XPost: sci.astro.research   
   From: jthorn@astro.indiana-zebra.edu   
      
   [[   
   Meta-comment: This discussion started in sci.physics.research, but   
   its "natural home" is in sci.astro.research.  I've cross-posted this   
   article to both newsgroups, and set the Followup-To: header so further   
   discussion should be in s.a.r.   
   ]]   
      
   Gary Harnagel  wrote:   
   > I'm having trouble picturing why we should see the CMBR at all.  Since it's   
   > traveling at the speed of light but we're moving somewhat slower, shouldn't   
   > it have passed us long ago?  I know, the FLWR metric must have something to   
   > do with it, but ...   
      
   To try to answer Gary Harnagel's question:   
      
   <<>>   
      
   Imagine an infinite static Euclidean universe (i.e., flat spacetime,   
   no gravity involved) filled with (stationary) fog which both emits and   
   scatters (visible) light, and consider a (stationary) observer in that   
   fog.  Now suppose that at a time which we will label t=0, two things   
   happen:   
   * all the fog suddenly condenses into larger water droplets, and   
   * those water droplets no longer emit light.   
   Since the scattering cross-section of large water droplets is vastly   
   smaller than that of fog, the result is that at t>0, the sea-of-droplets   
   is mostly transparent to light (certainly much more transparent than the   
   original fog was).  In other words, at times t>0 light basically travels   
   in straight lines, with little scattering, emission, or absorption.   
      
   What will our observer see at t=1 year?   
      
   Since at t>0 there is minimal scattering, emission, or absorption, we   
   see that at t=1 year our observer will see (receive) those photons, and   
   only those photons, which were   
   (a) exactly 1 light-year away from her at t=0, and   
   (b) travelling directly towards her at t=0.   
   This holds in any direction our observer looks.  In other words, at   
   t=1 year our observer will see a uniform glow on her "sky".   
      
   At t=2 years our observer will will see those photons, and only those   
   photons, which were   
   (a) exactly 2 light-years away from her at t=0, and   
   (b) travelling directly towards her at t=0.   
   This holds in any direction our observer looks.  In other words, at   
   t=2 year our observer will see a uniform glow on her "sky".  But that   
   glow is comprised of a *different set of photons, emitted at a different   
   set of events* than was the glow she saw at t=1 year.   
      
   Etc etc for any other time t>0.   
      
   <<>>   
      
   As you can see, this analogy reproduces many of the features of the   
   CMBR.  It doesn't reproduce the CMBR's temperature -- for that you need   
   a cosmological redshift between the last-scattering time (t=0 in the   
   analogy, approximately 0.5 million years after the big bang in standard   
   cosmology) and today.  But the analogy does produce an all-sky uniform   
   glow seen by all observers, even at far-future times.   
      
   I hope this makes things a bit clearer (no pun intended).   
      
   --   
   -- "Jonathan Thornburg [remove -animal to reply]"    
      Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA   
      currently visiting Max-Plack-Institute fuer Gravitationsphysik   
                         (Albert-Einstein-Institut), Potsdam-Golm, Germany   
      "There was of course no way of knowing whether you were being watched   
       at any given moment.  How often, or on what system, the Thought Police   
       plugged in on any individual wire was guesswork.  It was even conceivable   
       that they watched everybody all the time."  -- George Orwell, "1984"   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)