When Did the Universe Start Accelerating?

[June_cosmo]

Homework Statement

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?

Homework Equations

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?

 

[Orodruin]

Accelerating expansion means that the second derivative of the scale factor $a(t)$ is positive.

 

[June_cosmo]

Accelerating expansion means that the second derivative of the scale factor $a(t)$ is positive.

Accelerating expansion means that the second derivative of the scale factor $a(t)$ is positive.

I see. Should I use the deceleration formula?