1. The problem statement, all variables and given/known data Calculate the angular size of the comoving horizon at the z=1100 last scattering surface, as projected on to the current (CMB) sky. Assume flat FRW cosmology and no cosmological constant. First calculate angular diameter distance to last scattering, then the particle horizon at last scattering. 2. Relevant equations Particle horizon dH = a(t) ∫0t dt'/at' = a(η)η Angular diameter distance dA = R0Sk(χ)/(1 + z) = dL/(1+z)2 3. The attempt at a solution I have used a = 1/(1+z) to go from z = 1100 to a = 9.08 x 10-4 Looking at the definition of particle horizon, I need to find what t is at z = 1100, so I tried to get this using the first Friedman equation, and rearranging it to get da/dt = a √(8∏Gρ/3) 1/a da = √(8∏Gρ/3) dt ∫ 1/a da = √(8∏G/3) ∫ρ(t) dt I don't know how to go any further with this though to find t at some a. Looking at the angular diameter distance, I have a flat unverse, so Sk(χ) = χ. I think R0 and χ are both just arbitrary measures though, so I have no idea how to do anything useful with that definition!