CMB energy

Michal Frackowiak michalf w
Śro, 25 Lut 2004, 09:07:51 CET

Bartosz Lew wrote:

>On Tue, 24 Feb 2004, Jarek Rzepecki wrote:
>>On Tue, 24 Feb 2004, Bartosz Lew wrote:
>>>The correct answer is: Nothing happened to that energy. 
That is not the correct answer. A single photon loses its energy.

>>>It's still there
>>>(unless photons gets older or other crap like that), the only thing that 
>>>changed is that we no longer see hot photons because, as we've been tought 
>>>in high school, we're floating away from everything else according to the hubble law.
>>>(So the answer is essentialy in the question ;)
>>>The "doppler effect" which can easily be derived from SR transformaiton
>>>formulas causes  that we see everythig red. 
SR do not apply to cosmology in this case as I believe.

>>>The only difference from the
>>>situation described above with observers looking at identical photons (and
>>>yet seeing them in different colors) and
>>>complication I see here is that it is not possible to find a recerence frame
>>>in which we will see the whole CMB photons at different temperature - color
>>>etc. This is of course because it's the space that expands and there is nothing
>>>we can do about it, so changing reference frame won't help in understanding
>>>that it is just our obserwational effect not a true energy theft by some
>>>more less unidentified process. But still I belive that  this reddening can
>>>be ballanced once when Universe start to collapse.
>well, your question, as I understood it, was about what's with the energy
>that seems to be missing because of the redshift. 
>So I disagree. The cosmological redshift
>isn't caused by gravitational effects - i.e. gravitational reddening, but by 
>combined effect of time dilatation and lenght change which comes from
>transformation from one reference frame to another - and together we call it
>"a doppler effect".

O really? I always thought calling photon reddening a "doppler effect" 
is the most naive one.
A agree it comes from the transformation..... but it is far more 

If fact the problem as I see it is: photon field energy + energy of the 
gravitational field is conserved (when only those 2 fields are present). 
Have you never solved Friedmann's equations? Come back to the source, to 
the lagrangian density for the tensor grav. theory (Riemann tensors 
etc.) - I believe a conservation rule can be obtained at this level. As 
I remember the energy of the photon field (as well as any other form of 
the energy) is (depending on its equation of state) transfered between 
the grav. field and this particular field.

Why do you think photon field is trying to slow down the expansion??? 
Because of this interaction.

My answer is: do not treat photons as separate objects for the photon 
field is not an isolated system - the photon field interacts with the 
grav. field.
If GR does not give you the answer - that would be a problem.

>Sahs Wolfe effect is a minor one here, and fully understood in terms of GR
>However I do realise that my explanation is a bit outstretched, because SR
>do not allow space to change in time, or at least doesn't say anything about
Forget about SR in cosmology!!!!!!!!!!!!!!!!!!!

>But on the other hand a single photon, or any other particle (eg.
>proton, neutrino) can't  (or can ? - I think not :) feel it travels trough 
>space that is actually expanding. 
have you attended prof Rudak's lectures on cosmology?

>If it did, we all would grow in time, atoms would
>have become greater etc. So eventually it makes no difference weather space
>is expandind or observer is receeding, except the problem I said before >>
we do not grow in time because subatomic, atomic (electromagnetic) and 
even gravitational interaction on small scales are MUCH STRONGER than 
the effect of space expansion.

GR is not a "perfect" and "final" theory. But for sure all the modern 
cosmology (Friedmann's cosmology) is based on it. Search for the answer 
in GR.

regards - michal


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