Recently in my reading of PJE Peebles Principles of Physical Cosmology I
came across an intersesting if not inadequately explored issue.
I quote:
"...since the volume of the universe varies as a(t)3 (cubed), the net
radiation energy in a closed universe decreases as 1/a(t) as the universe
expands. Where does the lost energy go? Since there is no pressure gradient
in the homogeneously distributed radiation (Cosmic Background Radiation,
CBR) the pressure does not act to accelerate the expansion of the
universe...The resolution of this apparent paradox is that while energy
conservation is a good local concept and can be defined more generally in
the special case of an isolated system in asymptotically flat space, there
is not a general global energy conservation law in general relativity
theory."
-Principles of Physical Cosmology by PJE Peebles, pg.139.
Now I may just be ignorant, and I know relativity overturned alot of
newtonian physics, but I have read Einstein, Fenyman, Hawkings, and others
and this is the first I've heard of the law of conservation of energy being
a local phenomenon!