Emergent de Sitter Cosmology from Decaying AdS
Souvik Banerjee, Ulf Danielsson, Giuseppe Dibitetto, Suvendu Giri, Marjorie Schillo
TL;DR
The paper confronts the difficulty of obtaining a small positive vacuum energy in string theory by proposing a braneworld scenario in which 4D cosmology emerges on a shell that mediates decay between AdS5 vacua, avoiding the need for scale-separated de Sitter vacua. It develops a concrete shellworld framework where the shell tension sets an effective Λ4 via Λ4 = Λ_ext − σ, includes bulk mass contributions as a radiation term, and accommodates shell-localized matter from strings ending on the shell, yielding a realistic 4D evolution. It demonstrates gravity localization on the shell through a graviton zero mode, yielding a 4D Newton constant G4 in terms of the bulk parameters k_±, and shows the necessity of string sources for a consistent localized propagator. Finally, it provides a string theory embedding via Romans vacuum decay mediated by a spherical (p,q)-5-brane, discusses fluxes, brane tensions, and the conditions σ_BPS < σ_ext for a finite-action decay, and outlines a full stringy realization with future work to address stability and backreaction.
Abstract
Recent developments in string compactifications demonstrate obstructions to the simplest constructions of low energy cosmologies with positive vacuum energy. The existence of obstacles to creating scale-separated de Sitter solutions indicates a UV/IR puzzle for embedding cosmological vacua in a unitary theory of quantum gravity. Motivated by this puzzle, we propose an embedding of positive energy Friedmann-Lemaitre-Robertson-Walker cosmology within string theory. Our proposal involves confining 4D gravity on a brane which mediates the decay from a non-supersymmetric false AdS5 vacuum to a true vacuum. In this way, it is natural for a 4D observer to experience an effective positive cosmological constant coupled to matter and radiation, avoiding the need for scale separation or a fundamental de Sitter vacuum.