QotW: 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


Rays of lights travelling outward from a point



Listener Mark asked: "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...

Cameron - Waves are just one way in which energy can be transferred from one place to another, whether that’s light waves lighting up your room, or sound waves taking my voice into your ear. You may be aware that waves, like light and sound, can stretch. Listen to this, for example.

As the ambulance moves away from us, the siren seems to slow down, which means the sound waves appear stretched - this is the Doppler effect, and the same thing happens with light - we say that it ‘redshifts’, since the light loses energy and becomes more red.

But in Mark’s question something slightly different is going on. The stretching he mentioned is because our Universe is expanding - as space expands, so too does the light within it, stretching it and so losing energy. So, where does all this energy go? I spoke to Enrico Pajer, a cosmologist from the University of Cambridge, to find out.

Enrico - This is an excellent question, and in fact one which, in one form or another, is often a source of heated debate among practitioners. Many of us are familiar with the idea that energy is conserved: it might change from one form to another, but when we add up all forms of energy the grand total does not change with time.

Cameron - For example, when you boil a kettle, electrical energy from your home is converted into thermal energy to heat up the water - the total energy does not change. So, what does the light’s lost energy convert into then?

Enrico - The short answer to the question is that energy is not conserved in an expanding universe such as ours, and so the energy that is lost by light as it shift its frequency to the red simply disappears.

Cameron - So the idea that ‘energy can't be created or destroyed’ doesn’t actually seem to apply to our Universe on its biggest scales. James Fergusson, another cosmologist at the University of Cambridge, explains why:

James - Now, conservation of energy can be shown mathematically to be a consequence of assuming something called "time translation invariance”. This just means that we expect the laws of physics will be the same tomorrow as they were today. However, in expanding space the background isn’t constant in time, so time translation invariance is broken and the energy conservation doesn’t hold in quite the same way.

Cameron - Because our Universe is expanding, the Universe looks different from day to day and so too do our cosmological experiments - standard energy conservation disappears. So the answer to Mark’s question? Light does lose energy as the Universe expands, but that’s okay! Regular rules about not losing energy as light is stretched don’t hold in our expanding Universe, and instead, cosmologists can consider ‘spacetime’ as a whole to have absorbed the energy. This restores a conservation principle, but not, perhaps, as we knew it. Thanks to Enrico and James! Next week, we'll be looking into this question from Margaharet...

Margaret - Have frogs been seen scratching an itch?


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