What happens to light's lost energy?

As our Universe expands, light stretches and so loses energy... but where does this energy go?
13 September 2021
Presented by Cameron Voisey
Production by Cameron Voisey.


A large astronomical telescope against a dark starry sky.


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...


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