Is energy always conserved?

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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/

More resources:
http://motls.blogspot.com/2010/08/why-and-how-energy-is-not-conserved-in.html
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/hubble.html

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Видео Is energy always conserved? канала Physics Girl