Interesting Article on Cosmology...some good insights, no math, illustrations. Lots for us novices to consider. By Tamara M. Davis a Scientific American article] Is the Universe Leaking Energy? http://www.physics.uq.edu.au/download/tamarad/papers/SciAm_Energy.pdf My synopsis: Emmy Noether found that conservation of energy relies on time symmetry. The changing shape of the universe due to cosmological expansion means that spacetime is changing....is not symmetric..... and so conservation of energy does not apply. So the universe does not violate the conservation of energy; rather it lies outside that law’s jurisdiction. One interesting insight: Photons traveling in an expanding universe appear to lose energy via cosmological redshift. What about matter: You find that the de Broglie wavelength of particles increases by exactly the same proportion as a photon’s wavelength does! Thus light and matter seem to behave in exactly the same way when it comes to 'energy loss'. Some here may not like so much her description of Doppler shift and implied cosmic microwave background radiation redshift: In small enough regions the universe makes a pretty good approximation of flat spacetime. But in flat spacetime there is no gravity and no stretching of waves, and any red-shift must just be a Doppler effect.... so the relative motion of the emitter and observer means that they see photons from different perspectives and not that the photons have lost energy along the way.