Hi, Been doing some cosmology revision and have a few questions owing to my terrible notes. One of the past papers asks the question "The density parameter of the Universe today has the value Ω = 0.25. Using the value of Hubble’s constant from (b), calculate the mean density of matter today. Estimate the matter density at the epoch of recombination (which occurred at a redshift z 1000)." In b) got value of H at 69.3km/s/mpc. Using pcrit=3H^2/8piG got the value 8.949*10^-27, then using Ω=ρ/ρcrit got the value 2.235*10&-27kg/m^3. The second part of the question slightly confuses me. I know εb=ε0b(1+z)^3 so would it just be 2.235*10^-27 *c^2 to give εb, and sub in z=1000. Would i be looking to solve for εb or ε0b, the notation is confusing me a bit. Second question is "Use the conservation of mass and photon number, and the scaling of photon energy, hν, with redshift, z, to calculate how the energy densities of matter and of radiation scale with (1 + z)." My notes don't cover this so any help would be appreciated. Sorry for the first post for help situation, thanks in advance.