Question: How do we know the size of our universe if it so big we can't reach parts of it?

Sarah Martin answered on 22 Jun 2012:

Hi ciarameehan!

It’s true that we have only so far explored parts of our tiny solar system, but we see stars that are much further away! But even here we can’t see the really far away ones, because their light hasn’t reached us yet. Some parts of the universe may simply be too far away for the light emitted from there at any moment since the Big Bang to have had enough time to reach Earth! In the future the light from distant galaxies will have had more time to travel, so some regions not currently observable will become observable in the future.
However we can estimate how much we currently can’t see by calculating how much energy is in the universe based on other observations apart from light – the cosmic microwave background and neutrinos for example, which give us an idea of how long ago it all started and how fast it’s moving apart – and when you know how long something’s been travelling, and how fast it’s going, you can calculate how far it’s travelled, right? Well, same for the outermost bits of the universe that we can’t see.

🙂 Sarah