when light redshifts where does the lost energy go?

Energy can not be created or destroyed right? And redshift is a change in energy of a photon right? shouldnt that energy be conserved?

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As it turns out, no.

The conservation of energy principle serves us well in all sciences except cosmology. In bound regions that do not expand with the universe (because they are dense compared with the average density of the universe), we can trace the cascade and interplay of energy in its multitudinous forms and claim that it is conserved. But in the universe as a whole it is not conserved. The total energy decreases in an expanding universe and increases in a contracting universe. Where does the energy go in an expanding universe? And where does it come from in an expanding universe? The answer is nowhere, because in the cosmos, energy is not conserved.
Cosmology: The Science of the Universe by Edward Harrison
Cambridge University Press, 2000; 2nd edition, page 349

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That explains redshift from expansion (and fits neatly with the idea that expansion implies "negative energy"), but not the redshift and blueshift due to relative motion without expansion. For those, I would note that the same amount of wave still hits the target; it just takes a different amount of time to do it.

Are you saying that a photon duplicates itself? Lets focus on one photon in a redshifted light beam. It should be at a lower energy state than when it was sent. This should be measured in number of planck units ( term ??? ).

Am I missing something?

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Is there anyone that disagrees with this? I am not happy with this answer.
In fact interesting thoughts are swirling around the expanding space in my head right now.

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Let's tune up our thought processes with a rough analogy in classical mechanics.

If you are receding from a gun at a bit less than the muzzle velocity, the bullet will not hit you as hard as it would have if you had been stationary. Nevertheless it had the same amount of energy in transit in either case. When you are moving away at impact, some of the energy remains in the bullet, while the rest is transferred to your body.

I realize it is not that simple with photons because quantum mechanics and relativity come into play. When you or your spectrometer absorb a photon, it is totally transformed into something else. I will have to leave it up to those who are up to speed in the necessary physics to follow up.

Once again, you are giving the appearance of attempting to understand an advanced topic without working through the exercises in math and physics that are the foundation for such understanding. Exercises in math and physics are brutally unforgiving of skipped prerequisites.

I think Edward Harrison is one of those people you want to learn from. He is an astronomer who is Emeritus Distinguished Professor of Physics and Astronomy at the University of Massachusetts. He was previously Principal Scientist at the Atomic Energy Research Establishment and Rutherford High Energy Laboratory.

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Yes but then the energy is not lost. The energy of the bullet is not decreasing with time ( in a vacuum ).

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Go back and review your own post to which I was responding. It clearly was posted in response to a scenario involving light striking moving targets in the absence of expansion.

Sorry I do not see this. I am merely asking about the apparent loss of energy due to redshift. I think I see your point but am not 100% there. Basically it is about the relative state of energy when it is sent. The energy loss is just because of our ( the observers ) momentum away from the light right?

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Shifting in frequency does not equal an energy loss.

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No energy is lost. The same energy that left the source arrives at the receiver but its arrival is spread out over a longer period of time than that over which it left the source.

Your problem is as follows:
1) the apparent loss of energy is in fact a relative loss of energy (it's all just relativity 101)
2) understand the difference between a closed and open system

These examples are not all inclusive(they are open systems), so energy does not need to be conserved.

huh? I disagree.

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I can almost understand this ... but how does this work on a photon / planck's constant level ?

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Conservation of energy (and momentum, including angular) is not a trivial thing in GR to say the least. Here's roughly how it plays out according to my understanding.

If the space-time is static, and I think just stationary. Static and starionary as used sort of backwards to the way I would think about it, but that's just me. Stationary means the space-time is not changing with time, and that's in an invariant sense. Static is further restriction of that where there is no frame dragging, again in an invariant sense.

In some coordinates (say rotating Born coordinates against flat space-time) you can have space-time cross terms in the metric that look like frame dragging. However you can globally transform those away (stop your silly spinning around in your swivel chair). With real frame dragging, no such transform is possible. There is real "tidal" frame dragging afoot.

And the same way with stationary. You can choose some coordinates where a stationary space-time has time varying terms in the metric, but those can be transformed away. In a non-stationary (truly dynamic) space-time, no such transform is possible, and the space-time is truly changing with time (everyone's time, basically).

At any rate, if the space-time is stationary, one can conserve energy and everything in a global sense in a given coordinate system. However that requires what Ken G. likes to call "cockamamie" physics -- things behave differently far away than locally because of the metric funny business.

However, if the space-time is truly dynamic, that fails and energy cannot be conserved globally in a given coordinate system. To do it, you have to add a notion of gravitational energy (and this is not anything like Newton, mind you. Nothing at all). But that is problematic because it cannot be made invariant. One observer can define a gravitational energy notion and conserve energy, but that will not be consistent with other observers.

The details of that are beyond me, understand, so don't ask me to 'splain any more than that. But that's the way it plays out to my understanding. I've read some stuff about this I could only barely understand about defining gravitational energy, and there are some differences in how things should be interpreted, as least to my understanding of that the argument was about.

DeSitter space-time gives us a pure Lambda expanding universe. But that space-time is actually static. So energy could be conserved globally there, even though you can choose FLRW style coordinates where "space is expanding" with time. But the space-time is truly static in an invariant sense, and energy can be conserved. How you do it exactly in co-moving, expanding space coordinates, I'm not sure, but I'm assured it can be done because of the invariant static nature.

However, add matter for the current LCDM model, and that space-time becomes truly dynamic. So, in our universe, our space-time, energy cannot be conserved globally as it can with stationary space-times. (And bound regions are locally stationary for all practical purposes).

-Richard

So basically this is a proof that matter / energy expands with expanding space.

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