  Recent measurements require modifications in conventional cosmology by way of
introducing components other than ordinary matter into the total energy density
in the universe. On the basis of some dimensional considerations in line with
quantum cosmology, Chen and Wu [W. Chen and Y. Wu, Phys. Rev. D 41, 695 (1990)]
have argued that an additional component, which corresponds to an effective
cosmological constant $\Lambda$ must vary as a^{-2} in the classical era. Their
decaying-$\Lambda$ model assumes inflation and yields a value for q_{0}, which
is not compatible with observations. We generalize this model by arguing that
the Chen-Wu ansatz is applicable to the total energy density of the universe
and not to $\Lambda$ alone. The resulting model, which has a coasting evolution
(i.e., $a \propto t$), is devoid of the problems of horizon, flatness,
monopole, cosmological constant, size, age and generation of density
perturbations. However, to avoid serious contradictions with big bang
nucleosynthesis, the model has to make the predictions $\Omega_{m} = 4/3$ and
$\Omega_{\Lambda}=2/3$, which in turn are at variance with current
observational values.
