Is energy always conserved?

This episode is brought to you by Squarespace: http://www.squarespace.com/physicsgirl 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 Help us translate our videos! http://www.youtube.com/timedtext_cs_panel?c=UC7DdEm33SyaTDtWYGO2CwdA&tab=2 http://physicsgirl.org/ ‪http://twitter.com/thephysicsgirl ‪http://facebook.com/thephysicsgirl ‪http://instagram.com/thephysicsgirl http://physicsgirl.org/ Creator: Dianna Cowern Editor: sefd.com , Jabril Ashe Writer: Sophia Chen Gravitational Wave Animations: LIGO Visuals: Pixabay.com Music: APM and YouTube