Gravity affects photon trajectories and energies. About 300,000 years after universe origin, when neutral-charge atoms formed, photons in higher-density regions lost more energy than photons in lower-density regions {Sachs-Wolfe effect}, because they had to overcome more potential energy. Sachs-Wolfe effect cancels gravity-photon effects.
If photons enter higher-energy-density regions and then exit them, universe space-expansion energy makes photons have higher energies than before {integrated Sachs-Wolfe effect}.
Physical Sciences>Astronomy>Universe>Cosmology>Radiation
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Date Modified: 2022.0224