I was reading an essay from Stephen Hawking's on the Space and Time warps and I was trying to make sense on some statements referring to the Casimir effect such as:
The energy density of empty space far away from the plates, must be zero. Otherwise it would warp space-time, and the universe wouldn't be nearly flat. So the energy density in the region between the plates, must be negative.
Could anyone tell me what's the logic about energy from a far away place from plates being zero? If that was not the case how would the space-time warp? I might lack the knowledge, but I would like to understand the reasoning or have an intuitive explanation about those statements. Thank you very much!