A Universe from Nothing: Why There Is Something Rather than Nothing

by Lawrence Krauss

Nevertheless, the data have had the last word, and the last word is in. Our observable universe is as close to being flat as we can measure. The Newtonian gravitational energy of galaxies moving along with the Hubble expansion is zero—like it or not.

Empty space can have a non-zero energy associated with it, even in the absence of any matter or radiation.