# Onset of Universe acceleration

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1. Oct 31, 2015

### June_cosmo

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?

2. Oct 31, 2015

### Orodruin

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

3. Nov 1, 2015

### June_cosmo

I see. Should I use the deceleration formula?