I’m quite confused with regards to photon emission throughout the creation of the universe.

From what I’ve heard, there was no light (of any frequency) in the universe until 300,000 years after the Big Bang. This because then the universe cooled down enough to allow for atoms. However, it seems that electrons were created at ~3 minutes after the Big Bang. In this big electron soup, where they are constantly being thrusted around, aren’t they being accelerated and hence would release radiation? Then there should be light due to the accelerated electrons (just like how accelerated electrons produce radio waves right?)


1 Answer 1


You heard wrong. There were photons, electrons, protons, and neutrons before 300,000 years. And before 3 minutes! (And before there were protons and neutrons, there were quarks.)

Before 300,000 years, the photons could not propagate freely; they were being constantly scattered by the charged plasma of protons and electrons. The universe was effectively opaque.

Around 300,000 years, the universe had cooled enough that protons and electrons could form hydrogen atoms. A few other light elements also formed, because protons and neutrons had earlier formed helium nuclei, etc.

Atoms are overall electrically neutral and do not scatter photons nearly as much as a charged plasma does. So, after 300,000 years, the photons could move right through the neutral hydrogen gas. The universe became transparent. Cosmic photons created in the Big Bang have been moving without scattering for billions of years since “recombination”, the formation of neutral atoms.

  • $\begingroup$ I understand that if photons are being scattered, you could not see what’s infront of you, however, if photons did exist, your “eyes” would get stimulated right (if they didn’t melt) $\endgroup$
    – John Hon
    May 6, 2019 at 4:03
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    $\begingroup$ @John It'd be like being inside a giant neon sign. You can't see very far in a universe of bright orange fog. But that's ok, since there's virtually no structure to see anyway, just minute deviations in the fog density. You can see an approximation of the colour of the universe when it was starting to become transparent at the end of this answer: physics.stackexchange.com/a/133943/123208 $\endgroup$
    – PM 2Ring
    May 6, 2019 at 4:10
  • 9
    $\begingroup$ I think I had a teacher once who said it would be like looking through milky water in all directions, you aren't realistically going to be able to see much. $\endgroup$
    – Tom
    May 6, 2019 at 9:13
  • 1
    $\begingroup$ @Tom That's a good analogy. Now imagine that, but very, very hot. $\endgroup$
    – wizzwizz4
    May 6, 2019 at 11:31
  • 1
    $\begingroup$ @Tom or a burning warehouse, except you crank the burning warehouse up to Michael Bay Nuclear explosion pressed against your eyeball. $\endgroup$
    – Aron
    May 7, 2019 at 4:00

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