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Big Bang for beginners-13: Does the Big Bang theory violate the law of conservation of energy?

Tuesday, 30 March 2010

Although the universe is mostly empty space (leaving aside for the moment dark energy and dark matter), there is quite a lot of matter in it. Some of it is in dense clumps that we call planets, stars, and galaxies. The rest is far more dilute and consists of interstellar gases and dust. And quite a lot of it is in the form of massless photons. So the question naturally arises: where did all this stuff come from? Doesn't it require a massive input of energy right at the beginning that violates the law of conservation of energy (also known as the first law of thermodynamics), one of the bedrock principles of science? The answer is simple: No.

By Mano Singham

The total energy of the universe consists of the energy due to the motion of all the particles (called kinetic energy), the energy that is stored because of the gravitational forces between the particles (called potential energy), and the energy associated with the mass of all the particles (usually referred to as rest energy).

The key feature to bear in mind is that the gravitational potential energy is a negative quantity. You can see this by realizing that in order to separate two objects, one has to overcome the attractive gravitational force and this requires one to supply positive energy from outside. This is why launching satellites into space requires such huge amounts of positive energy supplied by fuel, in order to overcome the negative gravitational potential energy of the satellite due to the Earth's attractive force.

This negative gravitational potential energy exactly cancels out the positive energy of the universe. As Stephen Hawking says in his book A Brief History of Time (quoted by Victor Stenger, Has Science Found God?, p. 148): "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." In other words, it is not the case that something came out of nothing. It is that we have always had zero energy.

Alan Guth, one of the creators of the inflationary universe model, points out that the fact that "in any closed universe the negative gravitational potential energy cancels the energy of matter exactly" has been known for some time and can be found in standard textbooks. (See The Classical Theory of Fields by L. D. Landau and E. M. Lifshitz, second edition, 1962, p. 378-379.)

But what made the universe and all its mass come into being at all? The suggestion is that the universe began as a quantum fluctuation of the vacuum. It used to be thought that the vacuum was truly nothing, simply inert space. But we now know that it is actually a hive of activity with particle-antiparticle pairs being repeatedly produced out of the vacuum and almost immediately annihilating themselves into nothingness again. The creation of a particle-antiparticle pair out of the vacuum violates the law of conservation of energy but the Heisenberg uncertainty principle allows such violations for a very short time. This phenomenon has observable and measurable consequences, which have been tested and confirmed. (The Inflationary Universe, Alan Guth, 1997, p. 272)

Guth says (p. 12-14, 271-276) that the person who first suggested that the universe and its associated space may have originated as a quantum fluctuation was Edward Tryon in 1973 in his paper Is the Universe a Vacuum Fluctuation? (Nature, vol. 246, p. 396-397, 14 December 1973.) As Tryon says in that paper:

In any big bang model, one must deal with the problem of 'creation'. This problem has two aspects. One is that the conservation laws of physics forbid the creation of something from nothing. The other is that even if the conservation laws were inapplicable at the moment of creation, there is no apparent reason for such an event to occur.

Contrary to widespread belief, such an event need not have violated any of the conventional laws of physics. The laws of physics merely imply that a Universe which appears from nowhere must have certain specific properties. In particular, such a Universe must have a zero net value for all conserved quantities.

To indicate how such a creation might have come about, I refer to quantum field theory, in which every phenomenon that could happen in principle actually does happen occasionally in practice, on a statistically random basis. For example, quantum electrodynamics reveals that an electron, positron and photon occasionally emerge spontaneously from a perfect vacuum. When this happens, the three particles exist for a brief time, and then annihilate each other, leaving no trace behind.

If it is true that our Universe has a zero net value for all conserved quantities, then it may simply be a fluctuation of the vacuum, the vacuum of some larger space in which our Universe is imbedded. In answer to the question of why it happened, I offer the modest proposal that our Universe is simply one of those things which happen from time to time.

Note that our universe likely came into being with just a tiny amount of matter. But after that initial fluctuation triggered the start of the universe, what caused the avalanche that created the massive amount of matter that currently comprise our universe? The inflationary model of the universe takes care of that problem too, although the explanation is a little technical. As Stenger says (p. 148):

[I]n the inflationary scenario, the mass-energy of matter was produced during that rapid initial inflation. The field responsible for inflation has negative pressure, allowing the universe to do work on itself as it expands. This is allowed by the first law of thermodynamics.

In other words, no energy was required to "create" the universe. The zero total energy of the universe is an observational fact, within measured uncertainties, of course. What is more, this is also a prediction of inflationary cosmology, which we have seen has now been strongly supported by observations. Thus we can safely say,

No violation of energy conservation occurred if the universe grew out of an initial void of zero energy.

In the first century BCE, the Greek philosopher Lucretius wrote that "Nothing can be created from nothing" and this assertion exerted a powerful influence over subsequent philosophers. For a long time, science just did not have a good explanation for the existence of all the matter in the universe and it was assumed that the existence of matter was just a given, an initial condition that we just had to accept and proceed from there. Religious people seized on this "How can something come out of nothing?" question to try and argue that the very existence of the universe violated of the law of conservation of energy and implied the existence of a creator who can violate such laws. In other words, it was a Deep Mystery that science has no explanation for and that could only happen by the will of a creator.

But the hope of religious people that they had finally found a safe niche for god where he no longer risked being flushed out by those pesky scientists has been dashed, just like all the other similar hopes of the past. The creation of the universe does not violate the law of conservation of energy. God is once again found to be superfluous.

Next: Does the universe violate the second law of thermodynamics?

This series is archived here.

POST SCRIPT: Baby Jesus prayer

From Talladega Nights.

(My latest book God vs. Darwin: The War Between Evolution and Creationism in the Classroom has just been released. For more on the book, see here. You can also listen to the podcast of the interview on WCPN 90.3 about the book.)