From: ethanb@ptolemy.astro.washington.edu (Ethan Bradford)
Newsgroups: sci.astro
Subject: Re: COBE result (was Re: Quantisation of REDSHIFT)
Date: 12 Jan 93 02:00:32 GMT
Article-I.D.: ptolemy.ETHANB.93Jan11180032
Organization: U. of Washington
NNTP-Posting-Host: kepler.astro.washington.edu
In-reply-to: markus@octavia.anu.edu.au's message of 11 Jan 93 00:31:49 GMT
TO: markus@octavia.anu.edu.au (Markus Buchhorn)


In article <1iqf5lINNilr@manuel.anu.edu.au> markus@octavia.anu.edu.au (Markus Buchhorn) writes:

   richmond@spiff.Princeton.EDU (Stupendous Man) writes:

   >   Speaking of astronomy, I read the article in today's (Friday's) NY Times
   > on a presentation of COBE results from the AAS meeting.  No one here at
   > Princeton could figure out what the article was saying.  It SEEMED to
   > say something like "COBE measurements of the microwave spectrum show 
   > a really, really good fit to a blackbody spectrum.  Therefore, nothing
   > exciting happened in the universe after the BB for at least one year."
   > Can someone at the AAS meeting make things more clear?
   > 

Nothing very powerful happened after the first year, which is when the
universe became neutral and thus released the photons from the matter
to stream to us unmollested.

Any energy released during the first year would just change the
temperature of the released radiation; energy released after that
would change the shape of the spectrum.

   The temperature that COBE sees now is the BB temp, redshifted for
   15Gyears (+/- whatever :-) ). If there had been other major releases of
   energy (even at the same temp as the Big One) at a later stage they would
   have an oh-so slightly different temperature now. Now, I can't draw the
   Blackbody curve in ASCII partuclarly well, so pretend it's a triangle :-)

   The peak of the triangle depends on the temperature, while the area under
   the triangle relates to the energy released in the event. If you see
   a temperature profile that looks like a triangle, you can measure its
   temp now, and the energy *in that event*. If there are other events, they
   will add other triangles, perhaps at other temperatures, and this causes
   bumps in your original triangle. If the event was large enough, it will
   deform your triangle quite drastically. If it happenened at another temperature
   there will be peaks away from the main peak.

   The AAS result says that they saw a triangle, and it was a very smooth
   triangle - no bumps at other temperatures, down to some very small level. Hence,
   if there were other events at a later time they were *very very* small.

   Does that help to make it clearer ?  {And to those who know, did I get 
   it right ? :-) }

Not exactly.  Very recent releases will leave bumps (which they didn't
see), but earlier releases will distort the spectrum in more subtle
ways.

If the releases are early enough, the spectrum will have a blackbody
shape, but with too few photons for the given temperature (it will
have the same distribution as a set of bosons with a small mass).
This is because the effects which move the photons around in
energy operate more efficiently (at low temperatures) than the effects
which change the number of photons.

Later releases of energy will cause a distortion which is like tilting
the curve slightly to higher energy.  This is due to some of the
background photons scattering off the hot matter (from the energy
release) and increasing their energy.  This is refered to as
"comptonization", since the photons Compton scatter off the hot
electrons.

All three kinds of distortions have been severely constrained.

-- Ethan
Newsgroups: sci.astro