On 4/9/2014 8:49 AM, Henry Baker wrote:
I don't see how it is possible for the number of photons to be conserved as the universe expands.
Experiment #1. Flat 3D space; no expansion.
A flash of green light at the origin expands in a spherical shell (as is usual in odd-dimensional spaces).
The intensity of this green light goes down as 1/r^2 as the spherical shell of radius r expands, but the light remains exactly the same green color.
The number of photons is conserved.
Experiment #2. Flat, but expanding 3D space.
A flash of green light at the origin expands in a spherical shell, and the light becomes redder.
Since red photons have less energy than green photons, there must be more of them to conserve the same energy from the initial flash.
Energy isn't globally conserved, or even well defined, if spacetime doesn't have a time-like Killing field. http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html Brent