B Confused About the Source of CMB Photons: Two Possibilities

[Nathi ORea]

TL;DR Summary

What is the actual source of the photons that are the CMB?

I am still so totally confused about the ultimate source of the photons of the CMB. I am getting really confused by online sources, who are either not very clear, or seem to contradict each other.

I feel like I have narrowed it down to two sources;

1. The early universe was full of high energy photons which could not travel very far without hitting something. The CMB is the moment when atoms formed and unlike a plasma, neutral atoms are transparent so those photons have been traveling in a straight line since.

2. The energy released when electrons combine with nuclei to form atoms. I guess just like in a discharge tube. Wouldn't this mean it would have emission bands in its spectrum?

Which one of these is right?

Thank you

 

[PeroK]

It's both, essentially. Point 1 explains why photons from the very early universe were absorbed. Point 2 explains where photons come from in the first place.

The CMB has the spectrum of black body radiation.

 

[Nathi ORea]

It's both, essentially. Point 1 explains why photons from the very early universe were absorbed. Point 2 explains where photons come from in the first place.

The CMB has the spectrum of black body radiation.

Thank you so much for replying.. I had a feeling it might have been both.. but I don't get.. when hydrogen recombines it makes the characteristic hydrogen spectrum, with the emission bands. Does the CMB have this?

 

[PeroK]

First, check out a thread on here entitled CMBR Origins from March this year.

The process of multiple scattering produces a blackbody spectrum. See, for example:

http://www.astro.ucla.edu/~wright/CMB.html

 

[kimbyd]

I'm pretty sure it's almost entirely (1). If it was (2), then the photons would deviate greatly from a thermal black-body spectrum. The CMB is, I believe, the closest thing to a perfect black-body spectrum that has ever been observed.

Essentially, prior to the emission of the CMB, the universe was a near-ideal gas of photons and electrons (the protons and helium atoms didn't play a large dynamic role due to their larger mass and, in the case of helium which had already cooled, neutral charge). Once this gas cooled below about 3000K, the electrons combined with the protons to form neutral hydrogen. This recombination did release energy, but not enough to impact the spectrum of the light released.

I imagine the emission spectrum was probably further dampened by the fact that the recombination process itself took a long time (hundreds of thousands of years, if memory serves), so that many of the photons emitted during recombination would have had time to possibly interact with still-free electrons, rethermalizing them.

As for why photons from recombination would not be thermal, those atoms recombine in stages: first, the electron enters an excited state, then the atom rapidly decays to the ground state. Thus much of the energy released during recombination ends up in the specific energy bands that stem from the transition of the atoms between various excited states and eventually to the ground state.

These energy bands would be spread out due to redshift over the duration of recombination, but the fact that they aren't visible at all tells me that this couldn't have been a major contributor.

 

[JimJCW]

I feel like I have narrowed it down to two sources;

1. The early universe was full of high energy photons which could not travel very far without hitting something. The CMB is the moment when atoms formed and unlike a plasma, neutral atoms are transparent so those photons have been traveling in a straight line since.

2. The energy released when electrons combine with nuclei to form atoms. I guess just like in a discharge tube. Wouldn't this mean it would have emission bands in its spectrum?

According to the Big Bang model, before the recombination era, the universe was filled with photons and plasma (mainly protons and electrons). The photon-to-baryon ratio in the universe is about one billion to one. The number of photons emitted during the recombination era was, therefore, relatively very small. I think Point 1 is the right description.

 

[Nathi ORea]

God... Thanks so much for these responses. That makes things so much more clearer. You guys are all legends!

 

[Drakkith]

2. The energy released when electrons combine with nuclei to form atoms. I guess just like in a discharge tube. Wouldn't this mean it would have emission bands in its spectrum?

There was a thread recently about this and I remember looking up the below info. From: https://en.wikipedia.org/wiki/Recombination_(cosmology)

• Direct recombinations to the ground state of hydrogen are very inefficient: each such event leads to a photon with energy greater than 13.6 eV, which almost immediately re-ionizes a neighboring hydrogen atom.
• Electrons therefore only efficiently recombine to the excited states of hydrogen, from which they cascade very quickly down to the first excited state, with principal quantum number n = 2.
• From the first excited state, electrons can reach the ground state n=1 through two pathways:
  • Decay from the 2p state by emitting a Lyman-α photon. This photon will almost always be reabsorbed by another hydrogen atom in its ground state. However, cosmological redshifting systematically decreases the photon frequency, and there is a small chance that it escapes reabsorption if it gets redshifted far enough from the Lyman-α line resonant frequency before encountering another hydrogen atom.
  • Decay from the 2s state by emitting two photons. This https://en.wikipedia.org/w/index.php?title=Two-photon_decay&action=edit&redlink=1 process is very slow, with a rate[7] of 8.22 s−1. It is however competitive with the slow rate of Lyman-α escape in producing ground-state hydrogen.
• Atoms in the first excited state may also be re-ionized by the ambient CMB photons before they reach the ground state. When this is the case, it is as if the recombination to the excited state did not happen in the first place.