"Two pieces of matter that are close to each other have less [positive] energy than the same two pieces a long way apart, because you have to expend energy to separate them against the gravitational force that is pulling them together," S. Hawking, "The Theory of Everything", New Millennium, 2002

Since it takes positive energy to separate the two pieces of matter, gravity must be using negative energy to pull them together. Thus, "the gravitational field has negative energy. In the case of a universe that is approximately uniform in space, one can show that this negative gravitational energy exactly cancels the positive energy represented by the matter. So the total energy of the universe is zero."

"If you drop a ball from rest (defined to be a state of zero energy), it gains energy of motion (kinetic energy) as it falls. But this gain is exactly balanced by a larger negative gravitational energy as it comes closer to Earth’s center, so the sum of the two energies remains zero." Alexei Filippenko and Jay Pasachoff, "A Universe From Nothing"

https://www.astrosociety.org/publications/a-universe-from-nothing/

The universe as a whole can be compared to this ball. Initially, before the big bang, the universe-ball was at rest. Now, after the big bang, it is falling: light and matter exist, and they are moving. And yet, because of the negative energy built into the gravity field created by these particles, the total energy of the universe remains zero.

The question, then, is why the ball started falling in the first place. How did something - composed of equal positive and negative parts, mind you - come from nothing?

Physicists aren't exactly sure, but their best guess is that the extreme positive and negative quantities of energy randomly fluctuated into existence. "Quantum theory, and specifically Heisenberg’s uncertainty principle, provide a natural explanation for how that energy may have come out of nothing," wrote Filippenko and Pasachoff.

They continued, "Throughout the universe, particles and antiparticles spontaneously form and quickly annihilate each other without violating the law of energy conservation. These spontaneous births and deaths of so-called 'virtual particle' pairs are known as 'quantum fluctuations.' Indeed, laboratory experiments have proven that quantum fluctuations occur everywhere, all the time."

Cosmologists have constructed a theory called inflation that accounts for the way in which a small volume of space occupied by a virtual particle pair could have ballooned to become the vast universe we see today. Alan Guth, one of the main brains behind inflationary cosmology, thus described the universe as "the ultimate free lunch."

In a lecture, Caltech cosmologist Sean Carroll put it this way: "You can create a compact, self-contained universe without needing any energy at all."

http://www.livescience.com/33129-total-energy-universe-zero.html

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