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CyberneticPanda t1_j26ilwq wrote

Reply to comment by Aseyhe in How does light factor into gravity models? by littleboymark

The CMB is light that was emitted, but not until about 380k years after the big bang. Before that matter was so dense in the universe that any light that was emitted was reabsorbed by other matter. There were also other elementary particles in the early universe ~1 second after the big bang like quarks and electrons. By 3 minutes after the big bang, quarks we're forming neutrons and protons. Around 24k years after the big bang, there was more matter than energy in the universe. At 380k years after the big bang, things had cooled Enough for electrons to get captured by hydrogen and helium nuclei, throwing off photons that we still see some of today, the CMB. The CMB doesn't dominate the energy in the universe. It's about 10 orders of magnitude lower than the average matter/energy density of the universe.

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Aseyhe t1_j26w43t wrote

> The CMB is light that was emitted, but not until about 380k years after the big bang. Before that matter was so dense in the universe that any light that was emitted was reabsorbed by other matter.

Since all photons are identical, there is no way to objectively say whether photons were absorbed/reemitted or scattered. Regardless, last scattering at ~370000 years did not cause any change in the energy density in photons (which continued to drop as a^-4 as usual), which is why for the purpose of this discussion, it is reasonable to ignore it.

> Around 24k years after the big bang, there was more matter than energy in the universe.

Matter is energy, but it's closer to 52k years that the energy density of matter began to exceed the energy density of radiation.

> The CMB doesn't dominate the energy in the universe. It's about 10 orders of magnitude lower than the average matter/energy density of the universe.

No, just 4 orders of magnitude today, as I mentioned.

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