The horizon problem is well known and has been well described by many. So rather
than re-invent the wheel and create yet another description, I instead present one of the clearer descriptions I've found. This is clip from
the The Universe - episode Light Speed1 where they
present a very clear description of the Horizon problem. For your
convenience I've also provided a transcript of the video below.
Transcript:
Narrator:
This breath taking shot is the Hubble space telescope's ultra deep
field. It's a massively detailed photo of an area of the sky a hundred
times smaller than the fool moon, yet containing ten thousand galaxies.
Some whose light has been speeding toward us for 13 billion years.
Beyond them is the cosmic background radiation from just 400,000 years
after the big bang. In NASA's color coded picture the radiation's glow
is pure green representing a distribution of matter so uniform, its
temperature varies varies no more than 1/50,000 of a degree. Nothing in
human experience is even close to this kind of uniformity. In fact
astronomers believe the universe should really be very different.
Michio Kaku (Author, Physics of the Impossible):
"By rights the universe should be lumpy. If you look in this direction
[pointing
forward] and you look in that direction [pointing backward] you should
see two
entirely different concentrations of matter, different temperatures. But
it's
extremely uniform. Therefore we have a puzzle.
Narrator:
"The puzzle has its roots in the universe's birth at the big bang. If
everything
flew apart from the beginning, why shouldn't it be uniform?
Alan Guth,
MIT:
"No kind of explosion that we know about leads to that kind of
uniformity. If you
imagine an ordinary explosion - an atomic bomb, a piece of TNT, it's not
really
uniform at all. There's pieces of shrapnel going off there, pieces of
paper going
of there, an extra piece of iron going off there. Its really very
non-uniform."
Narrator:
"So scientists believe the cosmic background radiation just shouldn't be
as smooth
and green as it is. We can find out why in an ordinary paint store."
Amy Mainzer, NASA/JPL:
Let's consider a universe that consists of different colored cans of
paint. In our
hyperthetical paint universe, we have a can of yellow paint and a can of
blue
paint. ANd at the instance of the big bang in this universe, two cans of
paint
start expanding apart from each other. In our hyperthetical paint
universe, one
side of it would look yellow, and the other side would look blue."
Narrator:
But as we learned, the cosmos looks green, whether it's the paint
universe or the
real thing. The two colors of paint represent the different particles of
the
infinite universe. To end up a uniform green, like the cosmic background
radiation, they had to be touching. But when scientist first calculated
the speed
of the big bang, they concluded that it blew everything apart faster
than the
speed of light. Meaning blue and yellow were too far apart, even at the
instant of
creation for any mixing to take place.
Amy Mainzer:
"Seeing a universe that's so uniformly green would be very strange. It
would be
like taking our can of yellow paint, pouring it out and having it be
green. Then
taking the can of blue paint, pouring it out, and having it be green as
well. It's
impossible."
Guth:
This horizon problem can be solved by a theory I worked on called
inflation. Which
is a twist on the big bang."