A Model of Vanishing Cosmological Constant
  We propose a model in which there exists a real scalar field $q$ satisfying a
condition $\dot{q} =MH$ and its energy density is given by
$(1/2)\dot{q}^2+V(q)$, where $H$ is the Hubble parameter ($H=\dot{a}/a$) and
$M$ is a mass scale characterizing the field. We show that the potential $V(q)$
of the field is uniquely determined by the condition. The potential depends on
the energy densities of background matters. We find that the vacuum energy of
the matters is cancelled by the potential of the field. As a result, the
minimum of the total energy density of the matters and the field vanishes and
is located at the infinite scale factor $a=\infty$. This remarkable property
results without a supersymmetry. We show that the present tiny dark energy is
caused by early inflation, while the energy is comparable to Planck scale
before the inflation. As our model is reduced to the $\Lambda$CDM model in the
limit $M\to 0$, it is a natural generalization of the $\Lambda$CDM model.
