  Constructing an explicit compactification yielding a metastable de Sitter
(dS) vacuum in a UV consistent string theory is an incredibly difficult open
problem. Motivated by this issue, as well as the conjecture that all
non-supersymmetric AdS vacua must decay, we discuss the alternative possibility
of realizing an effective four-dimensional dS cosmology on a codimension-one
bubble wall separating two AdS$_5$ vacua. The construction further elaborates
on the scenario of arXiv:1807.01570, where the aforementioned cosmology arises
due to a non-perturbative decay and is embedded in a five-dimensional bulk in a
time-dependent way. In this paper we discuss the relation between this scenario
and the weak gravity conjecture and further develop the details of the
four-dimensional cosmology. We provide a bulk interpretation for the dS
temperature as the Unruh temperature experienced by an accelerated observer
riding the bubble. A source of four-dimensional matter arises from a string
cloud in the bulk, and we examine the consequences for the particle mass
spectrum. Furthermore, we show how effective four-dimensional Einstein gravity
on the bubble is obtained from the five-dimensional Gauss equation. We conclude
by outlining some implications that this paradigm will have for holography,
inflation, the standard model, and black holes.
