  As an alternative to dark energy it has been suggested that we may be at the
center of an inhomogeneous isotropic universe described by a
Lemaitre-Tolman-Bondi (LTB) solution of Einstein's field equations. In order to
test such an hypothesis we calculate the low redshift expansion of the
luminosity distance $D_L(z)$ and the redshift spherical shell mass density
$mn(z)$ for a central observer in a LTB space without cosmological constant and
show how they cannot fit the observations implied by a $\Lambda CDM $ model if
the conditions to avoid a weak central singularity are imposed, i.e. if the
matter distribution is smooth everywhere. Our conclusions are valid for any
value of the cosmological constant, not only for $\Omega_{\Lambda}>1/3$ as
implied by previous proofs that $q^{app}_0$ has to be positive in a smooth LTB
space, based on considering only the luminosity distance.
  The observational signatures of smooth LTB matter dominated models are
fundamentally different from the ones of $\Lambda CDM $ models not only because
it is not possible to reproduce a negative apparent central deceleration
$q^{app}_0$, but because of deeper differences in their space-time geometry
which make impossible the inversion problem when more than one observable is
considered, and emerge at any redshift, not only for $z=0$.
