A Friendly Phantom: Late-time AdS-to-dS transition and cosmological tensions

Abstract: We present Ph-$Λ{\rm s}$CDM, a phantom-scalar realization within General Relativity of the sign-switching cosmological-constant idea, $Λ{\rm s}$CDM, in which a phantom scalar evolving on a bounded hyperbolic-tangent potential induces a smooth mirror AdS-to-dS transition in the late-time dark-energy density. The wrong-sign kinetic term, usually viewed as pathological, becomes the mechanism lifting the field from a negative- to a positive-energy vacuum-like regime. The construction also shows that the field can become repulsive while its energy density is still negative. The cosmology nevertheless remains controlled: total energy stays positive, the late-time attractor is de Sitter rather than a Big Rip, and the dynamics remain safely infrared. Ph-$Λ_{\rm s}$CDM thus offers a concrete late-time mechanism with the potential to address multiple cosmological tensions.