"The Universe must be flat. Why? Well, there is two reasons. There's the one I normally say, which is: it's the only mathematically beautiful universe. Which is true, but there's another reason I don't usually... talk about but I'll talk about here. It turns out that in a flat universe the total energy of the universe is precisely zero because gravity can have negative energy. So the negative energy of gravity balances out the positive energy of matter. What's so beautiful about a universe with total energy zero? Well, only such a universe can begin from nothing, and that is remarkable because the laws of physics allow a universe to begin from nothing. You don't need a Deity. You have nothing: zero total energy, and quantum fluctuations can produce a universe. So, if the Universe isn't flat we're worried, because then you've got energy at... the very beginning of Time."