It's not difficult to derive why the gravitational potential energy must be negative.[0] For the cosmological implications of this fact, there's Lawrence Kraus

It's not difficult to derive why the gravitational potential energy must be negative.[0] For the cosmological implications of this fact, there's Lawrence Krauss's book "A Universe from Nothing", also "The Inflationary Universe" by Alan Guth, and Stephen Hawking touches on it in "A Brief History of Time." [0] https://en.wikipedia.org/wiki/Gravitational_energy https://en.wikipedia.org/wiki/Gravitational_energy