Dynamical dark energy signatures from a new transition Om(z) parametrization in flat FLRW cosmology

We investigate a cosmic scenario using a new transition parameterization of the Om ( z ) diagnostic, O m ( z ) = z l ( 1 + z ) m , in the spatially flat Friedmann Lemaître Robertson-Walker (FLRW) framework. Using observational datasets such as Observational Hubble Data (OHD), Pantheon Plus (PP), and SH0ES, we analyze the evolution of the Om ( z ) function to probe deviations from the standard ΛCDM model and constrain free parameter space H 0 , l, m using Markov Chain Monte Carlo (MCMC) analysis with the emcee sampler. Our analysis reveals a clear transition in the slope of Om ( z ) from negative to positive at transition redshift values z t ≈ 1.41, 0.65, and 0.33 for the OHD, OHD+PP, and OHD+PP & SH0ES datasets, respectively. This behavior suggests a dynamical evolution of dark energy, indicating a transition from a quintessence-like phase to a phantom regime. From the combined OHD+PP & SH0ES dataset, we obtain a best-fit value of the Hubble constant H 0 = 73.01 ± 0.36 km s − 1 Mpc − 1 , which is consistent with the SH0ES calibration and supports the viability of our model. Additionally, our analysis indicates that the current age of the Universe is approximately 13 ∼ 14 Gyr from all available combinations of datasets, which is consistent with observational expectations. Further, we find that the deceleration-to-acceleration transition, which marks the beginning of cosmic acceleration, is inferred to occur within the redshift interval z t ∈ [0.5, 0.8], highlighting the emergence of dark energy as the dominant component in the Universe’s recent expansion history. Our transition Om ( z ) parameterization captured progressive cosmological changes and enabled seamless interpolation over cosmic epochs.