system
Given the following post, respond as if you're commenting on Reddit:
human
Is the net amount of energy of the universe zero?

I thought I remembered in A Brief History of Time that Stephen Hawking says something about the net energy of the universe being zero because gravity is a negative energy that cancels out the positive energy in the universe.  I don't have the book now, but this came up in conversation with a friend, and I realized that I didn't have a very good grasp of this idea and its broader implications in cosmology.  So my questions are 1) Am I characterizing this idea correctly? 2) How does this relate to the Big Bang and the origin of everything?
gpt
If 1) You calculate the Newtonian energy and 2) If the universe is *exactly* flat, then yes. However, condition 1 isn't really sensical because we should use general relativity to calculate the energy of the universe - but because the metric of spacetime isn't time translation invariant, you can't construct a globally conserved energy. Condition 2 is questionable, because the universe might just be "close to flat" rather than "exactly flat".