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So somebody please tell me why the sky is dark -- in view
of this discussion about "Old Stars, quasars, and Q's flashlight.
There are 4 possible suggested mechanisms (that I can think of)
as to why the sky would be dark in most directions.
1) The universe is finite in spatial extent giving only a finite
number of point light sources.
2) The universe is finite in time. Since it has only been around
for a finite amount of time there is a
light-travel-time-since-the-beginning horizon beyond which no
light can be seen.
3) The universe is expanding--causing the light from the
most distant sources to be red-shifted away to ever lower
(& dimmer) photon energies.
4) The luminous matter of the universe is hierarchically
arranged in a fractal arrangement whose Hausdorff dimension
is less than 3 so that the sources of light are a set of
measure zero in space (yielding an average density of zero)
allowing there to be not enough light sources to light up
the sky in most directions.
A few years ago a consensus developed as to which one was
more important, but I can't remember which way it came out.
(Maybe a legitimate list-resident general relativist can
help us out here.)
Mechanism 3) is responsible for red-shifting the wavelengths of the cosmic
background 1000-fold since those photons decoupled from the matter which
produced them so that they have a meager intensity in the microwave band
rather than a bright intensity in the visible one. Without this red-shift
the sky would be very bright everywhere from the cosmic backgound in the
visible wavelengths whether or not there are enough point sources (stars,
galaxies, quasars, etc.) to also make the sky bright.