
In the early Universe, pressures and temperature prevented the permanent
establishment of elementary particles. Even quarks and leptons were
unable to form stable objects until the Universe had cooled beyond the
supergravity phase. If the fundamental building blocks of Nature
(elementary particles) or spacetime itself were not permanent then what
remained the same? The answer is symmetry. Often symmetry is thought of as a relationship, but in fact it has its own identical that is preserved during the chaos and flux of the early Universe. Even though virtual particles are created and destroyed, there is always a symmetry to the process. For example, for every virtual electron that is formed a virtual positron (antielectron) is also formed. There is a time symmetric, mirrorlike quality to every interaction in the early Universe. How important is symmetry? When Nobel Prize winning physicist Richard Feymann was asked to summarize the more important aspect of modern science in one sentence he said "The Universe is made of atoms". When asked what he would say for a second sentence, he said "Symmetry underlies the laws of Nature."


Symmetry also leads to conservation laws, and conservation laws limit the
possible interactions between particles. Those imaginary processes
that violate conservation laws are forbidden. So the existence of symmetry
provides a source of order to the early Universe. Pure symmetry is like a spinning coin. The coin has two states, but while spinning neither state is determined, and yet both states exist. The coin is in a state of both/or. When the coin hits the floor the symmetry is broken (its either heads or tails) and energy is released in the process (the noise the coin makes as it hits the ground).


The effect of symmetry breaking in the early Universe was a series of
phase changes, much
like when ice melts to water or water boils to stream. A phase
change is the dramatic change in the internal order of a substance.
When ice melts, the increased heat breaks the bonds in the lattice of
water molecules, and the ice no longer holds its shape. Phase change
in the early Universe occurs at the unification points of fundamental
forces. The decoupling of those forces provides the energy input for
the phase change. With respect to the Universe, a phase change during symmetry breaking is a point where the characteristics and the properties of the Universe make a radical change. At the supergravity symmetry breaking, the Universe passed from the Planck era of total chaos to the era of spacetime foam. Spacetime was acquired during the phase transition. During the GUT symmetry breaking, mass and spacetime separated and particles came into existence.


Notice that as symmetry breaks, there is less order, more chaos. The
march of entropy in the Universe apples to the laws of Nature as well as
matter. The Universe at the time of the cosmic singularity was a time
of pure symmetry, all the forces had equal strength, all the matter
particles had the same mass (zero), spacetime was the same everywhere
(although all twisted and convolved).

Inflation:

The flatness problem relates to the density parameter of the Universe, W. Values for W can take on any
number between 0.01 and 5 (lower than 0.01 and galaxies can't form, more than
5 and the Universe is younger than the oldest rocks). The measured value is
near 0.2. This is close to an W of 1, which is
strange because W of 1 is an unstable point for the
geometry of the Universe.


Values slightly below or above 1 in the early Universe rapidly grow to
much less than 1 or much larger than 1 (like a ball at the top of a hill).
So the fact that the measured value of 0.2 is so close to 1 that we expect
to find in the future that our measured value is too low and that the
Universe has a value of W exactly equal to 1
for stability. And therefore, the flatness problem is that some mechanism
is needed to get a value for W to be exactly
one (to balance the pencil).


The horizon problem concerns the fact that the Universe is isotropic. No
matter what distant corners of the Universe you look at, the sizes and
distribution of objects is exactly the same (the Cosmological
Principle). But there is no reason to expect this since opposite sides
of the Universe are not causally connected, any information that is
be transmitted from one side would not reach the other side in the
lifetime of the Universe (limited to travel at the speed of light). All the Universe has an origin at the Big Bang, but time didn't exist until after the Planck era. And by the end of that epoch, the Universe was already expanding such that opposite sides were not causally connected.


The solution to both the flatness and horizon problems is due to a phase
of the Universe called inflation. Currently, inflation is the only theory
that explains why the observable Universe is both homogeneous and causally
connected. During inflation the Universe expanded a factor of 10^{54}, so that our horizon now only sees a small piece
of what was the total Universe from the Big Bang. The cause of the inflation era was the symmetry breaking at the GUT unification point. At this moment, spacetime and matter separated and a tremendous amount of energy was released. This energy produced an overpressure that was applied not to the particles of matter, but to spacetime itself. Basically, the particles stood still as the space between them expanded at an exponential rate.


Note that this inflation was effectively at more than the speed of
light, but since the expansion was on the geometry of the Universe
itself, and not the matter, then there is no violation of special
relativity. Our visible Universe, the part of the Big Bang within our
horizon, is effectively a `bubble' on the larger Universe. However,
those other bubbles are not physically real since they are outside our
horizon. We can only relate to them in an imaginary, theoretical sense.
They are outside our horizon and we will never be able to communicate
with those other bubble universes.


Notice how this solves the horizon problem in that our present Universe was
simply a small piece of a larger Big Bang universe that was all in causal
connection before the inflation era. Other bubble universes might have
very different constants and evolutionary paths, but our Universe is
composed of a small, isotropic slice of the bigger Big Bang
universe.


Inflation also solves the flatness problem because of the exponential growth.
Imagine a highly crumbled piece of paper. This paper represents the Big
Bang universe before inflation. Inflation is like zooming in of some
very, very small section of the paper. If we zoom in to a small enough
scale the paper will appear flat. Our Universe must be exactly flat for
the same reason, it is a very small piece of the larger Big Bang
universe.


Thus, inflation resolves both the horizon and flatness problems. There is good
reason to believe that the early Universe must go through an inflationlike event
due to phase changes from symmetry breaking. Curvature is forced to flat (k=0) which means matter density and the
cosmological constant must sum to one.
