What happens to light's lost energy?

Question of the Week, from the Naked Scientists

Listener Mark was curious to know: "Since the Universe is expanding, and light coming across it stretches as it does so, becoming more red, what happens to the lost energy when the shorter wavelength, higher energy light towards the blue end of the spectrum is shifted into lower energy, red wavelengths?" Cameron Voisey went off to figure it out... Like this podcast? Please help us by supporting the Naked Scientists