  The late-time behavior of our universe is one of accelerated expansion, or
that of a de Sitter space, and therefore motivates us to look for
time-dependent backgrounds. Finding such backgrounds in string theory has
always been a challenging problem. An even harder problem is to find
time-dependent backgrounds that allow positive dark energies. As a first step
to handle such scenarios, we study a time-dependent background in type IIB
theory, with four-dimensional de Sitter isometries, by uplifting it to M-theory
and then realizing it as a coherent, or squeezed-coherent, state over an
appropriate solitonic configuration. While classically such a background does
not solve the equations of motion, the corresponding Schwinger-Dyson equations
reveal that there are deeper issues that may even prohibit a solution to exist
at the quantum level, as long as the internal space remains time-independent. A
more generic analysis is then called for, where both the effective
four-dimensional space-time, the internal space, and the background fluxes are
all time-dependent. We study in details such a background by including
perturbative and non-perturbative as well as local and non-local quantum terms.
Our analysis reveals a distinct possibility of the emergence of a
four-dimensional positive curvature space-time with de Sitter isometries and
time-independent Newton's constant in the landscape of type IIB string theory.
We argue how the no-go and the swampland criteria are avoided in generating
such a background, and compare it with other possibilities involving
backgrounds with time-dependent Newton constants. These time-varying Newton
constant backgrounds typically lead to unavoidable late time singularities,
amongst other issues.
