The zero-energy universe hypothesis proposes that the total amount of energy in the universe is exactly zero: its amount of positive energy in the form of matter is exactly canceled out by its negative energy in the form of gravity. The zero-energy universe hypothesis proposes that the total amount of energy in the universe is exactly zero: its amount of positive energy in the form of matter is exactly canceled out by its negative energy in the form of gravity. Pascual Jordan first suggested that since the positive energy of a star's mass and the negative energy of its gravitational field together may have zero total energy, conservation of energy would not prevent a star being created by a quantum transition of the vacuum. George Gamow recounted putting this idea to Albert Einstein: 'Einstein stopped in his tracks and, since we were crossing a street, several cars had to stop to avoid running us down'. The zero-energy universe theory originated in 1973, when Edward Tryon proposed in the journal Nature that the universe emerged from a large-scale quantum fluctuation of vacuum energy, resulting in its positive mass-energy being exactly balanced by its negative gravitational potential energy. A generic property of inflation is the balancing of the negative gravitational energy, within the inflating region, with the positive energy of the inflaton field to yield a post-inflationary universe with negligible or zero energy density. It is this balancing of the total universal energy budget that enables the open-ended growth possible with inflation; during inflation, energy flows from the gravitational field (or geometry) to the inflation field—the total gravitational energy decreases (i.e. becomes more negative) and the total inflation energy increases (becomes more positive). But the respective energy densities remain constant and opposite since the region is inflating. Consequently, inflation explains the otherwise curious cancellation of matter and gravitational energy on cosmological scales, which is consistent with astronomical observations. Due to quantum uncertainty, energy fluctuations such as an electron and its anti-particle, a positron, can arise spontaneously out of vacuum space, but must disappear rapidly. The lower the energy of the bubble, the longer the duration it can exist. A gravitational field has negative energy. Matter has positive energy. The two values cancel out provided the universe is completely flat. In that case, the universe has zero energy and can theoretically last forever.