  Causality principle is a powerful criterion that allows us to discriminate
between what is possible or not. In this paper we study the transition from
decelerated to accelerated expansion in the context of Cardassian and dark
energy models. We distinguish two important events during the transition. The
first one is the end of the matter-dominated phase, which occurs at some time
$t_{eq}$. The second one is the actual crossover from deceleration to
acceleration, which occurs at some $t_{T}$. Causality requires $t_{T} \geq
t_{eq}$. We demonstrate that dark energy models, with constant $w$, and
Cardassian expansion, are compatible with causality only if $(\Omega_{M} -
\bar{q}) \leq 1/2$. However, observational data indicate that the most probable
option is $(\Omega_{M} - \bar{q}) > 1/2$. Consequently, the transition from
deceleration to acceleration in dark energy and Cardassian models occurs before
the matter-dominated epoch comes to an end, i.e., $t_{eq} > t_{T}$. Which
contradicts causality principle.
