Is the Universe infinitely big?

We put this question to Cambridge University astrophysicist Zephry Penoyre...

Zephyr - So, this old kind of stuff gets actually very worrying. It's even possible that time, although it's bounded by having a start, that as soon as the universe started, it may have instantly been infinite.

Chris - Hang on a minute. How does that work out? How can it be infinite if it's only just got started?

Zephyr - There are theories that it's quite possible that as soon as the universe was born, if you just went to one direction for long enough, you will get back to the same place; that the universe is essentially curved around on itself. Now, it's very difficult for us to see this because the universe now is so very large that we only see a tiny, tiny fraction of it. In our expanding universe, because light travels at a finite speed, the expansion of the universe can actually outstrip the speed of light and there's a finite horizon in which we will ever see or light from us or signals from us will ever be able to get - which is quite a worrying thought if you think about it, and even more worrying that horizon is shrinking. Our universe expands faster and faster and that horizon gets smaller and smaller compared to the total size of the universe.

Chris - So, there are literally some bits of the universe that we could never see even if we wanted to.

Zephyr - Likely, the vast majority, when we calculate things about the universe, we use the size of the universe, we see as a lower limit, but there may be no limit to how big it is. It's certainly so big that we're not seeing the overlap as we go all the way around once.

Chris - We also get the question quite a bit. People say, well, if the universe started with a Big Bang and the Earth has only been here for about 4.5 billion years, what are these echoes of the Big Bang, this light we're detecting as the cosmic microwave background radiation left over from the Big Bang? Why is it only just going past the Earth now if the universe was here and expanded away from us billions of years ago? How does that work?

Zephyr - So, the lovely thing about the cosmic microwave background is that it's almost completely uniform everywhere in the universe we see. This is actually really surprising result and it's the reason we invented inflation because we don't expect it to be in the form of inflation.

Chris - This is not the kind of inflation that Georgia Osborne is interested in. this is the universe getting bigger, isn't it?

Zephyr - Yes. The universe as it starts, grew incredibly rapidly, and then seemed to slow down a lot. And so, what this does is mix all of these cosmic microwave background radiation uniform everywhere in the universe. So, the light that we're seeing getting to us just now would be exactly the same as a light getting to any other point just now or getting to us at a different point in time. And so hence, the light we're seeing just now is completely uniform.