Basculement progressif du signe de l'énergie sombre : test du modèle ΛsCDM ✛ ✦

Résumé original

tel que rédigé par les auteurs, en anglais arXiv:2607.06735

Recent baryon acoustic oscillation (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI), in combination with CMB and supernova data, show a mild preference for dynamical dark energy over flat $$\Lambda$$CDM. They can also drive the best-fit effective neutrino mass to unphysical negative values, motivating models that mimic this effect through late-time expansion physics. The sign-switching cosmological constant ($$\Lambda_s$$CDM) model addresses this by introducing an AdS-like negative vacuum energy density at high redshifts ($$z > z^\dagger$$) that transitions to a dS-like positive density at low redshifts. While original fits assumed a discontinuous step function transition, we generalise the dynamics using a smooth hyperbolic tangent parametrisation, $$\Lambda(z) \propto \tanh[\Delta(z^\dagger - z)]$$. We constrain the transition redshift $$z^\dagger$$ and smoothness parameter $$\Delta$$ using background distance data from DES-SN5YR (with Dovekie recalibration) and DESI 2024 BAO. We further incorporate a compact redshift space distortion (RSD) $$f\sigma_8$$ compilation. The joint data do not prefer the smooth sign-switching model over flat $$\Lambda$$CDM, yielding nearly identical $$\chi^2_{\text{min}}$$ values, while information criteria penalise the additional parameters. The transition smoothness remains unconstrained and dominated by the prior volume ($$\Delta = 24.75^{+17.42}_{-17.48}$$), indicating that current data cannot distinguish between a sharp transition and a smooth dynamical crossover. This work does not directly constrain the physical sum of neutrino masses $$\sum m_ u$$; rather, it shows that current late-time background and growth data alone cannot resolve the transition details. We discuss the implications of this null detection and present projections for next-generation surveys.