# Photon energy in an expanding Universe

We are told that as the Universe expands, the 'temp' of it cools and we now have the microwave background radiation as a remnant. Fine.

Considering the energy density of the universe, energy is indeed conserved as it expands and cools.

However, what about an individual photon, who's energy is given by E=hf. Its wavelength has increased due to the expansion of spacetime, reducing its frequency, and hence its energy.

Where did its energy go?

Staff Emeritus
Gold Member
Start with an 'easy' problem - what happens to a photon's energy in a 'normal' redshift situation?

Hmmm...not exactly sure. Give me a while to think about it.....

heumpje
The energy density is decreasing to keep the total energy constant. So the frequency redshifts

..but without creating new photons, the OVERALL amount of energy decreases. So somehow energy is lost to other systems. Which sort of brings me back to where I started... Where did it go?

Staff Emeritus
Gold Member
One more time, in two-part harmony,
Originally posted by Nereid
Start with an 'easy' problem - what happens to a photon's energy in a 'normal' redshift situation?

I read somewhere on the web that:

The momentum and energy slowly decay into, respectively, wavelength (along the photon path in intergalactic space) and wavetime (the same time dimension as it travels through the immense universe).

Is there a simpler way of putting this (ie one I can understand!), or is it just a too difficult a topic for a non-physicist to grasp?

To put quite simply here is what happens.

As spacetime expands, it creates what is called a redshift. This essentially means that the amount of space a given photon is traveling through increases as it is traveling. The effect of this is to increase the wavelength, and hence decrease the frequency and subsequently, the photon's energy. But now, you ask where did the energy go? A valid question, after all, it has to go somewhere. Well, the answer is that the energy doesn't really go anywhere in a way we can define energy. See, this redshift is simply caused by the metric increasing (the way we measure distance roughly) and nothing at all to do with the photon. In other words, the photon does not do anything to spacetime. That is a bit off, but perhaps someone else can better explain it.

We are told that as the Universe expands, the 'temp' of it cools and we now have the microwave background radiation as a remnant. Fine.

Considering the energy density of the universe, energy is indeed conserved as it expands and cools.

However, what about an individual photon, who's energy is given by E=hf. Its wavelength has increased due to the expansion of spacetime, reducing its frequency, and hence its energy.

Where did its energy go?

just another problem with the big bang theory.
but as far as redshifts are concerned think about a rubber band.
Let's say I snap you with a rubber band- it hurts right? So,
you get smarter the next time and you pull your hand away as
I try to snap you with the same rubber band again. I snap it exactly
the same way (with the same energy) but this time it will hurt less
because you pulled away. The energy of the rubber band snap didn't go anywhere, it just appears to have struck you with less energy
because your hand moved in the same direction as the rubber band.