Value for redshift change in acceleration

[jc09]

If the universe was decelarating and then started to accelerate can we calculate when this happened, and if so how can we do this?

 

[mathman]

Cosmologists estimate that the acceleration began roughly 5 billion years ago. Before that, it is thought that the expansion was decelerating, due to the attractive influence of dark matter and baryons. The density of dark matter in an expanding universe decreases more quickly than dark energy, and eventually the dark energy dominates. Specifically, when the volume of the universe doubles, the density of dark matter is halved but the density of dark energy is nearly unchanged (it is exactly constant in the case of a cosmological constant).
-----------------------------------------------------------------------------
The above is from Wikipedia

http://en.wikipedia.org/wiki/Dark_energy