Observational constraints on early time non-phantom behaviour of dynamical dark energy

Abstract

We investigate dynamical dark energy models that admit non-phantom behaviour at early times, including thawing, scaling--thawing, and effective fluid extensions. Using current cosmological observations, we find that late-time background parameters remain stable across all models. Time-dependent parametrizations such as CPL show a $\sim2σ$ preference for phantom evolution at low redshift. Imposing non-phantom scaling dynamics at early times leads to strong lower bounds on the potential steepness, $λ\gtrsim 20$--$30$, constraining the early dark energy density to below the percent level at matter--radiation equality. Consequently, early scaling behaviour does not alleviate the Hubble tension and is penalised by Bayesian model selection. Our results indicate that while late-time dynamics can mildly improve the fit, early non-phantom scaling is strongly disfavoured by current data.