Where Does Energy go in Cosmic Redshift?

Is the energy really conserved or not ? In redshifts of light where does the energy goes
- Swayam (age 13)

For ordinary local physics treated within special relativity and quantum mechanics, energy is exactly conserved. Descriptions of cosmology using general relativity raise exactly the question you asked. Most redshifts (Doppler, gravitational) make no problem for energy conservation, so long as you stick with a particular reference frame. The loss of energy of photons as the universe expands and their wavelengths grow is more interesting. The physics is over my head, but I've heard that if you include a gravitational energy term the loss of negative gravitational energy as things expand just cancels the loss of local energy of the parts- at least if the universe is finite. 

Here's a much better answer: http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html.

Mike W.

(published on 11/20/2016)