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    Ah I understand. I thought by "spreading out" you meant as in light spreads out from the source causing a fall in intensity when you move away from it... sorry misunderstanding

    yeah this makes much more sense,

    I wonder what would happen to the number of photons, because this might cause the number to increase as each one has a lower frequency due to conservation / the wave spreading out might lead to photons splitting? Because you could experiment on this, theoretically...

    eg.

    have a source a long distance away which shoots one photon off, space expands and there is a photon recorder to record this, Id wonder what it says, eg. two photons of half the wavelength? cos there would be discreet numbers of photons, eg. you can only get frequencies of a half or a quarter of the original one, nothing in between!

    just thinking out loud
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    Sigh, for those who do Relativity, energy is not lost in Redshift, energy merely depends on where you stand (or happen to be traveling in terms of velocity)
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    what kind of redshift are you talking about?

    and from an observer on the Earth, where do you think the energy would go? and how would this differ from someone moving relative to the Earth? both would observe the sape speed of the light, would both observe the same frequency of the light fall due to cosmological redshift?
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    (Original post by Mehh)
    Photons losing energy? How? Photons don't lose energy unless they interact with matter, for example Reverse Compton Scattering.
    Space itself is expanding, in a process imaginitively known as, expansion. This should mean that the Big Crunch won't happen right? Well no, we don't know how long space will continue expanding for, that is if there is a finite bound to that question.
    The best example of the difference between redshift and c.redshift, is in the cosmic background radiation. These are photons from (I don't know how many seconds after the Big Bang), when the Universe became transparent to light. The thing is, the universe was quite hot at the time. In theory (without expansion, since it was everywhere to begin with, so it can't have redshifted) this should result in a 3000K backbody radiation. Which it is not. It is about 2.7K today. This is purely due to expansion.
    Photons couple to gravity and hence can lose energy when moving out of a gravitational potential. It's a consequence of General Relativity.
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    Nope we are talking about cosmological redshift, not gravitational redshift. In cosmological redshift any energy lost moving out of a gravitational potential, ie away from a galaxy will have been gained when moving towards the galaxy. Also this would indicate that a smaller galaxy than ours would be blue shifted and from a larger galaxy red shifted
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    Righto, cosmological redshift has a similar cause, the expansion of space leads to photons losing energy to the gravitational field. You cannot understand cosmological redshift outside of the framework of General Relativity, SR is not enough to explain it because that does not take into account the expansion of space.
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    Actually I've just spoken to a lecturer in astrophysics at cambridge, and have since changed my position on this, apparently this loss of energy has no acccepted solution, there are those that believe in the tired light thoery or those that the energy is somehow lost to a gravitational field, however neither are necessarily right
 
 
 
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