🚀 go-pugleaf

In article <1992Nov4.002644.14768@CSD-NewsHost.Stanford.EDU>, pratt@Sunburn.Stanford.EDU (Vaughan R. Pratt) writes...
> 
>Actually it was the explanation of Olber's paradox in your FAQ that
>prompted my question.  You account for the paradox in terms of
>redshifting starlight downwards (4) and the age of the universe (5).
>So it occurred to me that if indeed the bulk of the 6000K starlight
>that *has* reached us so far has been redshifted down so far as to
>appear to have vanished "into obscurity," maybe it hadn't completely
>vanished but had become the 3K background radiation.

I see.  I just don't know to what extent the distant starlight is red-shifted.
But it is not nearly enough to get from 6000K to 3K.  The most distant
objects we can see have redshifts in the range 3-5.  The red-shift of the CMBR
is much more than that.  Ben's statement also seems plausible to me, but
I'm no expert.  Remember, however, that it is really the finite age of the
Universe which eliminates Olber's paradox.  The redshift contributes less.

>In addition it raised a question in my mind as to why all the radiation
>from the "big flash" *was* characteristic of a single temperature, i.e.
>why has it all been downshifted the same amount.  

The early universe settled down into a (near) thermal equilibrium at some
time before matter-radiation decoupling (when what you called the plasma
era ended and atoms formed).  The radiation in the Universe at that time
was characteristic of this thermal equilibrium.  It has since redshifted
to lower temperature.  

It is easy to see that a classical thermal distribution, with Boltzmann factor
exp(-h*nu/k*T) will look thermal (with temperature T/Z) if the frequency of 
each photon is scaled by the same factor Z.

>Shouldn't the parts
>of the flash that appear further away from us reach us at a lower
>temperature?  Or was the universe so small back then that all parts of
>it would now appear equidistant to any modern observer?

I'm not sure I understand the question.  However, I will point out, in case
it helps, that in 10 billion years, we will be detecting photons from
much further away, at a time when the CMBR will be more redshifted than
it is today.  So, in a little while, photons from parts of the radiation field
that were further away will eventually reach us, at lower temperature.

>Furthermore as the plasma continued to cool, presumably it continued to
>radiate.  Why doesn't that contribution skew the shape away from a
>single-temperature shape?  Is it because the amount of radiation
>decreases with the 4th power of the temperature, and hence the skew
>must be negligible?

Once the plasma unionized, the photons were permanently decoupled from the
atomic matter, except at very specific absorption lines.  The CMBR records
photon distribution in the plasma at the time when it cooled to the point
that atoms could form.  After that, as the gas cooled, the CMBR was unaffected,
because most of the photons did not correspond to specific transition 
frequencies of the atoms in the gas.


BTW:  I'm not sure that I would characterize the matter-radiation decoupling
event as the "Big Flash."  There was no sudden production mechanism which 
produced the CMBR.  It's more like having the fog lift and all of a sudden
having a much greater visibility (mean free path before absorption or 
rescattering of a given photon.)  The radation was there all along, it was
just getting bounced around alot while the Universe was hot and dominated
by plasma.

-Scott  
--------------------
Scott I. Chase			"It is not a simple life to be a single cell,
SICHASE@CSA2.LBL.GOV		 although I have no right to say so, having
				 been a single cell so long ago myself that I 
				 have no memory at all of that stage of my 
				 life." - Lewis Thomas