We explore the implications for an anti–de Sitter (AdS) vacuum, equivalently a negative cosmological constant, in the dark energy (DE) sector using current cosmological observations. Our joint analysis uses dark energy spectroscopic instrument baryon acoustic oscillation, the dark energy survey 5 year supernova, and P-ACT cosmic microwave background (ACT-DR6 + Planck) measurements. We also use the kilo-degree survey weak-lensing measurement to cross-check consistency with the inferred value of $S_8$. Within the Chevallier-Polarski-Linder (CPL) parametrization for the evolving component of the DE, the inclusion of an AdS term provides a theoretically motivated mechanism that opens up a finite nonphantom region in the CPL parameter space while remaining compatible with current observations. A negative cosmological constant also implies a finite cosmic lifetime, thereby linking observational DE phenomenology to broader questions in quantum gravity and string theory.

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• Cosmological parameters
• Cosmology
• Dark energy
• Evolution of the Universe
• Large scale structure of the Universe