The zero-energy universe 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.
It 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.
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.
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.
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 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)
- "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.
- Edward P. Tryon, "Is the Universe a Vacuum Fluctuation?", Nature, vol. 246, p.396–397, 1973.
- 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, A Brief History of Time, p. 129.
- "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, "If There's an Edge to the Universe, There Must Be a God" (interview), in Renée Weber, Dialogues With Scientists and Sages: The Search for Unity, 1986. (Also partially reprinted in "God as the Edge of the Universe", in The Scientist, Vol. 1, No. 7, February 23, 1987, p. 15.)
- Berkeley Lab, Smoot Group - http://aether.lbl.gov - Inflation for Beginners, JOHN GRIBBIN "Quantum uncertainty allows the temporary creation of bubbles of energy, or pairs of particles (such as electron-positron pairs) out of nothing, provided that they disappear in a short time. The less energy is involved, the longer the bubble can exist. Curiously, the energy in a gravitational field is negative, while the energy locked up in matter is positive. If the Universe is exactly flat, then as Tryon pointed out the two numbers cancel out, and the overall energy of the Universe is precisely zero. In that case, the quantum rules allow it to last forever." archived, 2014