### energy of photons in FLRW universe

Note that for photons , unlike particles for which

we know that , , where is an affine parameter. Also, . And calling

say implying that

and let’s call

differentiating this equation, we get

we need to solve this perturbatively i.e where Written with StackEdit

we know that , , where is an affine parameter. Also, . And calling

say implying that

and let’s call

differentiating this equation, we get

we need to solve this perturbatively i.e where Written with StackEdit