# de Sitter Cosmology on an expanding bubble

## TL;DR

This paper explores a novel approach to realizing a metastable de Sitter universe within string theory by modeling it as a bubble wall between two AdS vacua, connecting higher-dimensional dynamics with four-dimensional cosmology.

## Contribution

It develops a detailed framework for embedding four-dimensional de Sitter cosmology on a bubble wall in a five-dimensional AdS bulk, linking non-perturbative decay, the weak gravity conjecture, and holography.

## Key findings

- Bulk interpretation of dS temperature as Unruh temperature
- Matter on the bubble from a string cloud in the bulk
- Derivation of 4D Einstein gravity from 5D Gauss equation

## Abstract

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.