  A matter-coupled scalar field model is presented in obtaining a scalar fifth
force when the constraint of the current cosmological constant is satisfied.
The interaction potential energy density between the scalar field and matter
has a symmetry-breaking form with two potential wells. The cosmological
constant is proven to be a value of the scalar-field's self-interaction
potential energy density at the minimum of an effective
matter-density-dependent potential energy density. The effective potential is a
sum of the interaction potential and the self-interaction potential of the
scalar field. The scalar field can stably sit at the minimum and then the
time-dependent cosmological `constant' behaves like a constant. The observed
cosmic acceleration can be accounted for the scalar field. The scalar field is
also extrapolated to account for inflation at the inflationary era of the
Universe. In this era matter fluid is relativistic and then the interaction
potential wells vanish. The unconfined quintessence therefore dominates the
evolution of the Universe. It is concluded that `Planck 2018 results' favour
the closed space of the Universe. The reasons are not only the measured value
of the current Hubble constant, but also the observed feature of a concave
potential in the framework of single-field inflationary models. By invoking a
pseudo-potential in the inflationary era, the concave feature can be attributed
to the pseudo-potential although the self-interaction potential is a convex
function. The pseudo-potential is defined by a sum of the self-interaction
potential and the energy density scale of the curvature of the Universe. It is
that the positive curvature leads to the concave feature of the
pseudo-potential.
