There are two major problems for the Big Bang model of the creation of the

Universe. They are

l the flatness problem

l the horizon problem

The flatness problem relates to the density parameter of the Universe, . Values for

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 of 1, which is strange because of 1 is an

unstable point for the geometry of the Universe.

Values of Omega slightly below or above 1 in the early Universe rapidly grow to

much less than 1 or much larger than 1 as time passes (like a ball at the top of a hill).

After several billion years, Omega would have grown, or shrunk, to present-day

values of much, much more, or much, much less than 1. So the fact that the

measured value of 0.2 is so close to 1 that we expect to find that our measured value

is too low and that the Universe must have a value of exactly equal to 1 for

stability. Therefore, the flatness problem is that some mechanism is needed to

produce a value for to be exactly one (to balance the pencil). A Universe of of

1 is a flat Universe.

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 (see 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 of the Universe has an origin at the Big Bang, but time didn't exist until after the

Planck era. By the end of that epoch, the Universe was already expanding so that

opposite sides could not be causally connected.

The solution to both the flatness and horizon problems is found during a phase of the

Universe called the inflation era. During the inflation era the Universe expanded a

factor of 1054, so that our horizon now only sees a small piece of what was once 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.

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