Anti-de Sitter spacetime

What the term means here

The maximally symmetric solution of Einstein gravity with negative cosmological constant. Its geometry has a timelike boundary at spatial infinity.

Why it matters to this question

Supersymmetric anti-de Sitter vacua are comparatively easy to obtain in string theory, and KKLT begins from one. The difficulty is the last step: lifting that vacuum to positive vacuum energy. The asymmetry between how easy AdS is and how hard dS is, is the shape of the whole problem.

What this does not establish

Our universe is not anti-de Sitter. The tractability of AdS in string theory is a statement about the mathematics, not about the observed spacetime.