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## Homework Statement

What is the average energy of the CMB photons, in electronvolts, for ##T=2.73K##?

## Homework Equations

## The Attempt at a Solution

I used the grand canonical ensemble for photons and after several calculations I get $$<E>=\frac{8\pi V}{c^3}\int_0^\infty \frac{h\nu^3}{e^{\beta h \nu }-1}$$ replacing variables and doing the integral I get: $$<E>=\frac{8 \pi^5 k^4 T^4 V}{15 h^3 c^3}$$ Now I am a bit confused. What should I use for ##V##? I found online that the value should be ##7.06 \times 10^{-4}## eV but I am not sure how to get there. Any suggestion?