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From the wikipedia page on dark energy, in reference to the total mass-energy of the universe:

The mass–energy of dark matter and ordinary (baryonic) matter contribute 26.8% and 4.9%, respectively, and other components such as neutrinos and photons contribute a very small amount

Why do photons contribute such a small amount?

The baryonic matter in the universe is the remains of what is generally quoted as a billion to billion-and-one ratio of antimatter-matter. If the billion on one side collided with the billion on the other, and all that mass-energy was converted into photon energy, why isn't there significantly more photon energy in the universe?

I understand that photons dominated the mass-energy of the universe in the photon epoch shortly after annihilation... but since the CMB still stretches to every point in space, I don't see how that energy became 'lost', and photons now only make up a small amount of the total mass-energy.

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    $\begingroup$ Short answer, because they have no rest mass. When everything was hot, the rest mass makes not too much difference and everything has about the same energy. As things cool and expand rest energy comes to dominate. $\endgroup$ – George Herold Mar 11 '16 at 14:30
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The argument is as follows. The mass density of ordinary or dark matter scales as $\sim a^{-3}(t)$, where $a(t)$ is the scale factor, which depends on time $t$. The reason for this scaling should be intuitive. In contrast, the energy density of photons/radiation goes as $\sim a^{-4}(t)$. This is because the number of photons is diluted by $\sim a^{-3}(t)$ and the wavelength is stretched by a factor of $\sim a(t)$.

Hence, the energy density of radiation decreases much faster than the energy density of ordinary/dark matter.

Does this help?

psm

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