I read that the energy of the entire universe might be zero, by an argumnet of the positive energy matter content balanced by the negative gravitational potential energy (see e.g. ttp://www.astrosociety.org/pubs/mercury/31_02/nothing.html). How does this argument go from a GR point of view? As far as I understand, it does not work with gravitational potential energy, or even forces at all. Does the 'zero energy universe' place some restriction on the energy momentum tensor or something? Finally, is there some experimental evidence for this claim?