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I sort of understand it but I am not sure if my understanding is correct.

So what I understand of the CMB is:

When the universe was created it was very hot, too hot for matter to form.

When matter tried to form a photon would smash the electron away.

This would scatter the electron and photon in random directions.

Over time the universe expanded and cooled. This means that the photons no longer had enough energy to break apart matter, this was the last time photons got scattered. After that the photons went in a straight line.

These photons were red shifted to be in the microwave band, these microwaves are what we now see as the CMB.

I have a few questions:

How did the photons lose energy, i get how as the universe expanded there would be less photons per km2 but wouldnt the photons still have enough energy to break apart matter, if the photons lost energy wouldnt that violate the law of conservation of energy?

Is my understanding of the CMB correct?

hbblue
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  • I'll post an answer in a few hours if someone else doesn't. Good question +1 – shai horowitz May 18 '21 at 09:08
  • Photons are not pointlike particles, so when you increase the volume of space, the photon spreads and dilutes, making it a longer wavelength and thus lower energy. On the funny (unrelated) side, the expanding universe itself does violate the conservation of energy. – tobalt May 18 '21 at 10:26
  • I like that the title says "Cosmic Background Microwave"! :-) Good question... does it make more sense to you if you do not think about photons, but electromagnetic waves, where energy of a photon corresponds to wavelength? – Koschi May 18 '21 at 11:54
  • So the expansion of the universe causes the electromagnetic wave to redshift wich lowers its energy? – hbblue May 18 '21 at 11:57

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