# Using mass of the observable Universe to estimate an energy equivalent

For quite some time now, physicists have been able to estimate the mass of the observable universe.

Reportedly it's around $10^{50} \:\mathrm{kg}$.

There is also general relativity, which states that $E=mc^2$.

If we can calculate the energy equivalent of the observable Universe, can we extend the same logic to the rest of it?

Let's leave electromagnetic radiation out of the picture for starters, since I know nothing about neither average photon density nor spectral distribution. Although if anyone does, please feel free to elaborate.

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And you certainly can't apply the special-relativistic $E=mc^2$ to the entire universe. Special relativity doesn't work when you consider large enough volumes of spacetime that gravity becomes important. –  Ted Bunn Jul 26 '11 at 17:07