  Different approaches to quantum cosmology are studied in order to deal with
the future singularity avoidance problem. Our results show that these future
singularities will persist but could take different forms. As an example we
have studied the big rip which appear when one considers the state equation
$P=\omega\rho$ with $\omega<-1$, showing that it does not disappear in modified
gravity. On the other hand, it is well-known that quantum geometric effects
(holonomy corrections) in loop quantum cosmology introduce a quadratic
modification, namely proportional to $\rho^2$, in Friedmann's equation that
replace the big rip by a non-singular bounce. However this modified Friedmann
equation could have been obtained in an inconsistent way, what means that the
obtained results from this equation, in particular singularity avoidance, would
be incorrect. In fact, we will show that instead of a non-singular bounce, the
big rip singularity would be replaced, in loop quantum cosmology, by other kind
of singularity.
