  We review various aspects of de Sitter spacetime in string theory: its status
as an effective field theory spacetime solution, its relation to the vacuum
energy problem in string theory, its (global) holographic definition in terms
of two entangled and non-canonical conformal field theories, as well as a
realization of a realistic de Sitter universe endowed with the observed visible
matter and the necessary dark sector in order to reproduce the realistic
cosmological structure. In particular, based on the new insight regarding the
cosmological constant problem in string theory, we argue that in a doubled,
T-duality-symmetric, phase-space-like and non-commutative generalized-geometric
formulation, string theory can naturally lead to a small and positive
cosmological constant that is radiatively stable and technically natural. Such
a formulation is fundamentally based on a quantum spacetime, but in an
effective spacetime description of this general formulation of string theory,
the curvature of the dual spacetime is the cosmological constant of the
observed spacetime, while the size of the dual spacetime is the gravitational
constant of the same observed spacetime. Also, the three scales associated with
intrinsic non-commutativity of string theory, the cosmological constant scale
and the Planck scale, as well as the Higgs scale, can be arranged to satisfy
various seesaw-like formulae. Along the way, we show that these new features of
string theory can be implemented in a particular deformation of
cosmic-string-like models.
