1. The problem statement, all variables and given/known data Assume the cosmological model with H0=72,Omega_M=1-Omega_lamda=0.3,(so dark energy with w = − 1) and a ﬂat universe.) a) Find the redshift z at which the universe starts accelerating (that is, when it transitions from decelerating to accelerating). b) How long ago did this happen? 2. Relevant equations 3. The attempt at a solution I don't quite understand the accelerating here. Does it mean that the expanding rate begins to accelerate? How should I start this problem?