<p>The CMB consists of photons that were emitted very shortly before and during the recombination of electrons with hydrogen and helium nuclei. [Side note - <em>why is</em> it called the epoch of recombination when the atoms were never "combined" to begin with?]</p>

<p>Those photons were then able to travel, unabsorbed across the universe until they encountered our microwave telescopes 13.7 billion years later. </p>

<p>Although the photons are unabsorbed, the expansion of the universe causes a redshift as they travel. Their current wavelengths are approximately 1100 times larger than when they were emitted and the universe is <em>almost</em> completely transparent to photons of this wavelength - that is there is very little chance of a photon being <em>absorbed</em> even if it travels across the diameter of the observable universe. </p>

<p>There are <em>scattering</em> processes that can deviate the photons or even increase their energy (and decrease their wavelength) via inverse Compton scattering (the <a href="https://en.wikipedia.org/wiki/Sunyaev%E2%80%93Zel&#39;dovich_effect" rel="nofollow noreferrer">Sunyaev-Zel'dovich effect</a>), but this does not affect the CMB spectrum a lot and is difficult to observe.</p>

<p>Our current ideas about the future of the universe are that it will continue to expand at an accelerating rate. The CMB photons will become even more redshifted - to radio wavelengths and beyond - and the universe will become even more transparent to them.</p>

<p>Related: <a href="https://astronomy.stackexchange.com/questions/16508/future-redshifting-and-effect-on-the-pitch-of-cmb-radiation?rq=1">Future redshifting and effect on the &#39;pitch&#39; of CMB radiation</a></p>

CC BY-SA 3.0
2016-12-29T13:13:09.417Z
2016-12-29T13:13:09.417Z
2017-04-13T12:59:06.910Z
ProfRob
<p>Is the CMB that we see the same, unchanged energy from this moment? Does the radiation of the CMB exist as Photons and are we only witnessing the photons that have not been absorbed or destroyed? Will there be a time we can no longer witness the CMB because it has all been absorbed? How does red-shift factor in to this?</p>

<redshift><cosmic-microwave-background><photons>
Is the CMB the photons that were created at the birth of the atom?
Kanero
# Is the CMB the photons that were created at the birth of the atom?

(Asked by: username_0 on 2016-12-29)

Is the CMB that we see the same, unchanged energy from this moment? Does the radiation of the CMB exist as Photons and are we only witnessing the photons that have not been absorbed or destroyed? Will there be a time we can no longer witness the CMB because it has all been absorbed? How does red-shift factor in to this?

**Tags:** redshift, cosmic-microwave-background, photons


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## 1 answers

### Answer by username_1 on 2016-12-29. Score: 4 | Accepted answer

The CMB consists of photons that were emitted very shortly before and during the recombination of electrons with hydrogen and helium nuclei. [Side note - *why is* it called the epoch of recombination when the atoms were never "combined" to begin with?]

Those photons were then able to travel, unabsorbed across the universe until they encountered our microwave telescopes 13.7 billion years later.

Although the photons are unabsorbed, the expansion of the universe causes a redshift as they travel. Their current wavelengths are approximately 1100 times larger than when they were emitted and the universe is *almost* completely transparent to photons of this wavelength - that is there is very little chance of a photon being *absorbed* even if it travels across the diameter of the observable universe.

There are *scattering* processes that can deviate the photons or even increase their energy (and decrease their wavelength) via inverse Compton scattering (the [Sunyaev-Zel'dovich effect](https://en.wikipedia.org/wiki/Sunyaev%E2%80%93Zel'dovich_effect)), but this does not affect the CMB spectrum a lot and is difficult to observe.

Our current ideas about the future of the universe are that it will continue to expand at an accelerating rate. The CMB photons will become even more redshifted - to radio wavelengths and beyond - and the universe will become even more transparent to them.

Related: [Future redshifting and effect on the 'pitch' of CMB radiation](https://astronomy.stackexchange.com/questions/16508/future-redshifting-and-effect-on-the-pitch-of-cmb-radiation?rq=1)


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