A volume containing a photon--baryon fluid is adiabatically expanded or compressed. The energy density of the fluid is ε=εγ+εbary, and the pressure is P=Pγ=εγ/3. What is dP/dε for the photon--baryon fluid? What is the sound speed, cs? In Equation 11.26, how large an error did we make in our estimate of λJ(before) by ignoring the effect of the baryons on the sound speed of the photon--baryon fluid?