Before discussing the intended topic, the reader
needs to understand the idea of the
Hubble radius.
This explanation will remain non-technical, but
necessary to comprehend the "real" topic.
The
Hubble radius describes
the current size of the
universe, based on the
(reasonable) assumption that
light from a
point-source origin of the universe (ie, the
big bang) travelled
radially outward from
that point.
Simply put, anything the big bang threw out
can only have travelled, at the
speed of light,
a certain finite distance. Our universe,
therefore, consists of a perfect
sphere having
a radius the exact distance that light can
travel since the big bang, roughly 12 billion
light-years. No more (which would require
something to travel faster than the speed of
light), and no less (since the big bang did
emit quite a lot of energy in the form of
light, that light can only have gone radially
outward at the speed of light).
This idea of a forced radius of the universe
due to light travelling outward works both ways,
however. Considering it from the opposite
direction gives rise to the idea of the
causal
horizon.
Most people realize that light from distant
objects takes some time to reach us. The
light we see from our
sun takes just over 8
minutes to reach us. This means we actually
"see" the sun as it existed 8 minutes
ago, not as it stands "now". As we get further
away, this seeing-into-the-past effect grows
larger -
Sirius, the brightest nighttime
star
in our sky, lies 8.7 light-years away (fairly
close, for a star), meaning that we see it as
it existed 8.7 years ago. Quite a lot further
away, we can see
quasars near the edge (as
defined by the Hubble radius assuming a
universe roughly 12 billion years old, of
course) of our universe, roughly 10 billion
light-years away.
Now, imagine the existance of something that
did not result from the big bang. Given the
speed of light as the fastest rate at which
information can travel through
space, in order
for us to detect such an object, a certain amount
of
time must have passed, just like it must for
the sun, Sirius, and those quasars. But consider
the quasars - At their distance, it took 10
billion years for their light to reach us.
Something even further from us would take a
proportionately longer time before we could see
it. What happens when it would take longer
than
the age of the universe for light from a
distant object to reach us?
Thus we encounter the essence of the causal
horizon.
Light from an object 20 billion light-years away
could not yet have reached us. This doesn't
just state that we can't see it - In any meaningful
way, such an object simply does not exist in our
universe.
Update to rebut Briglass' argument: I wrote
the above explanation from the purely classical
perspective. I didn't mention such ideas
as inflation, or the possibility of an
anisotropic universe, or the EPR paradox
of quantum physics, or Robertson-Walker,
or even (most of) relativity (other than the
idea that nothing can travel faster than the
speed of light), because I wrote it for the
average reader, rather than for physicists
and cosmologists.
I will grant that perhaps the word "perfect" goes
a tad too far, but even taking every possible
variation on the speed of light (short of it not
existing as an upper limit in our universe), the
geometry of the universe, and the flatness (or
lack thereof) of spacetime into consideration,
the core idea remains sound and the actual
distances involved would change very little
(percentage-wise).