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Jim G. wrote:
...
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.
There was a debate a while ago in cosmological circles as to
which of these two mechanisms, 2) or 3), is dominant in making the sky dark.
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.)