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In principle, the number density of photons include all photons, both of cosmic origin (e.g. the CMB) and of astrophysical origin (starlight, gamma-rays from GRBs, radio waves from QSOs, etc.). However, most of these photons are CMB photons, as seen from this figure (from Lacasa 2014): The second largest contribution to the photon density are the cosmic ...


$C$ stands for charge. Charge symmetry implies that if the charge of all particles were to be inverted - particles become anti-particles and viceversa - the universe would look exactly the same. A universe made of matter would be indistinguishable from one made of antimatter. So if a given process allows for two matter particles (say, a proton and a ...

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