When light passes through the universe and is redshifted by the expansion of space itself, how is energy conserved? The stretched light has a longer wavelength and therefore a lower energy. Is energy conserved? If so, where does it go?
Many thanks to Sean Carroll for his wonderful description of this question: http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/
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