1. The problem statement, all variables and given/known data if a(t) = a0(t/t0)^(alpha) how long does it take for a light ray to circle the universe if K = 1 2. Relevant equations The LFRW metric 3. The attempt at a solution it involves that metric and an integral at theta = pi/2 , phi [0,2pi], l [?] and ...... and something about proper time. also we know that for null geodesics Guv*dXu*dXv = 0 very interesting question but i cant seem to start it, any suggestions?