How do we measure energy density of radiation?

[binbagsss]

I have that the energy density of matter approx 0.3 , so this is measured by galaxy motion / gravitational lensing etc, that's fine.

I have that the energy density due to the vacuum is 0.7, so this is measured by the expansion rate- the common suspect being dark energy.

I have radiation contribution approx 10^{-4}. How is this measured? Or is this theoretical?

Thanks

 

[Chalnoth]

The radiation density is the energy density of the CMB, and has been measured directly.

 

[binbagsss]

The radiation density is the energy density of the CMB, and has been measured directly.

nothing else contributes? like when a star explodes, the EM radiation given off then e.g?

 

[Chalnoth]

nothing else contributes? like when a star explodes, the EM radiation given off then e.g?

Not to any significant degree. The CMB comprises nearly all of the radiation ever emitted, by energy density.