<p>I would expect that energy of photons in the <a href="https://en.wikipedia.org/wiki/Cosmic_microwave_background" rel="nofollow noreferrer">Cosmic ray Microwave Background</a> gets less and less because their wavelengths are stretched due to the expansion of space.</p>
<p>How can this be possible? Does it only look like their wavelength is increasing because we, as observers are not &quot;expanding along&quot;?</p>
<p>Where does the energy of the decaying CMB go?</p>

CC BY-SA 4.0
2021-06-08T16:51:50.007Z
See https://physics.stackexchange.com/q/259759/123208 & links therein.
PM 2Ring
CC BY-SA 4.0
2021-06-08T18:14:18.447Z
[It doesn't go anywhere](https://kaiserscience.files.wordpress.com/2015/08/is-the-universe-leaking-energy-law-conservation.pdf).
pela
CC BY-SA 4.0
2021-06-09T08:21:58.637Z
@planetmaker But the wavelength of each photon decreases. This means (the number of photons stays the same) that the total energy goes down. (as seen from our perspective).
Deschele Schilder
CC BY-SA 4.0
2021-06-09T08:33:47.260Z
@pela That's a very nice article!
Deschele Schilder
CC BY-SA 4.0
2021-06-09T08:41:27.540Z
@planetmaker In the article linked by Pela, Tamara Davis mentions that the apparent energy lost via redshift is *not* balanced by the creation of dark energy.
PM 2Ring
CC BY-SA 4.0
2021-06-09T08:46:26.240Z
@PM2Ring Is it just an apparent loss? Because *we* are not expanding, so to speak? So *it looks* as if the wavelength of the photons is growing? So seen from us dark energy *has* to be there to compensate?
Deschele Schilder
CC BY-SA 4.0
2021-06-09T08:53:35.697Z
@PM2Ring interesting... thanks!
planetmaker
CC BY-SA 4.0
2021-06-09T08:54:09.383Z
Barbierium, you should check out Prof Davis's [homepage](https://people.smp.uq.edu.au/TamaraDavis/). There's lots of great stuff in the Research section and videos in the Talks and Outreach section. FWIW, her PhD topic was on the expansion of space, and her papers and popular articles "Clarifying common misconceptions about the expansion of the universe and cosmic horizons" are *very* highly regarded.
PM 2Ring
CC BY-SA 4.0
2022-08-25T17:06:41.500Z
Wavelength of incoming photon is defined by the frequency of said photon. And frequency is defined relative to time. As space expands fast enough the time dilation explained by GR makes it appear that the photons are losing energy when we measure the frequency *relative to our local time*.
Mikko Rantalainen
<cosmology><light><cosmic-microwave-background>
Where does the energy of the decaying CMB go?
Deschele Schilder
# Where does the energy of the decaying CMB go?

(Asked by: username_0 on 2021-06-08)

I would expect that energy of photons in the [Cosmic ray Microwave Background](https://en.wikipedia.org/wiki/Cosmic_microwave_background) gets less and less because their wavelengths are stretched due to the expansion of space.

How can this be possible? Does it only look like their wavelength is increasing because we, as observers are not "expanding along"?

Where does the energy of the decaying CMB go?

**Tags:** cosmology, light, cosmic-microwave-background

> **Comments:**
> **username_1**: See https://physics.stackexchange.com/q/259759/123208 & links therein.  
> **username_2**: [It doesn't go anywhere](https://kaiserscience.files.wordpress.com/2015/08/is-the-universe-leaking-energy-law-conservation.pdf).  
> **username_0**: @username_3 But the wavelength of each photon decreases. This means (the number of photons stays the same) that the total energy goes down. (as seen from our perspective).  
> **username_0**: @username_2 That's a very nice article!  
> **username_1**: @username_3 In the article linked by username_2, Tamara Davis mentions that the apparent energy lost via redshift is *not* balanced by the creation of dark energy.  
> **username_0**: @username_1 Is it just an apparent loss? Because *we* are not expanding, so to speak? So *it looks* as if the wavelength of the photons is growing? So seen from us dark energy *has* to be there to compensate?  
> **username_3**: @username_1 interesting... thanks!  
> **username_1**: Barbierium, you should check out Prof Davis's [homepage](https://people.smp.uq.edu.au/TamaraDavis/). There's lots of great stuff in the Research section and videos in the Talks and Outreach section. FWIW, her PhD topic was on the expansion of space, and her papers and popular articles "Clarifying common misconceptions about the expansion of the universe and cosmic horizons" are *very* highly regarded.  
> **username_4**: Wavelength of incoming photon is defined by the frequency of said photon. And frequency is defined relative to time. As space expands fast enough the time dilation explained by GR makes it appear that the photons are losing energy when we measure the frequency *relative to our local time*.  

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## 0 answers

