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Re: Old Stars



This is another repost of a previous post of mine which seems to have
vanished without a trace into cyberspace.

Concerning the "Old Stars" thread Roger Haar wrote:

The extreme case of the original question is the microwave blackbody
radiation left over from the big bang. If this light was produced a few
hours ( or minutes ) after the bang, why did it take so long to get here?
...
I think that the answer is in general relativistic effects.

It is correct that the cosmic microwave background provides a more extreme
case of the original question asked by David Abineri. It is not correct
that "this light was produced a few hours ( or minutes ) after the bang".
The universe was opaque to EM radiation for about the first 300000 to
1 million years or so after the BB and during this time the mean free path
length for a photon was *VERY* tiny. This is because the temperature was
so hot that the matter was completely ionized as a dense plasma of separate
charged particles. The photons were continually being absorbed and re-emitted
before they propagated a significant distance during this time. After this
time ~500kyr (give or take a factor of 2) the temperature of the universe had
cooled sufficiently that neutral atoms (mostly hydrogen) could form. Once the
atoms formed the matter was electrically neutral and their interaction cross
section with photons plummeted. The universe became effectively transparent
to photons and the photons propagated unobstructed by the matter ever since.
Thus it is these relic photons which date from about 500kyr after the BB which
are seen today as the cosmic background. Also these photons did not
necessarily take "long to get here". They have been bathing the entire
universe--going in all different directions since they were formed. The ones
which we see now have been travelling some 10^10 yr before they were captured
by our instruments (COBE). Thus they were emitted and last interacted with
matter (when the universe was ~500kyr) at a distance of about 10^10 lyr from
here. It is true that the detailed explanation for how these photons have
propagated through the ever expanding universe (red shifting as they go) is
found in "general relativistic effects" however.

David Bowman
dbowman@gtc.georgetown.ky.us