How fast is the universe expanding?

Fran Chadha-Day took on this one, clarifying firstly how we know if the universe is expanding, and how we can measure it...

Fran- When we look at distant galaxies, we see that they're all moving away from us and the further away the galaxy is, the faster it's moving away from us. And we can imagine what's going on here by thinking about a balloon that I've put lots of dots on. As this is radio, you're going to have to do some visualization, but a balloon with a lot of dots on and I'm blowing it up and all of the dots get further and further away from each other, as I do that because the balloon is expanding. And if I imagine I'm sitting on one dot, the further away the dots are, the faster it looks like they're moving away from me. So that's what's going on. And one measure of how fast the universe is expanding, is how much faster the further away galaxies are moving than the closer-to-me galaxies. This is called the Hubble constant and its value is about 70 kilometers per second per megaparsec. So a megaparsec is about a million light years. So that means that something a million light years away from the earth is moving away from us at about 70 kilometers per second. Something a hundred million light years away from the earth is moving away from us at about 7,000 kilometers per second. So that is the best measure of how fast the universe is expanding. And it's a very important parameter in cosmology.

Chris - So why should something more distant be moving away from us more rapidly?

Fran - So is because all of the space between us and the galaxy is kind of increasing in size. So the more space there is, the more increasing size there is.

Chris - And hence the further you look or the furthest away you look, the greater the increment. But it hasn't always been the same number has it? Because if we look at the way that the universe is now, the only way we can explain the universe we have is if when it was very, very young, very, very small, it grew faster than a teenager and expanded very, very quickly. And then it slowed down a bit and now it appears to be speeding up again. So how do we explain that?

Fran- Yeah. So you're talking about initially there's a period of inflation and we need inflation to explain a lot of observations about the universe, notably why it's so homogenous. And no one really knows what drove inflation. It's one of the mysteries in physics, but we think there was probably some particle as yet undiscovered that drove this inflation. Now, it seems like the rate of expansion is accelerating again. Not to the same extent as it was during inflation, but a bit. And we think this is due to something called dark energy, which would be either kind of a constant in Einstein's equations, or perhaps a new particle or field that permeates the universe that is driving this accelerated expansion. As you can tell from the way I'm talking about it, we don't even fully understand what questions we should be asking.

Chris- Isn't the weird thing about this though, that um, space seems to make dark energy when it exists and the more space you make, the more dark energy you make. So it seems to be almost like it's coming from nowhere.

Fran- That's right. So with a normal substance like matter, as space expands, matter gets less dense. You know, if I've got stuff in a box and I increase the size of a box, it gets more dilute. But with dark energy, that is not the case. The density remains the same, so it just grows and grows as space grows.