String Theory has a “landscape” of solutions (space-time vacua) with negative cosmological constant (anti-de Sitter spaces) while fairly recent observations tell us that our universe undergoes an accelerated expansion, hence has a positive cosmological constant, corresponding to a de Sitter space. Obtaining such de Sitter spaces remains one of the major open problems in string theory. The most generic proposal starts from any solution in the landscape of anti-de Sitter vacua, and argues that one can transform it into a long-lived metastable de Sitter space by placing appropriate branes (extended dynamical objects that generalizes particle and strings). This mechanism gives rise to a huge landscape of string theory de Sitter vacua. I. Bena, M. Graña and collaborators at IPhT have shown that taking into account the dynamics of the branes always makes the solution singular, which invalidates the whole mechanism. This (unexpected) result suggests that string theory does not admit a landscape of de Sitter vacua, and opens the door for new inventive paradigms.
I. Bena, M. Grana, S. Kuperstein and S. Massai, "Anti-D3 Branes - Singular to the Bitter End", Phys. Rev. D 87, 106010 (2013).