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Does this intensity of microwave background radiation correspond to the huge amount of gamma photons that could be released during the theoretical annihilation of matter and antimatter at the time of the creation of our universe?

Qmechanic
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Krešimir Bradvica
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1 Answers1

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Sort of. The CMB are photons we detect from the last scattering surface, essentially when the universe went from being opaque to transparent (approximately at around $3000K$). The opaqueness was just a product of the Universe being hot and dense.

But yes, it's true that the majority of photons that constitute the CMB were created from matter/anti-matter annihilations in the early universe. However, since then the CMB photons have interacted and scattered many times (during the radiation era until recombination).

Eletie
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  • So the (once UV, now microwave)-photons from last scattering (the CMB) are the SAME photons that thousands of years earlier were gamma rays photons in thermal equalibrium when the universe's temperature was much higher? That is maybe up to debate, I would say, by this logic also the photons coming from the Sun were gamma ray photons (once upon a time). Still, the photons we observe today from the CMB have NEVER been gamma rays AFTER the interacted with matter at last scattering, right? – Koschi May 25 '22 at 10:57
  • Mostly, yes. Photons stopped exchanging energy with electrons when the Universe is about $T=10^5K$ and continue to scatter until recombination. Before then (at higher $T$), photons and charged particles were in equilibrium, but the majority of the photons would have been produced from particle/anti-particle annihilation at earlier times. The question seems to be about production, not about last-scattering (which I also explain in my answer, hence the 'sort of'.) – Eletie May 25 '22 at 11:04
  • @Koschi I don't disagree that the CMB originates at the last scattering surface. But the vast majority of the radiation/photons were still always present before this: this is how we do early Universe cosmology! – Eletie May 25 '22 at 11:08
  • I think it's a matter of semantics whether it's the same photon after it's been scattered. After all, they don't have serial numbers. ;) Personally, I prefer to think of it as a new photon, but I understand people adopting the other convention. – PM 2Ring May 25 '22 at 11:42
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    @Eletie I agree that now that I think about it, what you explain it probably what was asked by this question... 'where did all the radiation come from in the first place ?' – Koschi May 25 '22 at 12:05
  • @Koschi we most likely just interpreted the question differently. PM 2Ring is right that it's probably just semantics! – Eletie May 25 '22 at 14:05