Q1 [3 marks]: The energy density of photons in a blackbody radiation field is: fi df exp[hf/kT 1 '8h e(f)df - (1) _ Show that if the CMB has a blackbody spectrum now (with T = 2.73K), it also had a blackbody spectrum at redshift z = 3. What was the temperature of the CMB at that time? [Hint: remember how photon frequency f, and photon number density change with redshift in order to relate the energy densities at the two epochs] Q1 [3 marks]: The energy density of photons in a blackbody radiation field is: fi df exp[hf/kT 1 '8h e(f)df - (1) _ Show that if the CMB has a blackbody spectrum now (with T = 2.73K), it also had a blackbody spectrum at redshift z = 3. What was the temperature of the CMB at that time? [Hint: remember how photon frequency f, and photon number density change with redshift in order to relate the energy densities at the two epochs]