Before you can define what the universe is, you need to be clear on what exactly you mean. If you refer to everything that you can see, then you’re defining the observable universe. If you’re refering to everything that is, has been and will be, then you’re talking about the Whole Universe.
Q: Does the Universe have a centre?
A:For the observable universe, yes. The observable universe is everything that we can see from our perspective of the universe. This includes light from distant galaxies reaching us, so really, you can say that each person is the centre of their own observable universe.
For the Whole Universe, the answer is no. It’s hard to imagine, as the Universe is expanding, so how can it not have a centre? The problem is that the Universe doesn’t expand like a shockwave from The Big Bang; rather, it expands like the surface of a balloon. If you were to draw points on a balloon and then inflate it, all the points would move away from each other at the same rate, but with no centre.
Q: Is the Universe expanding?
A: For the observable universe this is true in two senses. Space is obvioulsy expanding, so that’s a yes. In addition, older light from galaxies too far away to be previously seen reaches us, so our observable universe expands in that sense too. The Whole Universe expands in terms of space.
Q: Can you measure how big the Universe is?
A: For the observable universe, yes. You could just asume that, if you knew the distance that light from the furthest galaxies took to reach you, which is about 13 billion lightyears, and double it to give you a diameter, then you’re all set. But as we know the universe is expanding, the objects that we see have become a lot further away than what we see. Really, as we have observed that object, our observable universe should be bigger than it is. And the Whole Universe is likely infinite, so you can’t measure it in terms of distance.
Kojo Otu, CGS Physics Learning Leader.