## FAQ on HW #6

*Eq. (1) doesn't look right.*

This is a yet-another typo in the homework problem to my embarrassment.
The numerator in dr integral should *not* have r^2. Thanks to Adam!

*What is Omega(gamma)?*

It is the ratio of the energy density of CMBR photons to the critical
density.

*The energy is conserved when electron, positron annihilate into
photons, right?*

Wrong. The energy constantly changes because the expansion of the Universe
constantly red-shift all the wavelengths. On the other hand, there is no
heat source from "outside" the Universe. Therefore, dQ = T dS = 0, and
hence what is conserved is the entropy per comoving volume.

murayama@lbl.gov