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The expanding universe and the conservation of energy

  1. Sep 15, 2010 #1
    The universe is expanding. We know this because light from distant stars is red shifted indicating that they are moving away from us.

    Here is the nugget that just occurred to me. Lower frequency light carries less energy so if all the light emanating from a star shifts further down in frequency the further it travels then energy is not conserved as a direct result of space expanding.

    From this I can surmise that there should be a relationship between space and light. Perhaps the relationship is more general, between space and energy. perhaps this relationship is similar to the relationship between energy and mater. If space, energy, and matter can be shown to be equivalent or interchangeable that would be a paradigm altering idea, at least to me.

    Does anyone know of any work done along this line of thought?
  2. jcsd
  3. Sep 16, 2010 #2


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    Staff: Mentor

    Hey, here is my view on this.

    In general, lower frequency light has a lower energy. But that wouldn't apply quite as well to accelerating bodies. Think of it like this. If you were just outside a galaxy that emitted light, and you were traveling with it at the same speeds, the light that hit you wouldnt be redshifted at all. (Or so little as to not be noticeable at least.) Now, imagine you were suddenly accelerated and reached a speed of lets say 20% the speed of light. Now the light coming from that galaxy is very redshifted. The actual photons are still carrying the same amount of energy, but were emitted in a different frame of reference, and when you see the light it enters your frame of reference and becomes red shifted since you were speeding away from it at the time. Now, if the theory is true, then the further away a galaxy is, the faster it is traveling away from us, hence the further redshifted the light is.

    Also, i've read that one theory suggests that over the billions of years that the light travels, the expansion of space itself has lengthened the wavelength of the light. I'm not sure about that myself, but who knows.

    See http://en.wikipedia.org/wiki/Redshift for more info.
  4. Sep 16, 2010 #3


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    That's not really a different theory, it's rather another description for what happens.
    In principle you're right, the vanishing has to do with reference frames. Instead of choosing different frames for emitter and receiver (which explains why the energy is not the same), one chooses a global non-static reference frame. Because that frame isn't static, energy is not conserved in it, and the explanation with stretching wavelengths makes sense in it.
    They are not interchangeable, but the ideas of time and energy or space and momentum are strongly connected. If you choose to describe the world via expanding space (instead of moving things, which works also), you necessarily have decaying momentum in such a frame. Describe it in a static coordinate system, and momentum is conserved again.
    Generally, the notion of energy in GR is http://en.wikipedia.org/wiki/Stress-energy-momentum_pseudotensor" [Broken].
    Last edited by a moderator: May 4, 2017
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