The
zero-energy universe theory states 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.
[1][2]
The theory originated in 1973, when
Edward Tryon proposed in the
Nature journal 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.
[3]
Free-lunch interpretation
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.
[4][5] 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 inflaton field—the total gravitational energy decreases (i.e. becomes more negative) and the total inflaton 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.
[6]
Quantum fluctuation
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 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.
[3][7]
Hawking gravitational energy
Stephen Hawking notes in his 2010 book
The Grand Design: "If the total energy of the universe must always remain zero, and it costs energy to create a body, how can a whole universe be created from nothing? That is why there must be a law like gravity. Because gravity is attractive, gravitational energy is negative: One has to do work to separate a gravitationally bound system, such as the Earth and moon. This negative energy can balance the positive energy needed to create matter, but it’s not quite that simple. The negative gravitational energy of the Earth, for example, is less than a billionth of the positive energy of the matter particles the Earth is made of. A body such as a star will have more negative gravitational energy, and the smaller it is (the closer the different parts of it are to each other), the greater the negative gravitational energy will be. But before it can become greater (in magnitude) than the positive energy of the matter, the star will collapse to a black hole, and black holes have positive energy. That’s why empty space is stable. Bodies such as stars or black holes cannot just appear out of nothing. But a whole universe can." (p. 180)
