Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Old Stars/Olber's Paradox



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.

Any combination of these will just make a dark sky that much easier to obtain.
Most cosmologists appeal to mechanisms 2) and 3) to explain the dark sky, and
those two mechanisms naturally are consequences of the BB theory. Even though
galaxies seem to be arranged in patterns of voids, bubbles, crumpled sheets,
etc. on a length scale of 100 Mpc (I think) the over all pattern is still
believed to be homogeneous with a finite density on the largest length scales
thus invalidating mechanism 4). Mechanism 1) is also not considered to be
relevant even if the universe is finite in spatial extent because its effect
is preempted by mechanism 2). The light-travel-time horizon is smaller than
the size of the universe even if it ends up being finite in extent. The, now
discredited, steady-state theory appealed to solely mechanism 3) to prevent a
blazing sky. 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.)

And in view of this
discussion, please tell me what importance the dark sky has.

To the extent that mechanisms 2) and 3) are responsible for the dark sky we
see that the fact the the sky is dark is evidence that the universe is indeed
of finite age and that it is expanding.

I think this is the same question as "Why can we now see background
radiation?", but what do *I* know?

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.

David Bowman
dbowman@gtc.georgetown.ky.us