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The zero-energy universe hypothesis states that the total amount of energy in the universe is exactly zero. The amount of positive energy in the form of matter is exactly canceled out by the negative energy in the form of gravity.
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. 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.
Quantum fluctuation 
Due to quantum uncertainty energy fluctuations such as 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.
See also 
- ^ "A Universe from Nothing". Astronomical Society of the Pacific. Retrieved 10 March 2010. by Alexei V. Filippenko and Jay M. Pasachoff
- ^ "A Universe From Nothing lecture by Lawrence Krauss at AAI". 2009. Retrieved 17 October 2011.
- ^ Alan Guth, The Inflationary Universe, (ISBN 0-224-04448-6) Appendix A) Since the negative energy of a gravitational field is crucial to the notion of a zero-energy universe, it is a subject worth examining carefully. In this appendix I will explain how the properties of gravity can be used to show that the energy of a gravitational field is unambiguously negative. The argument will be described [in the appendix] in the context of Newton's theory of gravity, although the same conclusion can be reached using Einstein's theory of general relativity.
- ^ Stephen Hawking, (page p129, A Brief History of Time)
- ^ We might decide that there wasn't any singularity. The point is that the raw material doesn't really have to come from anywhere. When you have strong gravitational fields, they can create matter. It may be that there aren't really any quantities which are constant in time in the universe. The quantity of matter is not constant, because matter can be created or destroyed. But we might say that the energy of the universe would be constant, because when you create matter, you need to use energy. And in a sense the energy of the universe is constant; it is a constant whose value is zero. The positive energy of the matter is exactly balanced by the negative energy of the gravitational field. So the universe can start off with zero energy and still create matter. Obviously, the universe starts off at a certain time. Now you can ask: what sets the universe off. There doesn't really have to be any beginning to the universe. It might be that space and time together are like the surface of the earth, but with two more dimensions, with degrees of latitude playing the role of time. Stephen Hawking
- ^ Edward P Tryon, Is the Universe a Vacuum Fluctuation?, Nature, vol 246, pp 396-7, 1973