Stephen Hawking is one of the greatest scientists and thinkers of our time, and is famous around the world because he writes and talks about cosmology - the science of the universe - in a way that people can understand. He has a terrible disease called ALS, and today he can only move and speak with the help of machines. But this has not stopped him working to try and understand our world better. Will people travel in time one day? Where did the universe come from? Most of us cannot even begin to think about questions like this - but Stephen Hawking is always looking for the answers.