CMB energy

Bartosz Lew Bartosz.Lew w
Wto, 24 Lut 2004, 17:33:23 CET

CMBR is almost perfectly well described by u(\ni) d\ni function - the
plack function which is the spectral energy density described with the
only parameter - temperature. Expansion changes the overall CMBR energy (as
we see it) like (1+z)^4 which comes from 4'th power T relation when we integrate
u(\ni) d(\ni) (to get Stefan boltzmann law) ( and there is stright
relation between temperature of boson gaz with it's enery - alghough
coefficients are different in classic and ultra relativistic regime).

Bosons are particles which participe positivly to universe's density and
pressure thus they only make expansion slower, hence IMHO it's incorrect to say
that "it takes some energy  to expand the Universe..." nor that the
expansion is derived by it's contents. In fact we don't know why BB
happened. In fact we don't even know if it happened. (although the
expansion rate is altered by the contents in various ways).

Recalling the energy density doesn't help much for the question since
there still remains 1+z factor left to explain.

It's the question why photons suffer from redshift? And is the fact
that different observers see, say two identical photons (each of them
observes his own photon) as two different photons actually a manifestation
of violation of energy conservation principle?

I've been thinking about this problem yesterday and what I concluded is...
shocking simple. :)

The correct answer is: Nothing happened to that 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. 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.

I hope that helps.


On Mon, 23 Feb 2004, Jarek Rzepecki wrote:

> Hi all,
>   CMB photons are red-shifted due to Universe expansion -> this means that 
> the whole CMB energy gets smaller... where does it go?
> pozdrawiam
> 	Jarek


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