# Photon energy in an expanding universe

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• sector99
sector99
Now...exactly when and how do CMB photons lose their 3000˚ K creation energy to become 2.7˚ K?

Mentor
exactly when and how do CMB photons lose their 3000˚ K creation energy to become 2.7˚ K?

This happens during the entire history of expansion of the universe from the surface of last scattering to now.

sector99
With respect ... how can CMB photons undergo any change ... since time (for the CMB photon) hasn't passed ( dτ0 = 0 ) ?

The above according to Einstein.

As for this relation to DM ... measurements of CMB energy have everything to do with this thread.

Mentor
There is no "time for the CMB photon" and such a thing is not necessary. The wavelength of the photons scales with the overall expansion of the universe. This is a prediction of general relativity.

As this has nothing to do with the original thread I split the post into a new thread.

Mentor
how can CMB photons undergo any change ... since time (for the CMB photon) hasn't passed ( dτ0 = 0 ) ?
The above according to Einstein.
This misunderstanding is so pervasive that we even have a FAQ for it (which you will find if you search the forum for "rest frame photon"). Einstein said no such thing; what he did say is that there is no inertial frame in which light is at rest. Yes, if you assume that such a frame did exist and naively apply the equations of special relativity while setting ##v=c## you would get a division by zero that could be understood as time not passing in that frame - but those equations are derived from an assumption that is equivalent to the assumption that there is no such frame so cannot be correctly applied in this way.

What's going on with the cosmic microwave radiation is no different than what's going on with the ordinary doppler effect - the same amount of energy is present but it's spread out over a larger volume of time and space meaning less energy delivered per unit time, which manifests as red shift. This will be less surprising if you remember that kinetic energy is always frame-dependent, even in classical physics.

sector99
Just today, Adam Riess clearly admitted that the community "we're missing something". That "something" is so big that it has misled researchers.

The disconnect (apparently from below) arises from the reality that time hasn't passed for the photon. Thus the freely propagating CMB photon carries all its creation energy and frequency until detection. The Fahr/Heyl implication effects (1) expansion rate and (2) vacuum energy density–as shown in the paper's introduction...

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• Fahr:Heyl:How are Cosmic Phons Redshifted?:2017.pdf
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sector99
There is no "time for the CMB photon" and such a thing is not necessary. The wavelength of the photons scales with the overall expansion of the universe. This is a prediction of general relativity.

As this has nothing to do with the original thread I split the post into a new thread.

As you can see from the abstract of Fahr/Heyl (keywords, above) the phrase "Dark Matter" appears and this is why I chose to post the above paper as well as my query RE: When exactly do CMB photons lose their creation energy from 3000˚ K to 2.7˚ K ? in the Dark Matter thread. It has everything to do with the current, apparently unsolvable impasse confronting cosmologists today.

Mentor
Just today, Adam Riess clearly admitted that the community "we're missing something". That "something" is so big that it has misled researchers.

This is not relevant to the topic of this thread.

The disconnect (apparently from below)

This paper just repeats the same misconception you gave in your OP. It is not in any way a valid explanation of the "disconnect" you refer to.

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As you can see from the abstract of Fahr/Heyl (keywords, above) the phrase "Dark Matter" appears and this is why I chose to post the above paper as well as my query RE: When exactly do CMB photons lose their creation energy from 3000˚ K to 2.7˚ K ? in the Dark Matter thread. It has everything to do with the current, apparently unsolvable impasse confronting cosmologists today.
If You moved towards the CMB radiation fast enough, the photons would get all their energy back.

Frequency is frame dependent so not an inherent property of the photon itself.

Any photon has different frequency and energy in different reference frames.

sector99
This is not relevant to the topic of this thread.

This paper just repeats the same misconception you gave in your OP. It is not in any way a valid explanation of the "disconnect" you refer to.

This isn't the only paper.

This question is certainly related to (1) expansion rates as well as (2) Dark Matter.

sector99
If You moved towards the CMB radiation fast enough, the photons would get all their energy back.

Frequency is frame dependent so not an inherent property of the photon itself.

Any photon has different frequency and energy in different reference frames.

The authors above remind that the energy/frequency changes occur only at the adibatic world clock detection time and place.

Mentor
This isn't the only paper.

Then you need to provide links to these other papers (which you will need to do by PM to me or one of the other Mentors, since, as you will see below, this thread is being closed); just an image of a bunch of paper titles tells us nothing useful.

Also, if these other papers make the same claim as the one in what you quoted, it's still wrong; repeating a wrong claim a bunch of times doesn't make it right.

This question is certainly related to (1) expansion rates as well as (2) Dark Matter

Neither of which is the topic of this thread. This thread's topic is the specific question you asked in the OP, which has been answered.