How Does Inflation Work?
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.
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.
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.
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.
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
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.
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
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.
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.
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