence currently is that this expansion is getting ever faster and the universe is going to carry on expanding forever. The speed at which it expands isn't affecting the eventual fate of the universe, but what it does affect is what we can see of that universe, and how much of it we can see because if a galaxy is moving away from us faster than the speed of light, that means any photons of light leaving it can't travel across the space fast enough. It can't outrun the expansion of space. And so we have something called the observable horizon. Anything that's expanding further than the speed of light away from us, we will never see the light from those objects. And as the universe gets bigger and bigger, that horizon is going to shrink and we're going to see less and less of the universe, and that's really what's going to be affected by this expansion speed.

Diana - But what about that crunch theory?

Carolyn - Whether or not the universe actually collapses back down on itself depends on how much matter there is in the universe and how much stuff there is to pull it back together under gravity, and really, the current cosmological thinking is that with the presence of dark matter and dark energy, the universe is not going to contract. It's going to carry on and expand. It's going to expand faster and faster, and never go back to that big crunch at the end.

Diana - Current thought is, the universe will just keep expanding. To sum up, objects are moving away from us in space as space stretches, meaning that they can travel away from us faster than the speed of light. But those objects cannot move through space faster than the speed of light.