Orateur
Description
The remarkable phenomenological success of the concordance $\Lambda$CDM model is accompanied by persistent tensions and anomalies, including those associated with $H_0$, $S_8$, the growth index $\gamma$, and cosmological inference of the neutrino-mass sum $\sum m_\nu$. These discrepancies may reflect unaccounted-for systematics, but they may also point to missing structure in the standard cosmological framework. In this talk, we discuss $\Lambda_{\rm s}$CDM, a sign-switching effective cosmological-constant scenario in which the dark-energy sector undergoes a rapid, cosmologically late sign transition near $z_\dagger\sim2$, from an AdS-like negative vacuum-energy contribution to a dS-like positive one. Our main focus is the theoretical status of this framework: whether such a transition, either in the abrupt limit or smoothed over a narrow redshift interval, can arise from a more fundamental description rather than being imposed as a phenomenological prescription. We review representative mechanisms and model-building settings capable of generating $\Lambda_{\rm s}$CDM-like dynamics, clarify the physical interpretation of an effective AdS-like--to--dS-like transition in a cosmological background, and discuss its main observational signatures in distance, expansion, and growth probes. For smooth and sufficiently rapid realizations, we also highlight the possibility of a transient intermediate acceleration phase near $z_\dagger$, whose onset can occur on the negative-density branch during the transition epoch. We conclude by commenting on recent observational and theoretical developments, emphasizing how this framework provides a minimal but structurally novel extension of $\Lambda$CDM and a testable route toward addressing cosmological tensions.