This is a part of my wider attempts at accounting matter in the observable universe.

The total light ever emitted by stars and other space objects is capped around 1% of baryonic mass (in energy equivalent).

How much light is there in space and how heavy is it?

How does the CMB compare to that? How would we calculate it?


The energy density of the Cosmic Microwave Background (CMB) is $4.19 \times 10^{-14} J/m^3$.

This value can be calculated from the CMB temperature being 2.728 K and

$energy$ $density = aT^4$

where $a$ is the Radiation Constant.

You can multiply the energy density by your volume of interest such as that of the observable universe if you want total energy.

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    $\begingroup$ For comparison the average density of matter/energy in the universe is around $10^{-9}$ J/m$^3$. $\endgroup$ – John Rennie Apr 19 '14 at 6:16

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