Constraints on alternative models to dark energy

Recent observations of type Ia supernovae strongly support that the universe is accelerating now and decelerated in the recent past. This may be evidence of the breakdown of the standard Friedmann equation. We consider a general modified Friedmann equation. Three different models are analysed in detail. The current supernova data and the Wilkinson microwave anisotropy probe data are used to constrain these models. A detailed analysis of the transition from the deceleration to acceleration phase is also performed.