Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Cosmological redshift



On Mon, 20 Oct 1997, Leigh Palmer wrote:

I'm afraid that I am not a cosmologist either, but I'll have a go at this.
David Bowman can probably shoot it down where it is excessively naive, but
I don't mind erecting exceedingly flammable straw men for him.

Imagine a box with rigid perfectly reflecting walls. The box is filled with
radiation derived from a blackbody source which has since been removed from
the box. The photons (if you like to think of photons) bounce around in the
box without interacting. Alternatively one can think of the box as having
many normal modes of electromagnetic oscillation, exch with nodes at the
walls of the box. Since the walls are rigid and perfectly reflecting,
whatever the occupation numbers are for these modes (the numbers of
photons in each mode) they will persist for eternity.

Now relax the constraining box to allow one wall to move perpendicularly to
its surface as a piston. As the box expands the radiation does work on the
wall, and all the eigenenergies of those normal modes decrease. This
process is quasistatic and adiabatic, and the result is a stretching of the
individual mode wavelengths. The occupation numbers remain as they were.
The space does expand.

Another way to look at this (which is quantitatively the same) is to
consider what happens to a photon when it is reflected from a wall moving
away from it. The reflected photon will be Doppler shifted to a longer
wavelength and will have transferred momentum/energy to the wall.

In the prevailing cosmological view this is what has happened to the fossil
fireball since it was uncoupled from the hot, opaque plasma of creation
300,000 years after the big bang. Since then the linear scale of the
universe has increased a thousandfold. This increase may have been in the
total volume of space itself (that is called a closed universe) or merely
in the local volume we call our observable universe.

I must hasten to add (David will probably not like this) that photons can
be written out of the script entirely. This effect is purely classical. It
seems to be fashionable now to write in a part for photons these days even
when it is gratuitous.

Leigh




What, if anything, are the photons reflecting from? This has always been
very confusing to me.

W. Barlow Newbolt 540-463-8881 (telephone)
218 Howe Hall 540-463-8884 (fax)
Washington and Lee University newbolt.w@fs.science.wlu.edu
Lexington, Virginia 24450 wnewbolt@liberty.uc.wlu.edu

"Prediction is very difficult, especially about the future."

Neils Bohr