How Does Inflation Work?

I will begin by giving a quick rundown of how inflation works. Some of these issues were already discussed by Sandra Faber, who I thought gave an excellent description. For completeness, however, I will start my explanation at the beginning, but I will try to go more quickly when discussing points that Sandra has already explained.

The key idea - the underlying physics - that makes inflation possible is the fact that most modern particle theories predict that there should exist a state of matter that turns gravity on its head, creating a gravitational repulsion. This state can only be reached at energies well beyond those that we can probe experimentally, but the theoretical arguments for the existence of the state are rather persuasive. It is not merely the prediction of some specific theory, but it is the generic prediction for a wide class of plausible theories. Thus, gravity does not always have to be attractive.

The gravitational repulsion caused by this peculiar kind of material is the secret behind inflation. Inflation is the proposition that the early universe contained at least a small patch that was filled with this peculiar repulsive-gravity material. There are a variety of theories about how this might have happened, based on ideas ranging from chaotic initial conditions to the creation of the universe as a quantum tunneling event. Despite the ambiguity of this aspect of the theory, there are two things to keep in mind. First, the probability of finding a region filled with this repulsive-gravity material need not be large. I will come back to this point later, and argue that it is only necessary that the probability is nonzero. Second, the resulting predictions do not depend on how the initial patch was formed. Once the patch exists, inflation takes over and produces a universe that ends up inevitably looking very much like the one that we live in.

The initial patch can be incredibly small. It need be only about one-billionth the size of a single proton. Once the patch exists it starts to rapidly expand because of its internal gravitational repulsion. The expansion is exponential, which means it is characterized by a doubling time, which for a typical inflationary theory might be in the neighborhood of 10^-37 seconds. So every 10^-37 seconds the diameter of the patch doubles, and then it doubles again and again during each 10^-37 second interval. The success of the description requires about a hundred of these doublings, but there could have been many more. In the course of this expansion, the patch went from being a tiny speck to a size at least as large as a marble.

So the patch of repulsive-gravity material expanded by a huge factor. Whenever a normal material expands its density goes down, but this material behaves completely differently. As it expands, the density remains constant. That means that the total amount of mass contained in the region increased during inflation by a colossal factor.

The increase in mass probably seems strange at first, because it sounds like a gross violation of the principle of energy conservation. Mass and energy are equivalent, so we are claiming that the energy of the matter within the patch increased by a colossal factor. The reason this is possible is that the conservation of energy has a sort of a loophole, which physicists have known at least since the 1930s,but haven't talked about very much. Energy is always conserved; there are no loopholes to that basic statement. However, we normally think of energies as always being positive. If that were true, then the large amount of energy that we see in the universe could not possibly have gotten here unless the universe started with a lot of energy. However, this is the loophole: energies are not always positive. In particular, the energy of a gravitational field is negative. This statement, that the energy of a gravitational field is negative, is true both in the context of the Newtonian theory of gravity and also in the more sophisticated context of general relativity.

So, during inflation, total energy is conserved. As more and more positive energy (or mass) appears as the patch expands at constant density, more and more negative energy is simultaneously appearing in the gravitational field that fills the region. The total energy is constant, and it remains incredibly small because the negative contribution of gravity cancels the enormous positive energy of the matter. The total energy, in fact, could very plausibly be zero. It is quite possible that there is a perfect cancellation between the negative energy of gravity and the positive energy of everything else.

For the theory to be successful, there has to be a mechanism to end the period of inflation - the period of accelerated expansion - because the universe is not undergoing inflation today. Inflation ends because the repulsive-gravity material is fundamentally unstable. So it doesn't survive forever, but instead decays like a radioactive substance. Like traditional forms of radioactive decay, it decays exponentially, which means that the decay is characterized by a half-life. During any period of one half-life, on average half of the repulsive-gravity material will “decay” into normal attractive-gravity material.

In the process of decaying, the repulsive-gravity material releases the energy that has been locked up within itself. That energy evolves to become a hot soup of ordinary particles. Initially the decay produces a relatively small number of high-energy particles, but these particles start to scatter off of each other. Eventually the energy becomes what we call thermalized, which means that it produces an equilibrium gas of hot particles - a hot primordial soup - which is exactly the initial condition that had always been assumed in the context of the standard big bang theory.

Thus, inflation is an add-on to the standard big bang theory. Inflation supplies the beginning to which the standard big bang theory then becomes the continuation.

Contributed by: Dr. Alan Guth