From: ram@zedat.fu-berlin.de   
      
   Bruchpilot Aki  writes:   
   >Into which form of energy is the energy actually converted, which   
   >photons lose through the cosmic redshift?  Could it be that "space"   
   >itself is a form of energy? But then the electromagnetic field would   
   >have to couple to the metric, right?   
      
     The electromagnetic field influences the metric via its   
     energy-momentum tensor; vice versa, photons are moving   
     on geodesics which are determined by the metric.   
      
     The physic FAQ has an entry by Michael Weiss and John Baez   
     titled "Is Energy Conserved in General Relativity?":   
      
   |The Cosmic Background Radiation (CBR) has red-shifted over   
   |thousands of millions of years.  Each photon gets redder and   
   |redder.  What happens to this energy?  Cosmologists model   
   |the expanding universe with Friedmann-Robertson-Walker (FRW)   
   |spacetimes.  (The familiar "expanding balloon speckled with   
   |galaxies" belongs to this class of models.)  The FRW   
   |spacetimes are neither static nor asymptotically flat.   
   |Those who harbor no qualms about pseudo-tensors will say   
   |that radiant energy becomes gravitational energy.   
   |Others will say that the energy is simply lost.   
   Quoted from an FAQ entry by Michael Weiss and John Baez   
      
     .   
      
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