Geometrical Origin of the Cosmological Constant
  We show that the description of the space-time of general relativity as a
diagonal four dimensional submanifold immersed in an eight dimensional
hypercomplex manifold, in torsionless case, leads to a geometrical origin of
the cosmological constant. The cosmological constant appears naturally in the
new field equations and its expression is given as the norm of a four-vector
$U$, i.e., ${\Lambda}=6g_{{\mu}{\nu}}U^{{\mu}}U^{{\nu}}$ and where U can be
determined from the Bianchi identities. Consequently, the cosmological constant
is space-time dependent, a Lorentz invariant scalar, and may be positive,
negative or null. The resulting energy momentum tensor of the dark energy
depends on the cosmological constant and its first derivative with respect to
the metric. As an application, we obtain the spherical solution for the field
equations. In cosmology, the modified Friedmann equations are proposed and a
condition on ${\Lambda}$ for an accelerating universe is deduced. For a
particular case of the vector $U$, we find a decaying cosmological constant
${\Lambda}\propto a(t)^{-6{\alpha}}$.
