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I Redshifts of galaxies and the expansion of the universe

  1. Jul 13, 2016 #1
    (Sorry for my poor English.) I was watching a PBS video on expansion of the universe and the guy says the wavelength of a photon emitted in a supernova becomes larger as it travels to the Earth. Is it because the photon lose energy (to space)?
    If so, is that energy contributing to the expansion of the universe?
     
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  3. Jul 13, 2016 #2

    Bandersnatch

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    No, it's just lost.
     
  4. Jul 13, 2016 #3
    What about the conservation of energy?
     
  5. Jul 13, 2016 #4

    Bandersnatch

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  6. Jul 13, 2016 #5
    Good material. But I still can't understand some things.

    From http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/:
    "The thing about photons is that they redshift, losing energy as space expands. If we keep track of a certain fixed number of photons, the number stays constant while the energy per photon decreases, so the total energy decreases."

    Where goes this lost energy?

    From http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html:

    "The Cosmic Background Radiation (CBR) has red-shifted over billions of years. Each photon gets redder and redder. What happens to this energy?"

    I didnt understand what really happens.
     
  7. Jul 13, 2016 #6

    Bandersnatch

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    Nowhere. It is just lost.

    Conservation laws depend on symmetries. If there's no time-translation symmetry (as with the expanding universe), there can't be energy conservation, so it doesn't make sense to ask where it went - why would it need go somewhere if there is no conservation law?
     
  8. Jul 13, 2016 #7
    I understood it now. Thank you.
     
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